AMA | July 2025

Welcome to the July 2025 Ask Me Anything episode of Mindscape! These monthly excursions are funded by Patreon supporters https://www.patreon.com/seanmcarroll (who are also the ones asking the questions). We take questions asked by Patreons, whittle them down to a more manageable number -- based primarily on whether I have anything interesting to say about them, not whether the questions themselves are good -- and sometimes group them together if they are about a similar topic. Enjoy! Blog post with questions and transcript: https://www.preposterousuniverse.com/podcast/2025/07/07/ama-july-2025/

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:27Speaker 01

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• 0:58Sean Carroll?

  Hello, everyone. Welcome to the July 2025 Ask Me Anything edition of the Mindscape Podcast. I'm your host, Sean Carroll. As you know, if you listened to last week's solo episode, I'm actually back now from traveling. Maybe you didn't know that. You knew I was traveling. Now I am back. I was visiting Santa Fe, the Santa Fe Institute, among other places. And now I'm back in the good old studio, so the audio quality should be back to where it normally is. And I thought I would start off the AMA with two anecdotes that I came back with from Santa Fe. They're only vaguely Santa Fe-related. They're of broader interest, I think. One is, when I was driving back to Albuquerque to catch the flight home to Baltimore, killing time, I was looking for a podcast to listen to.

• 1:42Sean Carroll?

  In the car is the only time I really have to listen to podcasts. And the algorithm served me up an episode of Amy Poehler's podcast called Good Hang. Not a sponsor of the Mindscape Podcast, but that's okay. I'm happy to recommend Amy Poehler's podcast if you like that kind of thing. It is not generally my kind of thing, but Amy Poehler had Idris Elba as her guest on the show. And I figured, you know, I'm just driving back to the airport. I'm not in the mood, it's very early in the morning, to think about anything very deep. Let's listen to some celebrity chit chat, and that would be fine. These are two actors, Idris Elba and Amy Poehler, who I enjoy immensely, watching on TV and movies,

• 2:24Sean Carroll?

  so why not listen to them casually gab? But actually, even though I wasn't expecting any deep sociological or scientific insights, I kind of got one, which was interesting. I mean, maybe it's something I should have thought of myself before. They were talking about The Wire. Those of you who have not seen The Wire, I'm gonna try very hard not to spoil it, but you should see The Wire. It's the best TV show ever made, and I say that not just because it is set in Baltimore. Idris Elba was one of the stars of The Wire, and in fact, it was his, basically his breakout role, as far as Americans are concerned. And he played a character called Stringer Bell. And Stringer Bell is interesting, because

• 3:04Sean Carroll?

  he was one of the leaders of a drug gang, but he aspired to be something more than that. He wanted to go legit and be a businessman. He took classes at a local community college on finance and economics and things like that, and he tried his best. And again, no spoilers, but it's difficult to change your role in the system in this kind of circumstance. That's one of the lessons of The Wire. And this is what Amy Poehler pointed out. She said, you know, "One of the things about The Wire is, the system will not let you go against its wishes in some very tangible ways." You know, sure, you may be one kind

• 3:46Sean Carroll?

  of person, and you may want to be a different kind of person, but there are constraints, and there are negative feedback mechanisms in the system that surrounds you, the system of the cities, the politics, the economics, the culture, and so forth. In fact, I remember David Simon telling a funny story about when he initially pitched The Wire to HBO, and he said, he couldn't quite bring himself to say, "You know, it's a story that explores the systematic troubles of the American city." He didn't think that HBO would go for that, but you know, happily they did, anyway.

• 4:22Sean Carroll?

  And interestingly then, I think that Amy Poehler was exactly right about that message. But Idris Elba, in response, said, "Yes, but the system is people, is individuals." You can say the system is holding you down, or not letting you do what you want, not letting you be different than your assigned role, but the implementation of those desires that you're attributing to this larger-than-individual system are actually carried out by the individuals. It's the individuals who constitute the system, and each individual is just acting according to their own conception of what their self-interests are, etc.

• 5:04Sean Carroll?

  So, they didn't use the word, but obviously what they are saying is, "The Wire is about emergence." It's about how the individual dynamics of lower-level constituent systems, in this case, people, come together to make a higher-level system, the system, as they were calling it. It could be the city, it could be the police department, it could be the drug gang, or whatever. And, of course, that's right, and in fact, it's, as Phil Anderson says, "More is different." There are things you get out of that collective behavior that you might not have naively expected, out of thinking about the individual behaviors. If you had been good enough to exactly simulate them, like Laplace's demon, you would have

• 5:47Sean Carroll?

  anticipated it, but there is an extra-understanding that you get more directly by thinking at the systems level. So, one more reason to go watch The Wire if you never have done that. Then the other anecdote comes from thinking about the Natural Philosophy Forum here at Johns Hopkins. You know, we're trying to figure out how to, what future things we should do for the Natural Philosophy Forum, and one thing that we are contemplating is a postdoc program. Of course, this requires money and things like that, but, you know, we can dream, and a postdoc program would be, you know, hiring people as postdoctoral researchers in natural philosophy. So, some of them would be philosophers, some of them would be scientists, but they would be philosophers who think der-

• 6:31Sean Carroll?

  Directly about scientific issues, or there would be scientists who think in a philosophical way about the natural world, and you know, they would be able to have other postdocs they could talk to, and it would be a unique environment. And in that way, it kind of resembles the Santa Fe Institute in the sense that it brings a heterogeneous group of people together who cross-cut disciplinary boundaries in an unusual way, and that leads to interesting synergies that might not otherwise have been there. So while I was at SFI, Simon Dedeo was also there. Simon, of course, is a former Mindscape guest, and I knew him back when he was a cosmologist. You know, he went to grad school in cosmology, got his PhD, and he was a postdoc at the University of Chicago when I was a faculty member there, just doing

• 7:15Sean Carroll?

  ordinary theoretical cosmology, much like me. But then his next postdoc, he decided to go to SFI. He thought that there was more interesting stuff going on than in cosmology, and now he thinks about social intelligence. And you can check out the podcast episode we did with him to hear about it. But he had been a postdoc at SFI, so I wanted to get, you know, the inside scoop about what that was like as someone who then went on to a successful academic career later. And he told me a fascinating and important story. The SFI has a scientific advisory board. So what that means is distinguished scientists who are not part of the SFI family come in occasionally, meet as a group, and learn

• 7:57Sean Carroll?

  about what's going on, and offer advice to the president, David Krakauer, another former Mindscape guest, as well as the researchers there, and so on. You know, it's good to get a reality check from the outside world. Responsible institutions do this. The physics department and the philosophy department at Hopkins have both done that just since I've been here. And what Simon says was when he was there as a postdoc, the advisory board, scientific advisory board, asked all the postdocs to do little presentations on their work. And which they did. And afterward, some members of the advisory board reached out to the postdocs and basically said, "You folks gotta stop doing that.

• 8:38Sean Carroll?

  You're doing this wacky interdisciplinary stuff and you will never get jobs doing this." Because remember, you know, SFI has this very active external faculty program where you can be a faculty somewhere else and you can be affiliated with SFI as external faculty, which means you occasionally visit and things like that. It's not a very strong connection necessarily. It can be as strong or as weak as you want it to, basically. And what that means is that almost all of the researchers, at least in the United States of America, who are sort of either doing complexity research or even just complexity-friendly, are already affiliated with SFI one way or the other. And for the scientific advisory board, they try to get outsiders.

• 9:22Sean Carroll?

  So the people who are on the advisory board are more mainstream. They're more normal. They're more thinking like a typical, successful scientist would be. And they just couldn't think that these young people doing, you know, wacky things like applying category to the category theory to the origin of language or something like that would be able to fit into academia in any comfortable way. Academia which has this system where we have departments and departments doing the hiring. And I'm very sympathetic to that. That's what made me think of it, because natural philosophy would have the same problem. Like, if you were a physics PhD and you then got a postdoc doing natural philosophy, would

• 10:04Sean Carroll?

  that make it harder for you to continue on as a physicist in a physics department? I don't know for sure, but I very much worry about that. So anyway, Simon says, "The advisory committee told the postdocs that." And in response, the postdocs just laughed. And they said, "Well, we like our research, so we're gonna keep doing it." And all of them went on to get jobs. It was fine. They did not actually get blackballed by the academic. System or anything like that, unlike Stringer Bell, they were more successful. And, you know, in retrospect, that does make sense for two reasons. One is, however, we might like to talk about academia as, you know, not always open to new ideas, et cetera. Academia is not a monolith. That's the thing about higher level emergent phenomena.

• 10:48Sean Carroll?

  They are made up of little pieces and little pieces are kind of quirky themselves. When you apply for jobs as an academic, all you need is for one department to see the vision and to say, yes, we would like someone like that here as part of our group. And the other thing is that by doing this interdisciplinary stuff, you can do interesting things in a way that you can't do interesting things if you're just doing the mainstream stuff. I formulated that sentence very, very clearly. I'm sure it's going to be misunderstood. It's not that you can only do interesting things if you're being interdisciplinary, but the way in which your stuff is interesting will be different if you're doing it in an interdisciplinary way than if you're doing it just in a normal mainstream disciplinary

• 11:33Sean Carroll?

  way. And that difference makes you stand out a little bit when you apply for jobs, and that can work in your favor. It doesn't necessarily work in your favor, but it can. And for these folks, it did, which makes me feel good, which makes me have a little bit of optimism for the fact, the understanding we have of the emergent system that we call academia. It is, in many ways, conservative in the small c sense of the word conservative. It is, in many ways, I almost want to say anti-intellectual because they're about doing their individual thing, not about intellectual breadth more broadly, and that can be often problematic. And yet, and nevertheless, it is not monolithic, it is not simple.

• 12:17Sean Carroll?

  It is not like, oh, this will never work, don't even bother. There's room within the system to try something new and maybe even succeed. To put a cap on it, Idris Elba mentioned that Stringer Bell, the character in The Wire, was inspired by a real person in Baltimore who was a drug dealer, sort of a drug gang leader, who did aspire to become legit and start businesses and things like that, and the real guy succeeded.

• 12:50Sean Carroll?

  The dramatic story was better if you don't succeed, but in the real world, it is possible to succeed. Don't take the pessimistic view too literally, you know? Keep fighting. This is a lesson that has broader application in the modern world, I think. Anyway, thanks as always to the Patreon supporters of Mindscape who make all of these episodes possible, especially the Ask Me Anything episodes. You could be a Patreon supporter. Just go to patreon.com/shawnmcarroll and sign up to support Mindscape. And we both appreciate the support and also you get ad-free versions of the episodes and you get the opportunity to ask questions in the Ask Me Anything episodes. That's where all these questions come from. They come from the brains of our Patreon supporters,

• 13:34Sean Carroll?

  which are some great brains out there. So we're gonna get some great questions. Let's go.

• 13:55Sean Carroll?

  Benjamin Freilich says, "Do you agree with David Deutsch that abstract knowledge invented by a human creativity and free will can alter the number of branches of the quantum multiverse that instantiate such knowledge?" I hope you're not gonna be disappointed that I'm gonna answer that it depends a lot on what you mean by alter and free will and things like that. I'm a believer, and I think that David Deutsch is also, but maybe not, who knows, that human beings are physical systems. So when you, and they obey the laws of physics, including the Schrodinger equation underlying the many-world interpretation of quantum mechanics and the quantum multiverse. So when you say that abstract knowledge invented by human creativity

• 14:37Sean Carroll?

  and free will can alter the number of branches of the quantum multiverse, alter compared to what? Given the situation that the people are in with all of the quantum states of their various particles and their entangled subsystems and so forth, the Schrodinger equation tells you how many branches of the quantum multiverse there will be and what is going to be inside them. Of course, it might- Not be. So, what can I say? I don't think that either David Deutsch nor I thinks that you need to change the laws of physics to take into account human creativity and free will. The laws of physics as we understand them. What he might mean by that, and I'm only responding to your question, I don't know exactly

• 15:19Sean Carroll?

  the source of the original claim, is that compared to not having a human being, right? Compared to like, if you replace a human being with a rock, then you might get less, a smaller number of branches of the quantum multiverse, or at least certainly, a smaller number that instantiates such knowledge, okay? So, I do believe, as we talked about in the solo podcast last time, that in the course of the evolution of the universe, there come to exist subsystems that manipulate information in very important ways. Ways that help those subsystems survive and thrive and reproduce and so forth. And human beings are among those subsystems.

• 16:04Sean Carroll?

  So if the talk about human creativity and free will is just a higher level way of saying that these physical subsystems of the universe latch on to informational capacity in the universe in a way that non-living ones do not, then I'm 100% in favor of that. So, can't really answer the question definitively without knowing exactly what the terms are supposed to mean. Tomas Handlova says, "By pure statistics and the number of AMA episodes you've done, I think there must be at least some non-zero number of physics questions you answered, or rather, statements you've made, that were objectively incorrect. Whether due to unintentional mistakes or genuine unawareness, would you dare to estimate what fraction percentage that might be? And how much of that would you attribute to slip-ups versus simply not knowing?"

• 16:49Sean Carroll?

  I'm sure you're right. I'm sure there are at least some non-zero number of physics questions that I've been wrong about. I don't think that I've been wrong... Well, there's different kinds of physics questions, right? A lot of the kinds of physics questions we get here on Mindscape AMAs deal with things like, as we just talked about, the many-worlds interpretation of quantum mechanics. So, I could be wrong in the sense that... Many worlds are just not the right interpretation of quantum mechanics. That's completely possible. In that case, a very large fraction of the things that I've said will turn out to be wrong. If, rather, you're thinking about cases where the physics underlying the question is completely known and agreed upon, and we are correct about the physics, but I personally have

• 17:32Sean Carroll?

  given the wrong answer about it, I'm sure that that does happen, but I suspect the answer is very low, just because I'm the one choosing what questions to answer. I don't think that I would try to answer a question unless I was pretty confident. If the question were about down-to-earth physics stuff, I would not try to answer that one unless I kind of thought that I knew the answer. And I'm probably usually right about that, but certainly not always. So in other words, I'm more likely to be right about a question I think I know the answer to than to a truly randomly chosen question about physics out there in the world. And in particular, I am very aware that I'm not especially good at answering messy, real-world questions about implementations of physics in everyday life and things like that, you

• 18:17Sean Carroll?

  know? Once you get friction and lots of different things involved, then my expertise, my intuition, is not very good. That's not the kind of physics that I'm good at, so I don't try to answer a lot of those questions. I don't get that many of those questions either, but the chances that if I did, I would somehow go wrong are high, but I know it. So I try not to answer those kinds of questions. I choose not to answer those kinds of questions. Niles Darr says, "In your view, is information more fundamental than physical stuff like fields and particles? Could our understanding of reality itself be better framed in terms of information rather than matter and energy?" Well, I think fields, particles, matter and energy are not the most fundamental way of talking about reality.

• 19:02Sean Carroll?

  As longtime listeners know, I think that reality is best talked about as a vector in Hilbert space, according to our best current physics. As we just said, that could always be wrong. That's definitely possible. But that's my guess, and the words like "matter" and "energy" are useful in some context at a higher level that emerges out of the bare-bones description, which is just a vector in Hilbert space obeying the Schrödinger equation. So this is always a problem for people who want to talk about the fundamental nature of reality. We have to admit that we don't know it, okay? And at the same time, we have to admit we don't know nothing. About it. If you go back to the podcast with James Ladyman years ago,

• 19:47Sean Carroll?

  James is a leader in the understanding of what is called structural realism. And he explains that, you know, you can have a scientific revolution that completely changes your view of what reality is at a deep level, like the revolution from classical mechanics to quantum mechanics. And yet, some things remain, even though you've completely thrown out your ontology of the world and replaced it with something different. The fact that the sun rises in the east every day, that's still just as true under quantum mechanics as it is under classical mechanics. Okay. So that structure that gets inherited and conserved as you improve your scientific understanding is what Ladyman and others

• 20:28Sean Carroll?

  would claim is the target of your realism. That is the thing you're realist about, not specific details of your ontology. Okay. So all of which is to say, when you say words, phrases like physical stuff like fields and particles, matter and energy, things like that, those are all subject to being thrown out as not very important. The question you're actually asking, Niles, is, is information a way of thinking about what is more important? I don't think so. I mean, I guess it depends on what you mean. Again, sorry to be always like that. But my view, and I'm happy to have other people phrase things in different

• 21:08Sean Carroll?

  ways or emphasize different things. But my view is that there is some universe out there, some physical real universe. The fact that the universe happens to be mathematically modeled by a vector in Hilbert space as opposed to a set of particles and fields moving in a classical spacetime, that doesn't change at all the idea that there's a physical tangible universe out there. It might be very different than what we learned about in elementary school, but who cares? Why should that be surprising at all? There is stuff. That's why I entitled my little paper about mathematical realism against mathematical realism. Reality realism. I think that reality is real, and we're going to learn better and better ways to model and describe reality, but it's real.

• 21:52Sean Carroll?

  Information is one of those ways in which we describe reality, okay? I think of information very much like I think of energy. You know, you learn in classical mechanics, in undergraduate, that if you have a particle moving in a potential, then it has an energy, and that energy is the sum of its kinetic energy, one half mv squared, plus its potential energy, whatever that is. And it's very tempting to say the energy is there, right? That the energy exists, and that's fine, you're allowed to say that. But also notice, you never have to talk about energy in that context. You could just tell me what the position is and what the momentum is, okay? Then it turns out, there's a convenient way of characterizing the state of the particle

• 22:35Sean Carroll?

  by talking about its energy, but I don't have to. What exists is the particle, and it has some properties, and one of those properties is its position, one is its momentum, one is its energy. I think of information like a property. It's a property that helps us describe the underlying world. So to think of it as more fundamental than the thing it's describing just seems a little bit wrong-headed to me. Matt Habberland says, "Would you make a good president? What aspects of the role would you think you'd do well at, and what would you have trouble with?" Um, I think I would make a pretty mediocre president, like mediocre in the sense of good, good enough, but not especially great. I think there's a, you know, I have respect for the skillset of being a good president

• 23:17Sean Carroll?

  that is very, very difficult to pull off, and certainly not the same skillset that you're trained to develop as an academic and researcher, or even as a podcaster, right? Um, I think I would do well in the debates, if there were presidential debates. I think I would do well at sort of sticking by my principled guns when it came to policymaking, that I would actually try to make the world a better place. But as a candidate, there's a million things that I would be bad at. Fundraising is a huge thing, balancing the needs of various interest groups, or donors, or whatever, you know, you, again, I have respect for it, I'm not downgrading the skill.

• 23:58Sean Carroll?

  Politics is a story of building coalitions and getting people to vote for you, right? It is in many ways the... Opposite of being intellectual, where your job is to stick to the truth and tell the truth no matter what, even if certain people don't like it. The skill set of a politician is to say, "Well, you know, these groups of people might not agree, but we're going to bring them together for the greater good." And I respect that skill set. I think it's important. It's not necessarily one that I am especially good at. The other aspect, of course, is that politicians are people-people. They have to persuade their fellow politicians and business people and community leaders,

• 24:41Sean Carroll?

  et cetera, to go along with what they want. That might involve schmoozing and a certain bit of extraversion or whatever it is, none of which I'm especially good at. You know, I can schmooze when needed in short bursts, but I don't love it. Like, a real politician is energized by that kind of thing, and that's just not me at all. Not to mention that I have none of the inner deep knowledge that comes with experience of being a politician. I don't, you know, know where the levers of power are located and how to pull them or anything like that. So I think it's a mixed bag, and I would be a pretty middle-of-the-road kind of president. All right, I'm going to group two questions together. Dennis says, "It would be great if you could comment on the science and philosophy of

• 25:26Sean Carroll?

  Peter Thiel, as presented in the interview with Ross Douthat in The New York Times today, 26 June 2025. Scares the hell out of me." And then Eric says, "A common claim you see from Peter Thiel, Marc Andreessen, and others on the tech right is that scientific and technological progress have stagnated since 1970. They usually point to a lack of progress in areas like medicine, space, and nuclear power to prove this point. I'm guessing they don't point to theoretical physics only because they deem it too niche." Peter Thiel does qualify this claim by acknowledging progress in AI, but he says that AI alone is not enough and that the problem of stagnation is severe and the treatment of this problem needs to be dramatic. Do you care to comment? So I'll confess, I did not read the entire interview with Peter Thiel, or I guess it was even a video. I certainly didn't watch the video.

• 26:12Sean Carroll?

  It just, I get bored. I just get tired of it. You know, Peter Thiel is a guy, like many people, who are successful in business. And I don't think this is anything new with Silicon Valley, or the tech right, or anything like that. But what are people who are successful in business good at? The answer is being successful at business. That's the very established correlation, okay? They're not necessarily good at anything else, but they kind of, sometimes, and not always, give the impression that they think they could be good at anything. Again, not that different than physicists, thinking, you know, after being a successful physicist, that they could be good economists or historians if they just put their mind to it, or spent 15 minutes figuring it all out.

• 26:55Sean Carroll?

  So Peter Thiel, when he starts giving pronouncements on grander cultural, political, social questions, just doesn't come off well at all. It's not, you know, I have to laugh at the, at people who, you know, point to successful entrepreneurs, like Peter Thiel or Elon Musk, etc., and say, "Wow, these people are just like the smartest people around." I know a lot of smart people, and these people are not them. You know, they're, they're very, very good in their particular area, but their area is not understanding the fundamental nature of reality, or history, or economics, or anything like that. That is not what their area is. You know, in particular, there is something that leads to success in business,

• 27:37Sean Carroll?

  which is a sort of impatience, a sort of conviction that you know the right answer, and that other people who don't immediately agree with you are kind of in the way, right? And you can see this brand of impatience turning into a sort of pseudo-sophisticated view of the world, where the masses are holding us down, and democracy is failing us, and things like that, when really, it's just that, you know, the government wants you to have safety standards at your automobile plant, or whatever it is. And I'm not that impressed by this view of the world. So in particular, the both questions here talk about this this idea that Peter Thiel brought up in the interview, happily it was near the beginning, so I did read it, of

• 28:21Sean Carroll?

  the stagnation in progress. And I'm just, again, completely unimpressed by this. You know, Thiel specifically talked about the rate of progress in cancer research and I think also Alzheimer's research. So if anyone cares about those issues, I would advocate listening to the podcast interviews we did with James Allison and Nicole Rust, both of whom are actual experts in related areas to this, and they will tell you a very different story. The thing about progress in different technological fields is that it's something that you can

• 29:04Sean Carroll?

  easily tend to think should have a natural rate of progress, right? Progress was a certain speed at this time. Progress should always be like that. Why don't we have our flying cars now? We went from not even having airplanes in the year 1900 to flying across the Atlantic in a few decades after that. Why did progress slow down? The thing is that progress is always going to be nonlinear and unpredictable because you take advantage of what there is to be taken advantage of. I will tell you an analogy I've come up with. I haven't had a chance to use it yet, but you know, you're constantly bombarded by people telling you that there's a crisis in fundamental physics because we haven't discovered new things, particle accelerators, or new ideas or new theories or whatever.

• 29:49Sean Carroll?

  Almost always, these people think that they've figured out the right theory, but forget about that, just think about the claim that progress has slowed down. The analogy is this, imagine that you are in New York City, and you have decided to take a train trip across the United States. You go west, and you don't, you know, you aim at Seattle, but otherwise you don't take anything specific by way of instructions. And what you really want is to see cities, right? You like skylines of cities. You're not kind of bored with all this nature stuff, these plains and mountains and whatever. What do you really like are cities? And you start west from New York City, and you see like, oh my goodness, there's Philadelphia, then there's Pittsburgh, and Cleveland, and Chicago, all these great cities with all these great skylines, it's great.

• 30:34Sean Carroll?

  And then as you go west from Chicago, suddenly, there's no more great skylines anymore, all the way across, like a thousand miles, until you hit the west coast. And you're disappointed, you're like, am I not looking well enough? Am I not doing the right thing to see the skylines out the window of my train? No, that's obviously not what's going on. What's going on is they're not there. You know, you're in a part of the world that doesn't have tall city skylines anymore, and it's not your fault, it's just where you are in the world. Progress in theoretical physics and experimental physics, which is what I'm most familiar with, is very much like that. It depends on what the universe has to offer you, not just on what you're trying to do.

• 31:17Sean Carroll?

  And sometimes, you will make a brand-new breakthrough that is not only itself important, but like quantum mechanics or something like that, leads to a whole host of other breakthroughs. So it suddenly seems like, oh, we're brilliant! Now, we are so smart, we're doing the right thing, we're making progress. And then you make that progress, and then you're back to like, normal science again. You're like, oh, now we're dumb, right? Now we're not able to do it anymore. I feel exactly something like this is going on when people think about technological progress. If you think about... You know, in the early 20th century especially, there's a huge amount of progress in transportation, right? Cars and planes and rocket ships, things like that. And it's very tangible. It's right there.

• 32:01Sean Carroll?

  You're moving more stuff and you're moving it faster. And it's very easy in your minds to just extrapolate, right? To say like, "It's just gonna keep going. We're gonna have faster and faster airplanes." But there's a problems with having faster and faster airplanes. There are problems with the economics of it, with the physics of it, with the sound barrier, with a million different things that get in the way. And so you have to be a little bit less naive than that when you think about the rate of progress. And again, the thing about this group of people is they really love being naive. They really don't wanna do the work to understand the things they're talking about at a deep level. That's why you constantly hear this group of people poo-pooing things like reading books, right? They're like, "Every book should just be a blog post," you know?

• 32:46Sean Carroll?

  Because they don't have the patience. They don't have the stick-to-it-iveness to really get into and appreciate the details. That's why they wanna move fast and break things. And so, like I said at the start, I'm thoroughly unimpressed. All I can say is it makes me very sad that people like this seem to be taking over the world right now.

• 33:04Speaker 03

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• 33:35Speaker 04

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• 34:05Sean Carroll?

  David Harper says, "How do I pick the right graduate school for physics? Should I go out somewhere outside the United States if I'm finishing my bachelor's here?" So there's two different sets of issues here, right? One are like forever issues about picking the right grad school in physics. The others are the current weird situation we're in in the United States. Should you go outside the United States if you're finishing your bachelor's here? I would take it seriously. I wouldn't necessarily say you have to do it, but I get it if you're taking that idea seriously. I think that if you apply to a grad school here, well, for one thing, it's harder to get in now because there are fewer spaces, because there's less money, because the government's taken away all the money.

• 34:47Sean Carroll?

  You would, it would make perfect sense for you to check two things. Number one, what is the level of stability of the faculty who you might want to work with, right? The single most important thing about a graduate school is who you're going to work with as your advisor. Are they going to stay here or are they looking at opportunities elsewhere? That's something you should always think about asking. In many ways, it's better as an advisor to pick someone who's like young and energetic and ambitious, but those people are also less stable. They're more likely to be stolen away by somebody else. So even not in the current weirdness, that would be a good thing to check up on. And the other thing is the stability of the funding situation. If you went to a certain place, how would you get a stipend? How would you get your tuition remission?

• 35:32Sean Carroll?

  Is that coming from grants that might be in danger? Or is that coming from the university, which might be a little more stable, but it's still not completely stable? What has the university done in other situations analogous to this? You should check up on all those things. The thing about going elsewhere is that, number one, it can be fun. Going outside the United States can be very educational, right? It's a different kind of experience. Number two, it's, there's a hassle involved moving to another country. And also, if you want to come back to the United States at some point, you're making it harder by going away. You're not making the connections and meeting the people and being introduced to the same sorts of issues and problems that you would be if you stayed. Here in the United States.

• 36:15Sean Carroll?

  And third, you know, other countries are not immune from the kinds of things we're going through in the United States right now. I mean, the United States was a very special place to be a scientific student or researcher. Because we had a lot of support and other countries traditionally don't have that much support. And we're sort of going down to their level and going even further below that level right now. But it's something where you shouldn't just say, well, the United States is bad, other countries must be better. You should really check in at a detailed level. So I can't give you a simple answer to that, but I'm, I'm willing to say, yes, taking the idea seriously is perfectly worthwhile. In the general question, though, how do you pick a graduate school?

• 36:56Sean Carroll?

  I like to say that choosing your graduate advisor, your PhD advisor, is the second most important choice that you voluntarily make about connecting yourself to another human being in your life other than your spouse. It's super duper influential on your future career, who your advisor is, because generally, it's not that they, not just that, they give you advice and they teach you how to be a good scientist, but they have an ongoing research program and you will join that. And sometimes you're just a genius and you figure out how to do things by yourself. That's great. If you're a genius, you don't need my advice. But most of the time, that initial exposure to interesting research problems

• 37:37Sean Carroll?

  stays with you the rest of your life and you start getting some inertia, some momentum that pushes you in the direction of more and more research in that area. And very often, people do for their whole career, research that was more or less set as an agenda back in graduate school. So take that possibility into mind and take very, very seriously who your advisor is going to be, which means number one, look for places that have specific advisors you like, like, don't just say, well, you know, Harvard, Princeton, Caltech, those all sound good, right? Look very, very carefully at, number one, who the people are who are there. Read their papers, okay, you know, look, it's very easy right now to look online and say, what papers have these people written in the past five years?

• 38:20Sean Carroll?

  Ask yourself, what are the papers that? I would have most liked to be a co-author on. And then number two, what is there's those people's style of working? Do they collaborate with graduate students a lot? What graduate students have they had? Have those graduate students gone on to be faculty members elsewhere? Do they write papers with their graduate students? Or they let their graduate students do their own thing? And these are things you can figure out in part by looking at the published track record, but also in part just by talking to people who are grad students or even the faculty there right now. And then the other thing is, I mean, I'm skipping the obvious thing. So like you can go visit and get a feeling yourself. But it is important, it's, you know, five or so years of your life

• 39:03Sean Carroll?

  if you want a PhD in most areas. And so, do you want to live there? What are the people like? Do you think it's easy to find a place to live? Are the other graduate students happy? Are they more or less pleased with their prospects? Are the requirements in terms of exams and things like that, all things that you think are reasonable? Or are the students grumbling about them? All those down-to-earth things, those are perfectly legitimate things to think about and worry about. So the overall short version of that is think very hard about who might be your advisor. And I guess I didn't finish one thought. One thought is, you know, make sure there's a person there who you'd want to be your advisor. But another thought is, make sure there's a plausible backup.

• 39:49Sean Carroll?

  Because maybe your first choice doesn't work out for one reason or another. Maybe they leave, or maybe you don't like them, or whatever. Having at least two people you might plausibly work with is a very good thing. So having that and having some idea that you might actually like being at that place, these are the things I would most keep in mind.

• 40:08Sean Carroll?

  All right, I'm going to group two questions together. Tyler Whitmer says, "In the Brian Van Norden episode, you had a discussion of Nagasena's chariot explanation of Anata, no self. And you noted that the similarity to the, there's a similarity to the concept of emergence. Did that make you want to learn more about that aspect of Buddhist philosophy?" And Adam Rotmil says, "In the episode with Brian Van Norden, it sounded like you were able to interpret some aspects of Buddhism and Chinese philosophy about identity, for example, as early precursors to what you might now call poetic naturalism. The idea of being called by a name for expediency, while the truth can be told is not the eternal truth." I wonder if you could expand on that common ground or anything else that stood out to you from reading Van Norden's book prior to the episode. I think it's just a fascinating

• 40:51Sean Carroll?

  connection to see between materialism and eastern philosophy. So I'll say two things. On the one hand, in principle, I think that there might be lots of connections between eastern philosophy and modern ideas, whether it's poetic naturalism or anything else, and those ideas are very interesting to think about. So, all in favor of that, in principle. On the other hand, I'm not actually that invested in any ancient philosophies at all, whether they're western or eastern or anywhere else. There is a feature of growing up in the western world that you learn more about western philosophy than eastern philosophy, so there are natural examples that I will draw on from Lucretius

• 41:33Sean Carroll?

  or Aristotle or whatever, whereas someone who was brought up in a Chinese philosophy tradition might bring up examples from Lao Tzu or Confucius or whatever. But they're just examples, you know. I'm not going to learn anything about quantum mechanics by reading any of those people. You might get some good ideas, but you might also get some good ideas by reading recent people, okay? So again, I'm not downplaying the importance of ancient philosophy. I'm saying this is not central, whether western or eastern, to my particular brand of doing philosophy. You know, I'm not interested in the history of it. The history of philosophy is fascinating to me for historical, personal reasons. It is more or less not useful to me for occupational or research level reasons.

• 42:17Sean Carroll?

  Denver Naker says, "Recent discussions on StarTalk about the possibility of our universe being inside of a black hole. Are you... I'm curious to know your take. What are the possibilities? Is it merely conjecture or could we prove it?" So I've talked about this before, but sometimes I just like to talk about some things again, because, you know, if it's new to you, then it's new. But, you know, you could always... there are various ways of searching the archives for me talking about these things. The brief story is, no. We do not live in a black hole. It's done. There you go. Up to, of course, the caveat that who knows, anything is possible, as we like to say here on Mindscape. There's no reason to think we live in a black hole. There's no evidence that we live in a black hole. How about that?

• 42:58Sean Carroll?

  For two reasons. Number one, black holes are, by their definition, objects in a bigger spacetime. They're not all of spacetime. They're inside somewhere. When you have a black hole, you have an event horizon that separates the black hole from the rest of the world. As far as we know, there isn't anything outside of our universe for it to be a black hole inside. There's literally zero evidence of anything like that. The second reason is that the whole point of being in a black hole is that in the future, you are going to smash into a singularity, one way or another. There's, again, zero evidence that that's going to happen here in our universe. In fact, the opposite. Our universe is expanding and accelerating apart,

• 43:41Sean Carroll?

  and it's not headed toward a singularity, as far as we know. It's headed toward empty space. That's the opposite of what you would expect if you were in a black hole. There is, as I do say every time I talk about this, a singularity in our past, according to the classical theory, or in whatever replaces the classical theory. There will be whatever replaces that singularity. And that makes it more like we live in a white hole, if anything. A white hole is the time reversal of a black hole. And just like black holes have singularities in your future, white holes have singularities in the past. So if you want, I don't know, I don't know why people like this idea so much. Like, this idea just keeps coming up over and over and over again.

• 44:21Sean Carroll?

  I don't know why, but people really want to push it, and I would, my advice would be, if you really want to push it, push the idea that we live in a white hole, not in a black hole.

• 44:31Sean Carroll?

  Matthew Hines says, "On the subject of universities versus Trump, it strikes me that an important debate needs to happen, which isn't happening, across the partisan divide. I listened to a recent podcast by Jordan Peterson, who defended the Trump administration's approach while lambasting universities such as Harvard for their DEI policies. As someone who's spoken out against the persecution of the universities, would you consider talking to someone from the Trumpist right, such as Peterson, and subjecting their arguments to a rigorous examination?"

• 45:00Sean Carroll?

  I think it's unlikely that I would really get anything out of doing something like that. What would the goal of that be, I guess, is the question to ask. Sometimes it's absolutely worth talking to the most crazy, wild, useless people across the aisle, not because you want to talk to them, but because you want to reach a broader audience, who might be more interested in listening to an interaction between two sides than just listening to one side or another. But when you say "subjecting their arguments to a rigorous examination", I think that that sort of presumes things that are not in evidence, namely, the idea that what they have are mostly arguments,

• 45:44Sean Carroll?

  and what would illuminate those arguments is something along the lines of a rigorous examination. I don't think that's it. I think that a lot of these people have conclusions, and they come up with arguments to get to those conclusions after the fact. And they do not actually enter discussions in good faith, trying to find the truth. They try to find justifications for things that they want to believe. And when people like that are out there, talking to them and trying to have a reasonable debate is not actually going to play by the rules. So it's not going to lead to the kind of place we want it to lead to. Again, sometimes doing things like that is useful.

• 46:25Sean Carroll?

  We need to reach out to audiences that don't already agree with us. That's absolutely crucial, and very much in favor of that. The question is, what is the best way to do that? What is the best way to reach audiences that don't already agree with us? And explain to them how we think, in the broad sense of the word "we", whatever that might mean for the particular issue under consideration. And sometimes that might be clashing with somebody who just completely disagrees with you, but other times, there's other ways that you can get there. And I'm not quite sure, in this particular thought experiment, what would be gained from that particular interaction. Casey Haskins says, "Could you summarize where things stand now with the muon?

• 47:09Sean Carroll?

  Has any discrepancy turned out to be significant?" I've not been following the muon, which is a little elementary particle, very closely. For those of you who know, the muon is the heavier cousin of the electron, and which makes it sort of really good to study for some reasons, really difficult to study for others. With Kari Tezarotti, we talked about building a muon collider, like a whole collider that was smashing muons together. The reason why that's a good idea is because muons are heavier, substantially heavier than electrons, you can put a lot of energy into a muon at the same velocity and therefore reach higher energies when you collide them together. The problem is the muons are unstable, right? They don't hang around forever, so they're kind of delicate to deal with.

• 47:52Sean Carroll?

  There have been recent experiments, both looking at decays of the muon and at the anomalous magnetic moment of the muon, that I did a solo podcast a while ago mentioning that there were anomalies, that there were experimental results that seemed to not agree with the predictions of the standard model of particle physics. Maybe these were evidence that we're finding some hints of new physics. My impression is that those have not held up to further scrutiny. In particular, the magnetic moment of the muon, there was already at the time a worry that there was a discrepancy between the theoretical prediction and the measurement, but there was also a discrepancy just within the world of the theoretical prediction.

• 48:37Sean Carroll?

  Okay? In other words, there were different ways of doing the theoretical prediction that were getting us slightly different answers, and one answer kind of agreed with the experiment and the other didn't. And my impression, not having followed very closely, is that there's more belief now that the prediction that already agreed with the experimental measurement is the one that is more reliable. So there never really was an anomaly there at all. So the decay thing, I don't really remember any progress on that. So anyway, the overall excitement that the muon might be teaching us something new, I think in the last couple of years, has gone down rather than gone up, which was always the way to bet.

• 49:19Sean Carroll?

  There's not because there never will be a new experimental discovery, but most halfway there experimental discoveries go away, rather than being, becoming more firm with time. So the smart way to bet is that any individual one will go away, even if sometime, at some point, we are bound to get one that doesn't. David Kudaverdin says, "I often hear you say that many worlds' interpretation of quantum mechanics follows directly from the Schrodinger equation always holding. Recently, I tried to explain this to a friend, but they asked an interesting question. Why couldn't the Schrodinger equation always be valid, yet macroscopic objects that strictly follow it end up in a state that seems collapsed to the quantum Laplace's demon?"

• 49:59Sean Carroll?

  Sure, there are two parts of saying that many worlds, that there are many worlds resulting from the idea of the Schrodinger equation and the quantum state, which is originally Everett's idea for what he called "the theory of the universal wave function." Number one, that the Schrodinger equation always holds. And number two, that the Schrodinger equation naturally leads to multiple distinguishable branches, okay? How do you know that second part is true? Well, you can just do the examples. There's examples where you can solve the Schrodinger equation exactly, and you can show that in a very robust, predictable way, you get multiple decohered worlds. In particular, whenever you're doing a measurement

• 50:43Sean Carroll?

  of a system that was in a superposition of multiple possible measurement outcomes, the absolutely everyone agrees that the Schrodinger equation by itself predicts that the universe evolves into a superposition. In one part of the superposition, there are some people, some scientists, who think that the measurement came out one way, and in other parts of the superposition, there are some scientists who think it came out the other way. The only question is, do both those parts actually survive and go on to be separate worlds? Everett says yes, other people disagree. David says, "Former Mindscape guest Raphael Bussel has published a preprint extending the Penrose wall result that spacetime singularities exist, not just in classical general relativity, but even in a semi-classical world, allowing for some approximate backreaction from quantum

• 51:28Sean Carroll?

  fields onto the spacetime manifold. Bussel worked in a limit of infinitely many quantum fields. This somehow fixes the semi-classical geometry, allowing for a firm definition of time, Schrödinger equation, etc. I'm interested in the method more than the result. It seems like a DSCFT, a great toy model allowing deep insights, but that might be hard to make directly relevant to our world. Does this feel like a one-off gimmick, or might it have broader lessons as Maldacena's idea has had?" So I saw the title of the paper in the abstract. I haven't read Raphael's recent paper, so you'll forgive me if I am a little cautious or imprecise in talking about it, but I will say a couple of things. The idea of having infinitely many quantum fields to sort of

• 52:09Sean Carroll?

  suppress quantum fluctuations and make things look classical, that's a perfectly legitimate idea, especially if what you want to do is prove some theorems, right? The problem with quantum gravity is it's hard to prove theorems because you don't know what the axioms are. You don't know what the theory itself is. So proving believable theorems is very, very difficult. You might therefore want to make an unrealistic assumption or a set of approximations that allows you to get a rigorous result. And then you can't expect to sort of just take this result and extend it to circumstances where your approximations aren't valid. So

• 52:48Sean Carroll?

  I would kind of doubt off the top of my head that this method will be of great generality. It helps you get a result, but it's also not physically realistic, so I'm not sure what to think. The other thing I will say is that the very idea of having a theorem that shows you that there are singularities in certain circumstances, historically super-duper important, right? The Penrose-Hawking-Giroch singularity theorems was very important in the early days of black holes, but it can still be a little bit misleading because people think, "I have a theorem that says there's a singularity, therefore, conclusion, there is a singularity", right? But that's not the right conclusion.

• 53:31Sean Carroll?

  The right conclusion is, your theory has broken down. And that's the right conclusion just as much if you get this in a, what we might call a semi-classical world, where you have quantum fields on a classical spacetime, as it would be in a completely classical world. The conclusion you draw, the lesson you get from that, is that this theory is failing you at that particular point. It is not a theorem that says there definitely is a singularity there. So, really, the lesson here is that even with infinitely many quantum fields, you will hit a point where you need true quantum gravity and things are different. So, I was kind of already convinced of that, so I'm happy to see it verified.

• 54:12Sean Carroll?

  Maki Ojin says, "Does true randomness exist in our universe? Or are we just limited by our ability to model things correctly? Are quantum fluctuations only random as far as we know? Or is that randomness predicted by some mathematical logic? We know for sure it is random." Well, we don't know for sure that it is random. That's always true. There are very, very few things that we know for sure. What we can ask is, in the context of a specific picture of quantum mechanics, are there true randomness, right? And different pictures of quantum mechanics give very, very different answers to those questions. In stochastic, objective collapse models, like Penrose's model, or like what is called

• 54:54Sean Carroll?

  the GRW model, there is true randomness in the world, full stop. In things like pilot wave theories, hidden variable models, there is no true randomness. There is only ignorance, ignorance of the values of the hidden variables, okay? But apparently, that ignorance is absolute. There's no way to know ahead of time what those values are. So, in practical terms, there is really true randomness. Even though God doesn't think it's random, every human being or every other agent embedded in the universe does think it's random. That's the best you can do. Everett with many worlds, of course, is a weird hybrid, where the underlying dynamics are completely deterministic, much like hidden variable models.

• 55:39Sean Carroll?

  And, and there's nothing you don't know about the wave function of the universe. So that's different than in the hidden variable models. But things still are effectively random because of self locating uncertainty. Because there will come into being multiple copies of you and you don't know which one you are. So self locating or indexical uncertainty is precisely when you can know everything there is to know about the universe except who you are in it. So that's a kind of randomness that in Everettian quantum mechanics is inevitable. And since that's my favorite view of quantum mechanics, I like to say that quantum mechanics from the observer's point of view is absolutely random. There is randomness in the world. Maybe tomorrow we come up with a better theory that is not quantum mechanics and can get rid of that.

• 56:24Sean Carroll?

  Good luck with that. I'm a little skeptical, but who knows, could happen. P T Milo says, "You're typical, you typically push back somewhat on the importance of emergence being observer dependent by pointing out that the landscape of macroscopic observables is far from arbitrary. But is this enough? Even if there is an infinitely large swampland of configuration space that then..." Sorry, I think I'm parsing the sentence incorrectly. "Even if there is an infinitely large swampland of configuration space, then those can result in stable..." Yeah, there's a missing word here. "Then those that can result in a stable macroscopic compression. There could still be an infinite set of possible observer macrostate compressions.

• 57:06Sean Carroll?

  Right now, it's easy to imagine intelligence with no more use for temperature than Laplace's demon, but like us, generates highly stable intersubjectivity and compression. Doesn't it seem that if some of the best explanations turn out to be essentially anthropic, they would require us to say the most we can about the measure of the selection space that varies both the observed macrostate and the observer macrostate?" So I hate to say it again and again, but it's possible. There are things that are possible. Let's talk about what we have evidence for or reason to believe. You know, we human beings look at the air in the room around me and we say, "I can describe this in terms of macroscopic variables like average velocity, temperature, pressure, density, humidity, things like that."

• 57:49Sean Carroll?

  Okay, and we know that underneath the surface, there's really molecules and atoms bumping into each other. And so the question is being raised, there's a particular map going from the space of all possible configurations of the atoms and molecules to this emergent higher level description, temperature, pressure, density, etc.

• 58:09Sean Carroll?

  And here's the interesting thing, not only can you define those variables, but once you've defined those variables, two amazing things happen. One is, you can more or less describe the dynamics of the air only in terms of those variables. For most of the possible things that can happen, not everything, because you could go outside the realm of applicability. But for most things that will happen, you don't need to know the microscopic state of the molecules in order to say what will happen. You only need to know those macro variables. And number two, those macro variables are macro observable. They are surveillable in David Albert's formulation. I can measure them. I can have a temperature gauge and a barometer and things like that, okay?

• 58:54Sean Carroll?

  So I both have this higher level emergent description that is kind of autonomous. It works in its own right, and its quantities are graspable and measurable by me, okay? So the question is, could there be other very different sets of variables that have both of those properties, or even one of those properties but not the other? Yeah, maybe, there could be.

• 59:21Sean Carroll?

  My surprise is not that there is only one, but that there is even one. I mean, maybe there's others if you just want to coarse grain even more or something like that. But I think it's kind of amazing in the set of all possible physical behaviors that there exists the ability to tell what the future behavior of the air is going to be like without knowing all of the details. Certainly, if I just randomly, generically threw away information about the microscopic state, I would be very quickly left completely unable to make predictions. The fact that I can throw away so much of it and retain my ability to make predictions is amazing to me. Is it possible there's a completely different set of variables that would do just as well?

• 1:00:03Sean Carroll?

  Sure, it is completely possible. I would be very, very interested to see an explicitly worked out example of some microscopic dynamics, even if it's completely unrealistic, right? Just a completely made up model of microscopic dynamics with two completely different coarse granings that give me equally good understanding of what is going on. Okay? That would be great. I don't, as far as I know, I've never seen such a thing. So I encourage people to try to get it. I would be fascinated if it were true.

• 1:00:31Speaker 03

  Coca-Cola. For the big, for the small, the short, and the tall. Peacemakers, risk takers. For the optimists, pessimists. For long-distance love. For introverts and extroverts. The thinkers and the doers. For old friends and new. Coca-Cola. For everyone. Pick up some Coca-Cola at a store near you.

• 1:01:02Sean Carroll?

  Matthew Hall says, "Why were so many people surprised in 1998 by measurements that indicated a non-zero positive cosmological constant and the existence of dark energy? Didn't cosmic inflation suggest a geometrically flat universe? And wasn't the density of dark energy just what was needed to make the universe flat when added to the estimated densities of ordinary and dark matter?" Yeah, this is a perfectly reasonable question to ask, and I was there and I was one of those people who were surprised, so I'm well-positioned to answer it. People did recognize in 1997, before we found the acceleration of the universe and the dark energy, that it would be good to have a cosmological constant not be zero, okay? It would solve some problems. The most obvious problem it would solve, like you said,

• 1:01:47Sean Carroll?

  is that we weren't able to find the right density of matter to make the critical density, which was predicted by inflation, and even without inflation is still sort of a more natural density for the universe to have. But it was, okay, the fact that we couldn't find the density of matter that we needed wasn't by itself taken as very, very strong evidence. Maybe it should have been taken, but you have to understand the history here. As we did cosmology in the 1970s, 80s, and 90s... The more, the better we got at measuring the amount of ordinary and dark matter in the universe, the more of it there was. You know, there was just a tiny amount of ordinary matter and then you realize, oh,

• 1:02:32Sean Carroll?

  there's dark matter in galaxies and you realize, oh, there's dark matter in clusters and you're discovering more and more. So cosmological observations in the 80s and 90s were not at the very reliable, high-precision stage that they are today. And, you know, we didn't even know the Hubble constant to within a factor of two, but when I was your age, some people thought it was 50, some people thought it was 100. Okay? So factors of two in cosmological observations were thought like, yeah, you know, we gotta, gotta keep those possibilities in mind. And so there were plenty of people who held out hope that as we looked more and more for more and more matter in the universe, dark matter and ordinary matter, we would find enough to make the critical density. I do think that in retrospect, some people, Nettebecall in particular, were persuasively

• 1:03:17Sean Carroll?

  making the case that that was not going to happen. But it was always a prediction, right? It was always like, I'm predicting that we won't find more matter as we look at more and more places. And it's easy to be skeptical about that. Okay? So that's one thing. The other thing, I guess there's three things that I can mention. The second thing is maybe inflation is wrong, or maybe the universe just is open, negatively curved, right? That was absolutely on the table. But the third thing was, people took the cosmological constant problem and the coincidence problem very, very seriously. So to remind you what these are, the cosmological constant problem is, once you admit that in effective quantum field theory, there can be a vacuum energy and you can estimate what

• 1:04:00Sean Carroll?

  size it should have from naturalness perspectives, the answer is much, much, much, much, much bigger than what we actually observe. That's the cosmological constant problem. The coincidence problem is that if the cosmological constant is of the same order of magnitude as the energy in matter and radiation, which we think it is right now, same order of magnitude, it's about two or three times bigger, so that's within a factor of 10. That's a very temporary phenomenon, cosmologically speaking. If they're the same order of magnitude now, then at a redshift of 1,000, which is still long after the microwave background, they were different by a factor of a billion, right? That's a lot.

• 1:04:42Sean Carroll?

  So, people thought that was a coincidence and that would be weird for the cosmological constant not only to be non-zero, but to be just the right value to be observable today, but not in the past, okay? And the cosmological constant problem, even though we didn't know the solution to it, certainly did not. People thought that in the space of all possible solutions we haven't thought of yet, for why the vacuum energy is so much smaller than we think it might naturally be, it's easier to imagine that someday we'll find a mechanism that makes it exactly zero, some symmetry, right, some principle, some dynamics, than it is to find a principle that makes it exactly 10 to the minus 120, the natural value, okay?

• 1:05:25Sean Carroll?

  So, and that's perfectly reasonable, all of that stuff. Indeed, the only mechanism at the time, in 1998, and maybe even still today, that quote-unquote "naturally explains" the observed value of the cosmological constant is the anthropic principle. Steven Weinberg had written a paper that predicted pretty straightforwardly. If the reason why the vacuum energy is small is for anthropic reasons, then we should expect to find it at a non-zero value, which is same order of magnitude as where we actually did find it. But people don't like the anthropic principle, right? So, that they thought that, well, you know, if we find... Joe Polczynski told me if we found a non-zero cosmological constant, he would quit doing physics. Because the only, and this was before 1998, he said, "The only possible explanation is

• 1:06:10Sean Carroll?

  the anthropic principle, and I can't stand it." He changed his mind, as we all do, when we're confronted with data, but that was the opinion a lot of people had. You know, in retrospect, you can say, "We should have known," but there was also other alternatives. You know, there were cosmological worries, but maybe the dark matter is warm dark matter rather than hot, maybe it's a mixture of hot and cold, maybe there is spatial curvature, maybe there's tilts in the spectrum of density perturbations that are fooling us somehow. So, the cosmological constant was one good idea, but it was far from the only idea that people were contemplating at the time.

• 1:06:46Sean Carroll?

  Brendan Barry says, "I was wondering if you could further discuss the distinction between quantum wave function and the matter fields in quantum field theory. For example, in a double slit experiment with electrons, prior to interacting with the screen, are the quantum wave function for the position of the electron and the amplitude to the electron field across space identical? Are they peaks and zero points? Or is the quantum wave function a superposition of localized peaks in the electron field?" Well, I worry a little bit about those localized peaks. So, in general, the quantum wave function is certainly not a superposition of localized fields, of localized peaks that look like electrons. The more accurate thing to say is that the quantum wave function is a superposition

• 1:07:28Sean Carroll?

  of various different possible numbers of electrons, right? It's a superposition of the part of the state where it's the vacuum state, and there are zero electrons. There are states where there is one electron. You have to be careful, because the total electric charge should still be zero, so there's a proton somewhere else, or a positron somewhere else canceling out, but that's okay, still only one electron, right? And there's part of the state where it's two electrons, three electrons, etc. This is called Fock space. This is a way of sort of giving some tangibility to the wave function of the electron field. If it weren't for field theory, you just have a certain number of electrons and they'd be stuck. They couldn't change.

• 1:08:11Sean Carroll?

  In non-relativistic field theory, the number of electrons doesn't change over time. And so your wave function is just a wave function of a certain number of particles. In field theory, your wave function can be thought of as a superposition of zero particles, one particle, two particles, etc. But if, within that wave function, if you measure or somehow, for some other reason, know how many particles there are, like you know how many electrons there are, you've made an electron, okay? That's good for you. You now have a wave function of the electron field, but specialized to the case where there is just one electron there. Then, the wave function just looks like the wave function of a point particle, okay?

• 1:08:55Sean Carroll?

  You don't need to remember that it's a field anymore. The only thing the field is doing for you is telling you how many electrons there are. Once you know how many electrons there are, I mean, the field is still doing it for you because the field is doing everything, but you don't have to think about the field anymore. You can just think about electrons. So you can think in the one-electron sector of the wave function as just being an ordinary wave function of an electron, that is to say, psi of x, where the electron is, okay? And there's nothing about that psi of x that says that it's a localized lump. The electron wave function could be spread out all over the place, as far as you know. That's completely compatible with quantum mechanics. So, I find it most useful to think in these two stages.

• 1:09:38Sean Carroll?

  The first thing is that the wave function of the field can be thought of as a superposition of different numbers of particles, and then within any part of that superposition with a definite number of particles, it's just a wave function of a number of particles. Noah Worcester says, "Priority question." Remember that, once in the lifetime of every Patreon supporter, you can ask a question that I will do my best to answer, and if you label that a priority question in the AMA. Noah says, "I feel like my understanding of decoherence is incomplete. Perhaps you could correct me. We know that decoherence, and as a result, ever-ready and branching, is caused by an individual quantum system becoming entangled with the wave function of the wider universe. But what about isolated systems, like a black hole?

• 1:10:22Sean Carroll?

  Does everything inside remain in a superposition? If so, would many worlds branching even occur?" There is a sort of simplistic answer to this, and a more sophisticated answer that I'm less confident about, so let me give you both. You know, if you truly have an isolated system, then it generally remains in a superposition in some way of talking. I mean that by, often, your isolated system will also kind of have an environment, right? If you're really thinking about Schrodinger's cat inside the box, it's perfectly allowed to think of that as a system, which is the cat, an environment, which is photons and air molecules and things like that in the room, and in that case, even in the box before

• 1:11:07Sean Carroll?

  you open it, branching has already happened, okay? And then, here's the tricky part, okay? So, anyway, that is just to say...

• 1:11:17Sean Carroll?

  I'm allowed to talk of the box in which Schrodinger's cat is in as having a superposition. I'm also allowed to talk about it as if there are two different branches. Branches are always ways of talking. They're always convenience. There's always another way of talking, which is the whole wave function of the system. Right? And that can always be a superposition. So, how should you talk about it? Well, it's up to you. The usual way of talking about it would be that, once you open the box and look in, then now you become entangled with the cat. And if there were two branches inside before, there's now two branches outside. One with an observer who sees the cat awake, and the other observer sees the cat asleep.

• 1:11:58Sean Carroll?

  Did that outside world have two branches already, even before it became entangled? That's up to you. You are allowed to talk about it either way. So I would say, the reason I'm talking about Schrodinger's cat is, I would say, much the same thing goes for the black hole. You know, like you say, there can be events inside the black hole that we would ordinarily think of as leading to branching of the wave function. Let's say I throw in Schrodinger's cat into the black hole, and I make it so that, you know, the Geiger counter doesn't click until after it's in the event horizon. So now, inside the black hole, there's a superposition of cat awake and cat asleep. Are there two different branches outside?

• 1:12:42Sean Carroll?

  Well, you know, I think that you can think of it either way, but there's no reason to think of it as being a... Well, let's put it this way. There is nothing that you can do outside that could resolve the question, "Am I on the branch where the cat's awake or the cat's asleep inside the black hole?" Okay? You can't open the box. That's the important thing about it being in the black hole. So I would argue that, in that case, the sensible thing to do is to treat it as the whole wave function, right? To not break it into branches inside the black hole. And that has nothing to do with Schrodinger's cat. All sorts of quantum mechanical things going on.

• 1:13:24Sean Carroll?

  Would have that property inside the black hole that from the outside point of view, it's best to treat it still as a superposition of many things going on. I would say the same thing about our cosmological horizon. You know, sometimes you'll see pictures of the multiverse where in, you know, we live in some little bubble of the universe and then outside, there's all these other bubbles, a true vacua that have gotten created by bubbles nucleating out of false vacua. And in some very real sense, that's all nonsense because what's really outside our horizon is a quantum superposition of many, many different possibilities. And there's no sensible way to identify what branch we are on.

• 1:14:07Sean Carroll?

  So I guess to me, and this is kind of tentative, I don't 100% think that this is something you have to agree with, but I think that to me, in the Schrodinger's cat in the box case, the fact that I know a very definite procedure that will entangle me with what's inside the box licenses me to think about already being branched. You know, there's, I'm already on one of those branches or another, I just don't know which one I am yet. I'm allowed to talk that way. Whereas if a system is inside a black hole or outside the cosmological horizon, there's no way operationally for me to actually branch by becoming entangled with what's inside. So therefore it makes more sense for me to think of that as an existing superposition.

• 1:14:51Sean Carroll?

  But I'm open to thinking about this in more sophisticated ways. Marcin Chaddy or Chady asks another priority question. Eternalism posits that all moments of time are equally real and the general theory of gravity states that energy bends spacetime. So if both are true, then the Earth a second ago and the Earth a second in the future, both affect my spacetime as much as the Earth right now. I've reread the first volume of The Biggest Ideas in the Universe and the formulation of the intermendipenser seems to allow for that interpretation, but you don't state this explicitly. Is this because it remains in the domain of interpretation, i.e. energy can be considered either as concentrated in one instant or spread over the time axis? And if not, doesn't it conclusively settle the question of whether eternalism is true?

• 1:15:35Sean Carroll?

  I, I certainly don't think that it conclusively settles the question of whether eternalism is true. I'm not quite sure how that would be, but I think that there's an earlier misconception here because you say both. The Earth a second ago and the Earth a second in the future both affect my spacetime as much as the Earth right now. That is certainly not part of eternalism at all. Eternalism says all moments in time are equally real. It doesn't say that moments in the future and moments in the past affect moments in the present in exactly the same way as things going on in the present do. As far as I know, nobody who thinks seriously about either eternalism or presentism thinks

• 1:16:18Sean Carroll?

  there is some experimental observational distinction between them. Physics works exactly the same in both pictures, it's just a matter of what you attribute reality to. It's a philosophical stance, okay? The things, the dynamical equations, let's put it that way, that tell you how the gravitational field around you right now are created by other parts of the universe are exactly the same under an eternalist point of view or a presentist point of view. I think that one of the reasons why presentism becomes less popular over time is that in a Newtonian universe, it might have made sense, right? In a Newton-- I mean, it didn't necessarily, it wasn't necessarily correct even in a Newtonian

• 1:17:03Sean Carroll?

  universe, presentism, but it might have made sense because there is a preferred way of slicing spacetime into space and time. In special relativity, and certainly in general relativity, if you try to say, "Well, only the present moment is real," people are gonna look at you like you're a little wacky. Like, who defines what the present moment is? There's no obvious way to slice spacetime into a preferred present evolving in time. So, it's not that there's some experimental difference between presentism and eternalism, it's just that once relativity comes along, presentism just becomes very difficult to imagine that it's being a fundamental, useful way of thinking about reality. Tim Converse says, "In the May AMA, you were asked about the last time you said

• 1:17:45Sean Carroll?

  or thought WTF. What the fuck I think that's supposed to mean. I'd like to ask a similar question, but restricted to science. Over the timeframe of your professional career, what was your biggest WTF moment and/or your moment of biggest surprise, if that's different, about a new scientific development? It could be a theoretical development or an experimental one, and within physics or outside of it." I mean, I'm not even going to go outside it about physics, because there's a huge number of things where they're surprising in the world. I mean, my goodness. But in physics, I think there's a very obvious answer, which is exactly what we were just talking about, the acceleration of the universe in 1998. You know, I was a co-author on a

• 1:18:26Sean Carroll?

  review article with Bill Press and Ed Turner on the cosmological constant. I think it got published in 1992, or something like that. So, a good number of years before the discovery of the actual acceleration of the universe. And, you know, I was in charge of the theoretical side of things. Ed was in charge of the observational side of things. Bill was in charge of the analytic side of things. He wrote these wonderful computer codes to show how structure formation and things like that would be different in the universe with the cosmological constant. And, so as part of the, as the person who was supposed to write the theoretical parts, I wrote the parts that had to do with, "Here's

• 1:19:07Sean Carroll?

  why we don't actually think the cosmological constant would be real." Okay? And I emphasized, I talked about the cosmological constant problem, but also the coincidence problem. And all of the, you know, none of these are definitive, right? I mean, you, so what I did, what I thought was kind of cute, is something that was very common to do among people who cared about these things, was to plot omega lambda. So, omega is the proportion, the fraction, of the critical density of the universe, which is made up by any one particular kind of constituent of the universe. So, to say omega equals 1 total is to say the universe has the critical density. That is to say, the amount of energy density it

• 1:19:47Sean Carroll?

  needs to be spatially flat. If omega's greater than 1, the universe is positively curved. If omega's less than 1, it is negatively curved. But if omega equals 1, you can divide up omega into omega matter, omega radiation, omega lambda for the cosmological constant, etc. Okay?

• 1:20:05Sean Carroll?

  And this coincidence problem, the fact that the cosmological constant density remains constant even though the matter density is changing as the scale factor cubed, which is a huge difference in rates of change, let people, you know, there's a natural graph to draw on a log scale, so logarithm of time versus omega lambda. And omega lambda would be 0000, and then, you know, at one moment it would tend to shoot up, and now it's going to be 1 forever on this log scale. So it looks like in the early universe, you have negligible cosmological constant. In the late universe, it's dominating everything, which is exactly what it does. So if you plot that, starting in the early universe and going toward the future, it looks, the coincidence problem looks very vivid, right?

• 1:20:50Sean Carroll?

  You draw a line where we are supposed to be here today, and it's right smack on this very quick rising slope of the almost step function behavior for omega lambda. What I did, which was, which I was very proud of at the time, I was very young and impressionable, was I took the derivative of that. So rather than just plotting omega lambda, the fraction of the critical density given to the cosmological constant, I plotted, how fast is that changing versus time? And then instead of getting a step function, or an almost step function, you get an almost delta function, and up a big peak, right? It's 000, and then it rises up very quickly, and then it falls very quickly, because the

• 1:21:32Sean Carroll?

  derivative is 0, both in the era where omega lambda is always 0, the earlier parts of the universe, and when omega lambda is always 1, the derivative is still 0. So just, it's exactly the same information, just shown in a different way. And then when you plot that and put us there again, we're just like lying smack in the middle of this very, very narrow peak. And Mike Merrifield, who was my office mate at the time, I was a grad student, looked at him, and he said, "Oh, look, the universe is giving us the finger." Now, it wasn't, the universe wasn't giving us the finger yet because we didn't think it was true. I was trying to arsenal, what is it, marshal the arguments to say that this is unlikely,

• 1:22:17Sean Carroll?

  okay? We all knew it was possible, that's how we were spending time writing a review article about it, but we were being honest and we're saying, yeah, you know, it might be, but probably not. The other thing to mention, which we talked about in the paper, is this wasn't the first time, the 1990s were not the first time, when people said, "Well, if only we had a cosmological constant, that would solve our problems," right? That was what Einstein said in the, what, 1917? Or whenever it was. The problem he had was, why is the universe not stationary over time? Why is it supposed to be expanding or contracting? There was another era in the 60s where it looked like there was sort of a hesitation, and/or there's like too many galaxies with the same redshift,

• 1:23:00Sean Carroll?

  and they said, "Well, if there was a little cosmological constant, it could make the universe sort of start accelerating and then stop, and then you could explain this phenomenon," but then it just went away when the data became better. So again, you know, real scientists working in the moment are very aware of the historical trajectories of these things, not just the objective arguments for and against. And they knew perfectly well, the cosmological constant had been advocated before, and it always had gone away. And there were much less wild ideas that could explain the data, and so, and I was on that bandwagon. You know, I took those arguments perfectly seriously. I knew about proposed mechanisms like Sidney Coleman's

• 1:23:43Sean Carroll?

  that would set the cosmological constant to exactly zero. I knew there was no good argument other than the anthropic principle that would set it to small but not zero, so I thought it was zero. And it was my, my good friends from grad school and elsewhere who discovered that it was not zero. So, it certainly helped my career when they found that it was not zero, but I was still very surprised.

• 1:24:05Sean Carroll?

  Albin says, "I know Unruh and Hawking radiation are kind of similar, but if we apply the equivalence principle strictly, do they actually become the same thing? Like how E R equals E P R connects two seemingly different ideas, could this be a similar case?" So for those of you who don't know, Hawking radiation, of course, is Stephen Hawking's idea from the 1970s that in the vicinity of a black hole, the quantum mechanical vacuum will lead to the emission of particles from the black hole, Hawking radiation. And you can calculate the temperature it has to do with the mass of the black hole. When you look deeply into what's going on there and you try to ask yourself questions like, "Where does the Hawking radiation come from?" Bill Unruh wrote a wonderful little paper where he said,

• 1:24:47Sean Carroll?

  "I can imagine a very similar effect in empty space, a space with no black hole at all, no curvature, just Minkowski space, flat spacetime, the vacuum of the quantum field, so there's no particles around whatsoever." And Unruh said, "What if I have a detector?" So a particle detector, something that if there were a particle, it would detect it. And I say, "I put this particle on an accelerator, this detector, on an accelerated trajectory." So this is not really physically realistic because there's no rocket engine or whatever. And even if you did have a rocket engine, it would run out of fuel at some point. But imagine you had a detector that was accelerating at a constant acceleration forever, okay?

• 1:25:31Sean Carroll?

  Then it's interesting because you're accelerating forever. You go faster and faster. And you can define a future light cone that you never leave. There's basically a horizon. It's much like a black hole. There is a set of events that if you, if someone else were to pass this horizon, they could never catch back up with your perpetually accelerating detector. So a perpetually accelerating detector is basically next to or in the vicinity of an event horizon for all practical purposes, even though it's in perfectly empty space, okay? So, interestingly, Unruh showed that that detector would detect radiation. Even though it's in the vacuum state,

• 1:26:13Sean Carroll?

  you're supposed to define the vacuum state as the state in which there are no particles, right? It's the lowest energy state, but the detector detects particles. How is that possible? Well, you know, we can go into the details of the quantum field theory here. The, what a vacuum state looks like to an accelerated observer is just fundamentally different from what a vacuum state looks like to an unaccelerated observer. So when you define a vacuum state, or a zero particle state, that is always with respect to some notion of what it means to be standing still. And by definition, an internally accelerated detector is never standing still. It's accelerating. It's not on an inertial trajectory, so it's okay for it to detect particles.

• 1:26:55Sean Carroll?

  And there's a very close formal analogy between this so-called Unruh radiation and Hawking radiation. Indeed, if you look in the last chapter of my general relativity textbook, Space, Time and Geometry, I talk about quantum field theory and curved spacetime, but we really don't do curved spacetime. I do the Unruh effect. So you can see how thinking carefully about the quantum vacuum state can affect what you observe, depending on how you're moving through spacetime. And if you think about what it would mean to sort of live on a platform that was constructed around a black hole, or even actually, let's, let's bring it more down to earth. Think about what it's like to live in your house.

• 1:27:37Sean Carroll?

  Your house is in some sense, a perpetually accelerated reference frame, right? Because it is accelerated with respect to the geodesic that would be falling down toward the center of the earth in the vicinity where you are right now. You don't detect any Hawking radiation, because you don't get Hawking radiation just from being in an accelerated reference frame. You, you need a horizon, okay? So is there Unruh radiation that you would detect? Probably you would detect some Unruh radiation, but there's no event horizon in, on the earth, so it's not exactly the same as Hawking radiation. Anyway, that was a digression. What, what Albin is getting at is are they secretly exactly the same thing? They're not exactly the same.

• 1:28:21Sean Carroll?

  Because of the phrase that you used, "Apply the equivalence principle strictly." That's very, very difficult to do, to apply the equivalence principle strictly, because the equivalence principle says that certain things are equivalent in small enough regions of spacetime, namely that you can't tell the difference between acceleration and a uniform gravitational field in a small enough region of spacetime. Which is to say, it's a limiting procedure. If you strictly apply the equivalence principle, it's only true in a region of spacetime of zero size. So you can't detect anything at all. So there's kind of no such thing as applying the equivalence principle strictly. There's a very close formal relationship

• 1:29:04Sean Carroll?

  between the Unruh effect and the Hawking effect. Indeed, they're both basically versions of saying, um, what do, what do certain kinds of detectors detect in certain vacuum states in certain spacetimes? But the detectors are different and the spacetimes are different, so they're not strictly, uh, the same. I mean, all of this is still ongoing stuff. Just earlier this year, as I mentioned, Chris Shalou and I submitted a paper on what Hawking radiation looks like as you fall into a black hole. And the answer is you have to be very, very careful about your detector and how exactly it's moving and how exactly you switch it on and switch it off. You basically don't have enough time when you're crossing the event horizon of the black hole to set your detector to on

• 1:29:48Sean Carroll?

  long enough to detect any appreciable radiation.

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• 1:30:52Sean Carroll?

  P. Walder says, "Do you have a view of whether conscious experience is substrate independent or does biology make a difference? In a recent paper titled 'The No-Body Problem,' Anna Chiak... I don't know who this person's name is. Shyun Ika addresses the hard problem of consciousness by suggesting that qualia align with the idea that living systems are fundamentally driven by a homeostatic imperative to maintain their existence. The paper argues that these imperatives are what distinguishes machines based on silicon substrates from agents based on biological substrates. The paper further suggests that the substrate differences may be what accounts for this subjective experience, including qualia of biological systems such as humans from machines.

• 1:31:34Sean Carroll?

  Ultimately, the paper suggests that it is the existential context, as signified by unavoidable death, that necessitates the... that creates the necessity of qualia to provide the incentive to maintain homeostasis and thus avoid death. Does this seem like a plausible set of ideas? I'm always very careful trying to comment on papers I haven't seen. I have not seen this one. So it sounds close to things that I would see plausible, but from this particular paraphrasing of it, maybe pushing a little bit too far. It's actually very close to a recent paper by Anil Seth, who was a Mindscape guest not too long ago, on biological naturalism and consciousness and AI.

• 1:32:14Sean Carroll?

  And Anil's paper I actually have read, and one of the people who was invited to respond to it in the journal, which is still an ongoing process. But Anil's point was, uh, sounds very similar to what you say, uh, she, uh, C I A U N I C A, Chonika, Anna Chonika says, uh, in her paper. And the way that I would say it in sort of the way that sounds most reasonable to me is the following, it's not an anti-physicalist idea, right? It's still totally physicalist, but Anil is arguing against the idea of substrate independence, much as Chonika is.

• 1:32:52Sean Carroll?

  And Anil's angle on it is that we too often focus on what is called computational functionalism. We think of the brain and the mind as performing a calculation. And as I interpret his point, sure, they do that, but they do other things as well. And you're missing those other things if you just try to mimic a brain or a mind as an AI agent. They're based in biological systems where you get a lot of inputs, uh, other than simply, you know, the text that you're reading or listening to in spoken word. And, um, again, there's, uh, biological imperatives to stay alive.

• 1:33:35Sean Carroll?

  Uh, as Antonio Damasio would have said, there are feelings that you have in order to maintain your homeostasis. Um, there's a lot going on over and above what you are modeling in the form of an LLM or something like that. And I'm actually pretty sympathetic to this. I've said things like this before, that the differences to me between current versions of AI and biological intelligences are much more striking than the similarities, uh, exactly because of this biological embodiment. I don't think I would go so far as to think that these arguments really lead us to, um, a complete rejection of substrate independence.

• 1:34:19Sean Carroll?

  I mean, all the arguments are about functions, right? About, uh, trying to maintain your survival, trying to maintain your inner workings and things like that. I don't see any reason in principle why you couldn't mimic these in a different kind of substrate, right, in a mechanical one rather than a biological one. I take the sort of the most persuasive version of the argument to be something like, if you really tried to embody something that was conscious, or just even intelligent, or sentient, as Jonathan Birch would call it, if you really tried to put in all the ingredients over and above the sort of computational power, you would basically end up reinventing biology.

• 1:35:00Sean Carroll?

  I think that's plausible in some sense, and I really don't know the answer one way or the other. But certainly, I think it's a very good sort of pushback on the idea that we can just write a computer program and declare it to be conscious. Spock says, "Are you open to Mindscape guest suggestions from listeners? What catches your eye? I'm curious if I was close with one who brings his own martini and sparks unique insights." Yeah, no, I'm 100% in favor of suggestions of guests to Mindscape from listeners. Several, I don't even know what the list would be, but certainly several actual guests have been because a listener suggested them, and I looked them up and said, "Yes."

• 1:35:43Sean Carroll?

  There's enormous number of factors that go into me picking guests. There's lots of guests out there, and many good ones who I'm sure I've never even heard of before, so that's why I like to get suggestions. I get a lot of suggestions already, so if you make a suggestion and I don't follow up on it, there is a limit on how many I possibly can, so don't feel bad. But, look, I'm looking mostly for people who have ideas, and especially people who have actually, you know, created something with their ideas. Like, if someone is just generally smart, able to comment on anything, things like that, that's actually kind of a turnoff to me, if that's their claim to fame. I want to know what they've done. Have they written a book? Have they pioneered a field?

• 1:36:27Sean Carroll?

  Have they made a discovery or something like that? The substance of it is much more what I want to talk about. I like talking to academics, but certainly I'm not limited to that. Something like the martini thing, you know, I just had a podcast about cocktail theory and the thing is, as soon as I have a podcast, I always get a bunch of suggestions of somewhat similar podcasts, not exactly similar, but you know, along the same theme. But you should know, that's exactly what I don't do. If I've just had a podcast, I'm probably not going to do a similar topic for a while, usually, especially if it's a quirky, fun topic like cocktails or whatever. Potential guests have to be people who aren't good at being a podcast guest, ideally, right? Which means that, you know, they know their

• 1:37:12Sean Carroll?

  stuff, but also they're good at explaining it. You know, I can get them for an hour and a half to talk and, and things like that. So there are many things that go into it, but sure, absolutely, send your suggestions along through, through comments on Patreon, through comments on reposterousuniverse.com or whatever. Moritz says, "Priority question: Does the fact that information needs to be physically instantiated in finite space pose a problem for determinism? E.g., could the Big Bang really contain all the information needed to determine the entire future of the universe? What's your stance on determinism and free will?" So, that's more than one question. You're not allowed to ask more than one question, even for a priority question, but I think

• 1:37:53Sean Carroll?

  there's a theme to the question that I'll try to get into. I don't even know what it means to say, "The fact that information needs to be physically instantiated in finite space." I don't know why that is true, why I should believe that, whether it's true. I don't believe space is fundamental, so I'm not even sure I know what that means to say something like that. "Could the Big Bang really contain all the information needed to determine the entire future of the universe?" Yes. Why not? I mean, remember, the Big Bang, in the classical theory of general relativity, is a singularity that is just not describable at all. A moment after the Big Bang, the universe could have been infinitely big. So there's no limit. There's no obvious boundary on how much information

• 1:38:34Sean Carroll?

  you can have in the Big Bang. It seems like there's plenty. As far as determinism and free will is concerned, plenty of places to look up on the internet of what my opinion about that is. I've written about it many times. It is that I'm a compatibilist about the two ideas. Eric de Vigi says, "Under what circumstances, if any, do you believe that violence is justified in combating institutional evil? Quick example: If a gas company were about to drill in a pristine natural area and the project was certain to destroy a great deal of wildlife, what criteria would the situation have to meet for me to be justified in blowing up the gas company's infrastructure?" I think that there is, on the one hand, there's no absolute prohibition against doing things

• 1:39:15Sean Carroll?

  like that, but I think that the preponderance of feeling should be that that's usually, almost always, a bad idea. You know, violence to achieve political ends is exactly the kind of thing that can be kind of romantic and attractive to the right kind of person under the right kind of circumstances, but that doesn't mean it's effective. And what you have to do is think about, number one, are you 100% sure, or at least as metaphysically close to that as you can get, that you have correctly judged the moral calculus here before you start blowing up some infrastructure and potentially injuring people?

• 1:39:55Sean Carroll?

  Are you absolutely convinced of the correctness of both the physical facts on the ground and of your personal ethical weighing of what would be good and bad? But then, much more importantly, and also much more difficultly, what will be the consequences of your action? Is the gas company gonna stop being a gas company because you've blown up some of its infrastructure, or will they just raise prices on gas? Will you very dangerously set a precedent for increased violence and the justification of violence by people who maybe are not as morally correct as you? Remember back, way back when, very early in the podcast, I did this interview with Ed

• 1:40:38Sean Carroll?

  Watts, who is a historian of ancient Rome, and it's one of those podcasts that has absolutely stuck with me years later, and he talks about the decline in the Roman Republic and puts a lot of blame on the Gracchi brothers, these two aristocrats in ancient Rome who had the idea of assembling a mob to basically intimidate the Senate into doing what they wanted, and they didn't really get what they want, but more importantly, they set the precedent that when we don't get what we want, we assemble a mob. And that's a precedent that anyone can use, not just you. So when you start blowing things up to achieve some political aim, you have to realize, you know, okay, now the thing to do to achieve a political aim is to blow things up, right? And maybe the people who you don't agree with are gonna agree with

• 1:41:23Sean Carroll?

  that strategy and start using it themselves. Again, I think there's just a huge bias or inclination that it's the last thing you should do. It can potentially be justified, but it's probably not as justified as you initially think. David Maxwell says, "I just watched the NATO Secretary General publicly flatter the President of the United States like a scene from The Godfather. Of all the things to worry about right now, how much should we worry that world leaders must debase themselves to avoid US enmity? Is it harmless diplomatic speak or something far more insidious?" Yeah, I think that of all the things to worry about right now, that's a very, very tiny one. I mean, it makes the United States look foolish that we have leaders who are susceptible

• 1:42:07Sean Carroll?

  to this. It makes the other leaders look foolish because they're doing it, but the looking foolishness is really just not something that is high on my list of worries, just because there's so many other more tangible things going on. Yazan Al-Hajari says, "People often say there's a touch of madness in every great mind. How do you balance that intense logical side of your work with your emotions and that little bit of creative madness? Do you have any outlets that let that side of you shine?" Well, I don't quite agree that there's a touch of madness in every great mind. Either that, or maybe my mind is not all that great. I think that among physicists who I know very well, you know, some of them you could absolutely kind of perceive a touch of madness.

• 1:42:49Sean Carroll?

  Others are kind of, you know, they're kind of logical and straightforward and, you know, matter of fact about the work they do. I think that I'm, you know, relatively under control when it comes to those things. I can occasionally be creative, but I don't think it necessarily leads to anything or is involved with anything that anyone would recognize as madness. Maybe that's just what a mad person would say, but that's my self-diagnosis anyway. Henry Jacobs says, "I'm troubled by canonical quantization." It's good to have people listening to Mindscape who are troubled by things like canonical quantization. I think that's just charming. "It appears," Henry continues, "that the process is, number one, write down a classical Hamiltonian as a real valued function of canonical coordinates.

• 1:43:34Sean Carroll?

  Number two, close your eyes and pretend that each of the Qs and Ps are not coordinate functions but are actually operators on Hilbert space, satisfying some specific commutation relationship. Now you have a quantum Hamiltonian. And step three, forget that step one ever occurred, so that the only memory you have is of the operator obtained in step two. Perhaps this can be done with a good knock to the head." "Seriously, this is the way?" That's Henry's question. Apologies that this is a little technical for many of the people listening, but I'll try to be quick, and it's a really good question. It's a point that I often make. Yes, Henry, that is the way. That is what people do. But justification for it at the end of the day is ultimately pragmatic, okay? There's two things you need to keep in mind.

• 1:44:18Sean Carroll?

  Number one, there is not an algorithm, a map, a well-defined procedure, for taking a classical theory and making the one true quantum theory out of it, okay? There's just not a unique map from classical theories to quantum theories. For any classical theory, there are generally multiple quantum theories that kind of resemble it. For some quantum theories, there are no classical theories that it comes from. And for some other quantum theories, there are multiple classical theories that it comes from. So there's just not a good relationship between classical mechanics and quantum mechanics. And that is fine. There's no reason why there should be. What there is, when you are lucky, is a classical limit of your quantum theory.

• 1:45:03Sean Carroll?

  So you start with a quantum theory, and in certain circumstances, you can show that you get classical behavior out of it. In others, you can't. If you have a single qubit, there is no classical limit to that. But when you have a big macroscopic collection of stuff, there might be. But that's okay if what you keep in mind is ultimately I just want to find a quantum theory and test it against experiment. This procedure that you outlined is not supposed to be some airtight procedure for finding the correct quantum theory. It's just a way of getting a quantum theory and then you have to ask yourself if it works, okay? If there's another way of getting the quantum theory, then use that. The nice thing about canonical quantization is that if you do all of this magic that

• 1:45:44Sean Carroll?

  you just talked about, then usually, in well behaved circumstances, the classical limit of that quantum theory that you get will be either exactly or very close to the classical theory that you started with. I don't know a theorem that says when exactly that happens and when it doesn't, but something like that is often the case as a matter of fact. So it is sort of not quite legitimate from a purely logical point of view, but it works in practice and to physicists, that's usually what they care about. George says, "I teach high schoolers physics and math and I'm often met with why questions about the fundamental nature of the world. Why is the mass of an electron what it is and not something else, for example?

• 1:46:26Sean Carroll?

  I don't fall into the camp that physics only answers how and not why questions. However, when the why questions apply to something especially fundamental or seemingly axiomatic about our universe, I really struggle to come up with an answer that satisfies the itch that accompanies such a question. How would you respond to these questions in a way that a student might walk away satisfied with the answer?" Well, first I would say insisting or even aspiring that the student walks away satisfied is too high of a standard. You can never guarantee that students are going to be satisfied, even if you give people an entirely correct answer, because what counts as satisfying to them might be impossible for you to quite reach or might not be true. Maybe the world is not satisfying to them.

• 1:47:09Sean Carroll?

  When people ask about why questions in physics, I think you are completely correct that it is wrong to think that physics only answers how questions and not why questions. That's one of these like simple bumper sticker mottos that does not hold up to any scrutiny at all. Physics is answering why questions all the time. But at the same time, it doesn't Guarantee you an answer to every why question. There can be in the world brute facts. There almost need to be. If you imagine there could be different possible worlds and there's a question of why do we live in this world rather than another one? That's a brute fact right there. The weird thing is that when it comes to fundamental physics, we often don't know whether a specific question is answerable, a specific why question, whether a specific

• 1:47:53Sean Carroll?

  feature of the world has a reason why or is just a brute fact. Okay? The mass of the electron is exactly in that category. It is, on the one hand, perfectly respectable and frequently done to try to create theories that predict the mass of the electron. Not exactly the mass of the electron, because that's a dimensionful number, but something like the ratio of the muon mass to the electron mass. Okay? You can try. You can absolutely do it and maybe you succeed and then you've done it. You've explained why. But you can't demand ahead of time that there is an answer. You have to say, well, we're gonna try. If we win, that's great. If we don't, we accept it. Ultimately, what nature is, is the final answer, not what we want nature to be.

• 1:48:37Sean Carroll?

  Fulkman says, "I was thinking about the inevitability of the development of highly intelligent life forms on Earth and in the universe. Under what evolutionary conditions will it develop? Dinosaurs reigned supreme on Earth for almost 200 million years and, as far as I know, never developed advanced intelligence as we understand it. Had it not been for the asteroid that led to their demise, they might still be ruling Earth and mammals. Ultimately, primates, homo sapiens, might not have had the chance to flourish and develop intelligence." Intelligent life on Earth almost seems like a fluke. What do you think this means for the chances of intelligent life developing elsewhere in the universe? I think what it means is that we have no idea what the chances are of intelligent life developing elsewhere in the universe. On the one hand, you're right.

• 1:49:20Sean Carroll?

  We know it's possible for life to exist, even in highly complex forms here on Earth, for hundreds of millions of years in a row, without getting all that intelligence because it happened. You can't deny that it happened. And then there was a an event that was kind of unpredictable. And this often happens in evolutionary biology in particular, in complex systems more generally, that when you you put some external, you subject the system to some external jolt that changes things dramatically. It might suffer some temporary decrease in viability.

• 1:49:59Sean Carroll?

  But it also starts examining and exploring new parts of configuration space and parameter space, which turn out to ultimately be actually more robust and favorable and fit than the ones that it was already trying. So this is, it goes perfectly naturally with the metaphor of a fitness landscape. I talk about this in The Big Picture. And the fitness landscape idea, which has its limitations, it's not perfect, but it's pretty good for sort of getting some intuition. You imagine a rugged landscape as a function of what genome you have, okay? The problem is genomes are very big and we can only imagine sort of two dimensional landscapes, so that's already a problem. But let's imagine we can do it.

• 1:50:43Sean Carroll?

  And imagine that high peaks in the landscape have a lot of fitness and reproducibility and things like that. So populations of species naturally sort of move up via natural selection to the peaks of the landscape. Now, maybe there's a peak far away that is even higher, that would be even more fit. But in between you and there, there is some valley, right? The dynamics of natural selection give you no way to cross that valley because you just go by random motions and your most of your random motions are pretty small. So they will, if you're already on one peak, generally take you to places of even lower fitness. That's why the jolt from the asteroid is so important here. Now, what does this mean for the development of intelligence?

• 1:51:28Sean Carroll?

  I think that it seems to me very likely that under a wide variety of circumstances, developing intelligence is useful to biological fitness. On the other hand, it may be that there's a wide variety of circumstances where you don't have any intelligent beings in the ecosystem and you're more or less stable in whatever configuration you're in. And you're, you see no reason to undergo the dramatic changes that evolution would require before developing intelligence.

• 1:52:01Sean Carroll?

  Or maybe not. I mean, arguably, as we talked with Peter Godfrey Smith, arguably, octopuses are intelligent in a sense, and maybe if you gave them enough time, they would build spaceships. I really don't know. Maybe they can't build spaceships, no matter how intelligent they get, because of the differences in living in water versus living in air. So I try to be very humble about these things. People make these definitive sounding statements, but I think that there's a lot we don't know about exactly what went into the ability of human beings, not just to be intelligent, but to technologically influence the world around them, to really shape it and control it in ways that require

• 1:52:42Sean Carroll?

  not only intelligence, but a lot of other factors as well. Who knows how many contingent lucky events were part of that? You know, my evil twin Sean B. Carroll wrote a whole book emphasizing exactly that, and we had a podcast conversation about it, the important role of contingency and randomness, the development of life on Earth. So I think it's perfectly legitimate to simultaneously hold two ideas, that intelligence is generally adaptive and a good thing for evolution to find, and at the same time, there might be a tiny probability that it actually is found out there in the space of possibilities. I don't know. The probability might be large, might be small. I'm pretty open about that.

• 1:53:27Sean Carroll?

  Jeff Stone says, "Aside from my day job out there, somewhere out there in the sciences, I am 62 years young, very healthy and diet conscious, sorry, very health and diet conscious, semi-serious amateur athlete and international mountain sports adventurer. Over the past six weeks, my assumption of immortality has been challenged by a diagnosis of an advanced and serious form of cardiovascular disease, followed by cardiac catheterization and stenting. Now, rather than training and adventuring, I'm spending three sessions per week in a cardiac rehab program, exercising at walking speed indoors while EKG telemetry is continuously monitored in real time to keep me safe."

• 1:54:08Sean Carroll?

  My biggest issue is anxiety. By that I mean overwhelming panic attack, emergency department, visit level anxiety. I'm receiving professional help of every kind. I'm happy to report that the Mindscape podcast, your audiobooks and your sonorous voice are a welcome break from my own chaotic stream of consciousness. You already serve as a gentle supplement to other forms of mindfulness and contemplative practice. Do you have other specific words of wisdom for me? I'll just close my eyes and listen. Jeff, thanks for writing this and I'm sorry to hear about these kinds of problems from someone who is of a similar age cohort as you. I have not faced that exact situation yet, but we're

• 1:54:49Sean Carroll?

  eventually all going to come to it, right? I think that, I've said this before, that it's one of the great failures of our society, how we're bad at thinking about the fact that our lives are always finite, right? Not, I don't want to say even thinking about death, because that's a little bit too dramatic, but we do tend to think that we want to live forever and in fact, it's not going to happen, right? We kind of know intellectually it's not going to happen, but we ignore that fact. I did have, as early as episode 10 of Mindscape, I talked to Megan Rosenbloom about the death positive movement, a movement of people to

• 1:55:33Sean Carroll?

  try to sort of recognize the reality of death, the necessity of it, and accept it. And you know, not necessarily to think it's good, but to think that it's something that we need to accept, therefore we should learn to do it. And I don't even know, for Jeff's question, I mean, it's not necessarily a question of mortality and our facing limited time horizons, but also one declines in health and inability as one grows older. You don't bounce back as much as you used to. Those are all things that we have to deal with, but I do think that the finitude of our lives is the big thing, right? Is the thing that leads to these moments of anxiety and...

• 1:56:16Sean Carroll?

  inability to sort of know what to do because you're powerless, right? You are by definition powerless. You can maybe do the right exercises, do the right health routines and survive for longer rather than shorter, but it's all going to come to us. The end will will come to all of us. I think on the one hand, it's entirely legit to feel anxious in the face of that sort of powerlessness. It's not a mistake to feel anxious about it. It would be a mistake to let that anxiousness overwhelm you and prevent you from living the life that you have. I don't like giving out words of wisdom. I can suggest things to think about. I think that just thinking about it, I just think facing up to it and really think about

• 1:56:59Sean Carroll?

  what it would mean is already the single biggest step that you could take, which you're already obviously doing. It's very, very difficult to conceptualize death because by construction, we're, there's nothing it is like to be dead, you know? Thomas Nagel might think it's, there's something it's like to be a bat, which is different than what it's like to be a human. There isn't anything that it is like to be dead. There is no point of view. There's no perspective. We're not dead and looking back and going, oh, well, I regret doing that. There is only what we are thinking while we're here and we're alive. And, you know, very few of us really think, some will eventually come to the point, but most of us do not think, yeah, I've done it all, I'm done. Right?

• 1:57:42Sean Carroll?

  Like, there's more that you want to do, there's more fun you want to have, more people you want to talk to, more things you want to learn, whatever it is. So it's entirely okay while you're alive to feel like, at the very least annoyed, irked by the fact that you can't in fact be immortal and live forever. I do think that once you accept or come as close to accepting as possible the inevitability of your eventually not being here, that helps make your mental attitude toward that reality a little bit healthier in some very real sense. You can think about, okay, Have I been doing in my life the things that I sort of, maybe not everything I wanted

• 1:58:25Sean Carroll?

  to do, but the kinds of things that near the end would make me relatively satisfied with the life I've led? Of course, we all have ups and downs. No one is going to make all the right choices all the time, but we can come closer or less close, depending on the choices that we actually make. And, you know, are there still yet good things that I can do for myself or for other people? To try to sort of think about the people who you know who are no longer with us. Mindscape's been going around long enough now that several former Mindscape guests have died since we started it. And I think of them very, very fondly, right? And, you know, I would like to be thought of fondly after I'm gone, but I don't care

• 1:59:10Sean Carroll?

  that much because I'm going to be gone. But you can think while you're alive about whether or not the life you've been leading is one that other people will think fondly about, you know? I mean, I'm always, in the face of things you can't change, I'm always interested in thinking about the things that you do have some influence over or control over. You don't have any influence on whether or not you will live forever. That is not something that you can control. What you can control, or at least influence in some way, is what you do with a life you have, both, well, at the moment and in whatever future moments we each have. Is that helpful or useful information? I have no idea. I suspect it's very, very personal.

• 1:59:55Sean Carroll?

  I suspect that different people are going to react differently and think differently and have different useful strategies for coping with the inevitability of the end of our lives. I, again, I'm just going to repeat myself, it's not easy, it's not supposed to be easy. There's no simple answer. We're all going to deal with it in our own way, and it sounds like you're dealing with it in an extremely mature and good way, so congratulations to you. I hope that you get to keep treating it that way for several decades to come.

• 2:00:27Sean Carroll?

  Rod Edgerson says, "Does the rise of masked ICE terrorizing Americans and our guests concern you? Does the multiple murders, arrests, and censuring of elected Democrats concern you?" Yeah, I mean, obviously, it concerns me enormously. What can I say? You know, I think that we are... There's... It's impossible to be too alarmist right now, basically, you know? I mean, if you said 10 years ago, like, "Here is a bunch of things that if they happened, I think it would be a warning sign that our democracy is eroding and we're descending into totalitarianism," many of them have happened, just indisputably.

• 2:01:10Sean Carroll?

  The huge budget that was just passed for ICE, for non-Americans, that's the... What does ICE stand for? Immigration Something Enforcement. Who cares what it stands for? The ICE people are like a paramilitary police force with very little structure, or control, or limitation that is part of the federal government. And they've been... Again, for those of you who are outside the country, maybe, not following the news, without any coordination with law enforcement or whatever, these people dress up in sort

• 2:01:52Sean Carroll?

  of ridiculous outfits, wearing masks, no badges, no identification. Often, they put on camouflage, even if they're in the middle of a suburban shopping mall, and you're thinking, "What in the world are these people thinking?" They're dressing like they're in Fallujah, and they're in Des Moines, and carrying high-powered weaponry, and, you know, they pick up people and put them in vans and take them away. How can you not be scared by that? As Rob says, democratic politicians, and they don't even need to be Democrats. I'm sure that the administration would be happy to arrest any politicians they didn't like. They're not randomly arresting politicians, but there are politicians who are trying

• 2:02:34Sean Carroll?

  to do their jobs by, for example, examining, investigating detention centers, and they're being arrested for doing it by this weird paramilitary organization. It's very, very bad. What can I tell you? I think that we're basically building concentration camps in Florida right now. We're bragging about how many people can be sent there. We're inventing new rules where you can be an American citizen, but they can take that away from you by various strategies. There's no recourse to due process. They make fun of the idea of due process. They say, "If you had due process, we couldn't arrest and deport all these people nearly

• 2:03:19Sean Carroll?

  as easily as we are doing." It's terrible. It's going to be historically a black mark on the United States of America for centuries to come, whether or not the United States survives as a democracy. So, yes, I am concerned. I'm not quite sure what to do about it. What little I can do, I'm doing. Claim says, "How can we still see the cosmic microwave background nowadays? Why aren't all the photons already absorbed or moving away from us like everything else?" Well, moving away from us like everything else, you know, it depends what direction you're pointing in, right? The photons are red-shifted since they were first last-scattered, I guess is the right way to say it.

• 2:04:04Sean Carroll?

  But remember, or let me tell you, the cosmic microwave background comes to be at a moment called recombination, which is not quite a moment, it's an era. It's a sort of smooth, gradual process. Around 380,000 years after the Big Bang, when the universe becomes transparent because electrons and protons finally hook up to make hydrogen atoms. And it's not a place, the cosmic microwave background. It's a time. It's created at a time. It's created everywhere in the universe. So, starting from now, I can just ask the question, if I trace backward in time along a light ray, will I eventually hit that moment of time? Yes. Yes, I will. And that's where the cosmic microwave background photons come from.

• 2:04:49Sean Carroll?

  Now, why aren't they all absorbed? Because space is mostly empty and transparent, right? That's why if you look into the sky, it looks black rather than glowing bright. If we lived in a cloud of interstellar gas and dust that was opaque, then indeed, all the photons would already be absorbed. But most of outer space is not like that. It's perfectly, not perfectly transparent, but it's pretty darn transparent. Ted Anderson says, "Did Robert Frost suggest an answer to Princess Elizabeth?" Princess Elizabeth challenged Descartes to provide a mechanism for downward causation." Parenthetically, this is Sean talking now. This is an incident that is talked about in my book, The Big Picture.

• 2:05:30Sean Carroll?

  So Ted goes on to say, "Robert Frost's poem, 'The Road Not Taken,' seems to me to suggest an answer. 'Two roads diverged in a yellow wood, and sorry, I could not travel both and be one traveler, long I stood.' A thinking being encounters this situation all the time." This is Ted again, not Robert Frost. "A decision must be made, left or right." And Ted, you've added a lot of words to your question, so I'm editing you. Word to every future question asker. If you don't want me to edit the question, keep it short in the first place. "In the moment of the poem, the microstate is set, but both choices still seem to be possible." Indeed, both choices are possible in that moment if we simply posit that more than one macrostate can be compatible with a single microstate. How do you know that a given microstate can never be compatible with two different macrostates?

• 2:06:15Sean Carroll?

  Is this a requirement of physicalism? Or is it separate additional proposition? Can it be tested even in principle?" So I think that you have this a little bit backwards. So when you say, "In the moment of the poem, the..." Microstate is set, but both choices seem possible. The reason why both choices seem possible is because the microstate might be set, but it is not known. And it is certainly not fixed by the macrostate. What you know is the macrostate. You know, you know, here's a person, they're facing a fork in the road, et cetera, two paths diverging in the yellow wood. And what are they gonna do? Okay? It might very well be the case that there's a specific microstate of the person facing

• 2:06:56Sean Carroll?

  this choice, which if you knew it, and you had perfect calculational ability and knowledge of the laws of physics, you could say exactly what that person was gonna do. They would go either left or right. Okay? So both choices would not be possible, given all of that knowledge. But the reality, and this is the reality not just for Robert Frost, but for all of us, is that we don't have that microscopic knowledge. The idea that we are making choices, and that there are many things that are possible in the future, is because of our incomplete knowledge. So the issue is not that the microstate is set and it is compatible with multiple macrostates. That is just not how emergence works in the examples that we know and understand.

• 2:07:38Sean Carroll?

  It's that we know the macrostate before the decision is made, and is compatible with many, many different microstates, some of which correspond to going left, some of which correspond to going right. Sean B. says, "I am not a fan of the simulation hypothesis, but I often wonder about the mathematical structure of the universe. If we can model the origin mathem- the universe, sorry, mathematically and deterministically, doesn't that suggest we might be mathematical objects embedded within a larger mathematical framework?" Um, no. It does not suggest that. I don't know why it would suggest that. If you thought it suggested that, what about the people doing the simulating? Are they, uh, modeling their universe mathematically and deterministically?

• 2:08:23Sean Carroll?

  And are they for their, are they therefore likely to be in a simulation themselves? I don't think that there's any connection between the success of mathematics in describing the universe and the possibility we- Live in a simulation in either direction. So I don't think that if there were, uh, let's put it this way. If I did a simulation, I could easily inject in the simulation all sorts of violations of mathematical regularities and so forth. So that's completely compatible with living in a simulation. And the other way, the fact that there are mathematical regularities. I don't see a need to explain that by referring to a bigger simulation. Mathematical regularities are perfectly expectable, I think.

• 2:09:07Sean Carroll?

  This is a question we don't know the answer to. This is one of those questions we just talked about before. Is there a reason why our universe obeys laws of physics? I don't know, maybe it just does, but I don't feel the need to explain that by referring to a higher level who is simulating me.

• 2:09:24Sean Carroll?

  Victoria Lucas says, I'm a medical legal, sorry, medicolegal death investigator from Texas and an atheist, but some of my experiences have troubled me over the years. What are your thoughts on deathbed visitations? These are not near death experiences, but deathbed visitations on people that are in the process of dying. Why do you think dying people hallucinate dead relatives or friends and not living ones? Well, I think dead people hallucinate all sorts of things because they're dying, because their bodies and minds are failing. This is no more weird to me than why I have weird dreams at night. Like in, when I'm dreaming, my, my body and brain are perfectly healthy.

• 2:10:06Sean Carroll?

  I had very weird dreams last night where Jennifer and I were living together in Chicago, which we never have actually done, but I used to live in Chicago. And we were searching for an amplifier. I really don't know why we were searching for an amplifier. It makes no sense, but that's okay. It's not supposed to make sense because it's kind of random little associations bouncing back and forth in my brain. When you're close to dying, why do you remember people who you haven't seen for a long time? Again, this just seems like not at all implausible to me, especially when the alternative is supposed to be completely violating the laws of physics in order to make these, uh, former known people actually real somehow.

• 2:10:49Sean Carroll?

  Peter Schuler says, "How do you reach out to potential guests of the podcast when they don't know you? If you send an email, what do you say? Is there any tension between getting someone's attention, proving your email is worth reading and/or taking seriously, and coming across as self-aggrandizing?" Yeah, that's a good question. You know, as someone who receives a lot of emails myself, I like it when the emails are short. So, I don't like it when someone sends me an email and I have to read to paragraph three to find out what it is they're saying. So, I do send out emails myself, and they're generally very short, and they start with, "I'm writing to ask whether you'd like to be a guest on my podcast Mindscape," and then I give a link to the podcast. And then maybe another couple sentences about what the podcast is

• 2:11:32Sean Carroll?

  about, what we could talk about, things like that, and that's it. Sometimes it works, sometimes it doesn't. I have no problem at all with people who don't want to be on the podcast. That's fine. I say no to a lot of other podcasts. So, I don't generally Hector people. Like, sometimes, people will write back and say, "Oh, yes, sure. That sounds good. Let's do it." And then I'll write back to them and say, "Okay, let's pick a date," and I never hear from them again. Those people I will Hector a little bit. And again, it's probably not their fault. I get it. I've been there, too. Sometimes, you just don't get around to answering the emails. But I'm not gonna, you know, try to prove to someone that they really should be on my podcast. That's a choice that they're welcome to make or not. Joe Seibert says, "I think I subscribe to the notion that you can't derive an ought

• 2:12:16Sean Carroll?

  from an is, but then I wonder, where do aughts come from in the first place? Is there any kind of study of the nature of aughts that might point us towards some actual universals or nearer universals about morality?" Well, yeah, I mean, there's plenty of people who think that they have a way of getting universals or objectively true facts about morality. Russ Schaefer-Landau, who was a previous guest, is a leader of such people, but they don't have a unified front. There are plenty of different approaches to doing that. My answer is that there aren't any actual universals or near universals about morality. So the question, where do aughts come from, is from us, from individual people and

• 2:12:58Sean Carroll?

  collections of people trying their best to figure out how to reconcile their moral intuitions with how they think systematically about what is right and wrong. And we call that endeavor moral philosophy. And it might never be objective, it might never be realist in the sense that the physical world is real, but that doesn't make it any more useful for that. Blake McEwen says, "When my cat needs something and she gives me that look, I go and do the thing she needs and she responds as if we communicate. I believe she is conscious and I suspect you feel the same way for Ariel and Caliban. Where do you think the rock bottom is for consciousness? Clearly not a rock, but maybe bacteria."

• 2:13:38Sean Carroll?

  Well, my own view, which is not anything close to educated, or set in stone, or unwilling to be changed by further data or even persuasive argument, is that it's a continuum. I don't think that there is necessarily some phase transition where you go, oops, before you were not conscious, now you are. There might be a series of small phase transitions that give you different capacities that are part of being conscious. Maybe even microorganisms can have a certain amount of information processing, or self-awareness. Later down the evolutionary ladder, you have the ability to plan and imagine, as Malcolm MacGyver talks about. There's a whole bunch of things that go into being conscious. So I don't

• 2:14:23Sean Carroll?

  think that there is any such thing as where the rock bottom is. That might be, might sound like I'm cheating out of the question, but those are my honest feelings. Zach McKinney says, "What is your level of credence that consciousness is a substrate-dependent phenomenon? You often describe consciousness as an emergent property of information processing physical systems. However, when speaking with Christoph Koch, you took issue with his hypothesis that consciousness might arise from a state of quantum superposition on the grounds that superposition is not an objective fact about a system, but rather depends on the choice of informational basis to describe the system. Does it follow that you believe consciousness must be substrate-dependent, because the presence or absence of consciousness in a physical system is an ontological question, whereas, any definition of consciousness

• 2:15:08Sean Carroll?

  in terms of pure information processing will necessarily fall subject to the arbitrary choice of what properties of the system to count as information?" Well, no, that's not quite the argument I would give. I think that the quantum mechanics thing is a separate thing. Quantum mechanics is kind of special in this fact of superpositions and the fact that superpositions are not objectively defined, but rather relative to a basis. There's a lot, I'm not quite sure exactly how to describe, how to respond to the various points in the question here. Is consciousness ontological? Consciousness is a property that physical systems can have, and whether or not we choose to ascribe consciousness

• 2:15:53Sean Carroll?

  to them depends on the usefulness of that. I mean, this is the pragmatist in my philosophy coming out. What good is it to describe something as conscious or not? Does that, does saying that tell you something useful? Can you do something that is predictably correlated with what is happening in the world on the basis of that piece of information? So, I'm agreeing, I guess I would agree, that I've been persuaded by the arguments we talked about earlier in the AMA from Anil Seth and others, that there is more to consciousness than information processing. But I don't think that that takes it as far as saying it's completely substrate-dependent. Maybe it is easier to imagine it happening in certain

• 2:16:36Sean Carroll?

  kinds of substrates, biological, for example. But if it's not pure information processing, then I still don't see any obstacle to reproducing whatever are the features of consciousness in all sorts of different substrates. At the very least, I could imagine building larger objects, macroscopically big objects, that sort of interact with each other like atoms interact with each other, and therefore build a huge, brain-like thing that was more or less conscious. It might take decades, eons, or whatever, to have a thought, but I don't see the obstacle to that as a thought experiment. So I'm not willing to go so far as to say

• 2:17:18Sean Carroll?

  that consciousness is inextricably dependent on a substrate, but it might be as a matter of practice that biological substrates are just much more likely to get there. Brent Meeker says, "Why isn't there more interest in CP or T violation? Time reversibility is often invoked in physics thought experiments and Boltzmann brain puzzles, but CP violation says it is not time reversal invariant." It turns out, as you can read in various places that I've written, like From Eternity to Here, or one of the biggest ideas in the universe books, I forget which one it was, but it is time reversal invariant. It's just an accident of history that what we call T, the time reversal operator, is not the right one.

• 2:18:03Sean Carroll?

  What we should have called T is what we do call CPT. So for those of you who have no idea what I'm babbling about, in particle physics, there are three famous discrete symmetries. In fact, they are two-way symmetries, parity kinds of symmetries, that if you do the symmetry transformation twice, you come back to where you started. One is literally P, parity. You change right-handed coordinate systems for left-handed coordinate systems, or basically you look at the system in a mirror that switches right and left. Another is C, which roughly speaking, switches particles and antiparticles. And another is T, which is time reversal invariance. You might think that knowing what time reversal invariance is, is a completely obvious thing to do.

• 2:18:47Sean Carroll?

  There's not a lot of choices that go into it, but you would be wrong. The correct statement is that there is always, in every deterministic set of dynamical laws, a transformation you can do that reverses the direction of time and is a symmetry of the system. The real important thing is information conservation and reversibility. When you have information conservation and reversibility, you will always have some time reversal symmetry. So for example, in electromagnetism, it turns out that if you do time reversal, the electric field is unchanged, but if you, let's put it this way,

• 2:19:30Sean Carroll?

  I can make up whatever transformation I want, and then I can ask, is it a symmetry? So if I do a symmetry where I just change T to minus T, time to minus time, that would not be a symmetry of electromagnetism, classical Maxwell's electromagnetism. What I need to do is take the magnetic field and also reverse it. But not the electric field, okay? So E goes to E, B goes to minus B, and T goes to minus T. That is a good time reversal symmetry. Indeed, in Hamiltonian mechanics, where you have positions and momenta, the thing about Hamiltonian mechanics, which I do beat on about in the first volume of The Biggest Ideas: Space, Time, and Motion,

• 2:20:12Sean Carroll?

  is that you truly treat position and momentum as two independent variables. You do not define momentum to be the mass times velocity. You derive it, that's a fact, but in principle, those two things are separate. And the right time reversal invariance operator sends X to X and P to minus P, as well as T to minus T. It reverses momentum and time. So basically, you have to think about what would make a good time reversal operation. In the case of quantum field theory, the good time reversal operation that is a symmetry is called CPT. You change T to minus T, but then you also change the parity of the system and you charge conjugate it.

• 2:20:56Sean Carroll?

  You change particles to antiparticles. D. Dashewski says, "It seems like the number of mechanisms for faster-than-late travel, such as stabilized reversible wormholes or Alcuvier warpdrives, require negative mass exotic matter. Is there anything to this similarity, or is it mere coincidence? Also, is there any reason to interpret those results as anything more than a fancy way of saying, 'This could work if it was possible, but it's not, so it won't.'" For the latter question, I think that's more or less the way I think about it. This could work if it was possible, but it's not, so it won't. You know, we don't have any theorems to that extent, as far as I know. At least, we don't have any theorems that we completely believe, in part because we don't have the full theory of quantum gravity, right?

• 2:21:39Sean Carroll?

  So we're trying to work with classical gravity and some approximations about quantum mechanics and matter fields and stuff like that. So it's hard to know these things for sure. There's another problem, which is very much a problem of principle. For those of you who don't know, the Alcuvier warpdrive was an idea of Miguel Alcuvier, a Mexican physicist, wrote down a...

• 2:22:02Sean Carroll?

  Metric on spacetime, I don't even want to say a solution to Einstein's equation, because every metric is a solution to Einstein's equation for some kind of matter fields, right? The matter fields might not be realistic, but I can write them down. What does it mean for matter fields do not be realistic? Usually it involves negative energies or negative masses. If such fields existed and were sort of macroscopic and big and manipulatable, they would probably mean that spacetime itself was just dramatically unstable. Because you could make a big blob of negative energy and a big blob of positive energy, while conserving energy overall. And by quantum mechanics, that would happen all the time, and it clearly doesn't.

• 2:22:45Sean Carroll?

  So in realistic circumstances, you probably don't get appreciable amounts of negative energies. And things like the Alcubierre solution require some negative energies. So anyway, what Alcubierre did was he wrote down a metric that looks like the following thing. If you just imagine flat spacetime. So you imagine Minkowski space, and I have this thing called where I am now on Earth, and then Alpha Centauri is four light years away. And you would think that it takes me four years worth of travel to get to Alpha Centauri. That's hard to quantify, because you're in a certain reference frame, right? And in a different reference frame, it's different. But there's a number, which is how long it takes to go to Alpha Centauri and back. And that's at least eight years, right?

• 2:23:29Sean Carroll?

  That's the speed of light limit, and that's very well defined. So Alcubierre came up with a distortion of Minkowski space that was basically in the form of a bubble, okay? And this bubble looks like it zooms out to Alpha Centauri and it zooms back. And you could imagine putting a spaceship in this bubble, and the zooming out goes much faster than the speed of light. But to be super duper careful about that, it looks like it goes faster than light would have gone in the original Minkowski space. The point is, you have changed the metric on spacetime so that nothing actually goes faster than the speed of light.

• 2:24:11Sean Carroll?

  It's only a comparison between what actually happens and what would have happened in the completely different universe. So does that count as going faster than the speed of light? Well, you know, for all intents and purposes, you can, if you could build this so-called warp drive, you could look like you were going faster than the speed of light. It raises huge problems with the initial value problem of general relativity. That is to say, given the initial data, how do I evolve it forward in time? Things like that. People debate about whether or not this is possible even in our imaginations, much less in reality. I personally suspect things would go very, very wrong if you tried to do it. So all of which is to say, the short version of this answer is, I am sympathetic to the

• 2:24:56Sean Carroll?

  idea that there is something physically completely infeasible about all of these suggestions about faster-than-light travel in practice. But because we don't know exactly how to define the problem, and certainly we don't know quantum gravity perfectly well, it's hard to know for sure right now. Callan McAuliffe asks a priority question. "The Goldilocks fog conjecture posits that when the density of an evolving system's decision space exceeds its neurocomputational resolution, the inability to individuate its many choices is perceived as an illusory, edgeless sense of self, just as droplets form a fog. In other words, I reckon phenomenological phenomenal, sorry, phenomenal consciousness

• 2:25:41Sean Carroll?

  emerges within an evolutionary window of cognitive blurring, outside of which the decision space is too easily parsed, either because it is very simple, too few droplets to form the fog, or because the modeling resolution is sufficiently advanced, capable of resolving the droplets in the fog. One implication of this is that a superbrain might be as conscious as a bug, another is that I'm out of my depth. If I could, I'd frame this as a complement to AST's modeling and perhaps GWT's architecture, but that's well beyond where my amateur insights runs out of road. Is there any merit to this idea?" So the idea here, if I can sort of try to paraphrase it, is conscious beings, you know, have a certain way of thinking about the world, and they also have a certain

• 2:26:25Sean Carroll?

  way of thinking about the decisions they could possibly make, right? And basically, there are too many decisions for them to actually completely understand what is going on, and therefore, they have a feeling of making a decision, right? In other words, if you have no decision space, if you have no ability to even imagine the future, then you're not conscious. But also, if you have perfect Laplace-Deman knowledge of what's actually going to happen, so that there's a huge decision space, but you can actually perceive them all differently and know what's going to happen, then you're not conscious either. It's the fog, the Goldilocks Fog conjecture, as Callan calls it. I have no idea whether this is on the right track or not.

• 2:27:05Sean Carroll?

  I guess one thing I would say is, it seems to fall far short of really counting for everything you need for consciousness. I mean, maybe this ability, maybe there's some relationship between your ability to perceive what will happen and your ability to imagine different possibilities that is an important part of consciousness, but there's certainly other parts as well. Self-awareness, you know, things like that. I am conscious that it is warm in the room, right? I don't know where that would fit in to this description. So I think that you need to do a lot more work to really call this a good theory of or criterion for consciousness. Sam Hartzog says, "What is your take on fascism as a concept or label?

• 2:27:49Sean Carroll?

  It seems to be notoriously ill-defined from an academic standpoint, and only ever truly cemented around a particular regime in hindsight. In your estimation, do labels like this help or harm public discourse with regard to current world affairs?" I think that labels like this are not super-duper useful, honestly. You know, I mean, they can be useful for propagandistic reasons, rhetorical reasons, getting the people who are already on your side to get all riled up, which are all perfectly good reasons. Sometimes you want to do that. But in terms of like calm, careful deliberation about what's happening, they are less useful, which is why you'll never, I don't think you will ever hear me talking about, for example,

• 2:28:29Sean Carroll?

  the Trump administration as fascistic or so forth. I will talk about them as totalitarian or authoritarian or anti-democratic. Those are less loaded emotionally and more purely descriptive terms. I think that it's natural to understand why people want to leap to words like fascist or Nazi or whatever, precisely because they do convey an enormous amount of emotional punch. Words like genocide likewise, right? I mean, concentration camp, which I used before. But concentration camp, I think you can really argue is directly applicable in this situation. It comes with connotations that may or may not be completely

• 2:29:14Sean Carroll?

  there. And I think that's the problem also with fascist. You know, fascist had a clear meaning around the time of World War II and before, in the context of Italy and arguably Nazi Germany as well. And you can argue about what it meant, right? Was it only specific to that time, blah, blah, blah. And then, as soon as you start having that argument, you sort of lost the bigger argument. You know, the bigger argument is if you have a government that is even close, even somewhat resembling fascism and, you know, its authoritarian impulses, its use of secret police and things like that, that's what matters to me. It's not the label. And as soon as you call it fascist, instead of arguing about the facts,

• 2:29:57Sean Carroll?

  about all these terrible abuses and worrisome actions on the part of the government, now you're arguing about the word. And yeah, it seems like, it seems to be mostly not constructive if what you actually want to do is reach people who don't already agree with you. Red Antonov says, "As best as we can measure, the universe appears to be spatially flat. What does your gut say? Do you believe it's case closed, or is there room for a more precise measurement to deliver a surprise non-zero curvature result?" So there's two things to say here. One is that we can only see part of the universe, remember. So when people talk about the geometry of the universe as either being positively curved, negatively curved, or flat, for one thing, it's an approximation, because the

• 2:30:41Sean Carroll?

  universe is not perfectly smooth and isotropic. But for another thing, it only applies to the part of the universe we can observe. I see no reason to think that whatever is going on in the region that we do observe, it necessarily continues on forever into the region that we don't observe. So the very idea of the universe having a fixed simple geometry is one that is only sort of applicable in a certain regime. That's one thing. But within our observable universe, we do have an average curvature, and we've measured it, and it seems to be close to zero. You know, I was surprised when we discovered the cosmological constant, as I've already admitted earlier. So since then, I tried to have learned my lesson, and I tried to be open-minded. It's tricky, because you

• 2:31:26Sean Carroll?

  don't want to be too open-minded. You don't want to be so open-minded that you're paralyzed with indecision. I think that the chances that we detect a non-zero curvature in space are small, but not zero. We don't understand the space of possibilities well enough. We don't understand what happened at the Big Bang, or inflation, or whatever replaces that, to really say for sure. You know, if a model like the model that Jennifer Chen and I put forward, where we live in a baby universe that is a little bubble, if something like that is true, then you would expect there to be a small positive curvature in space. But we don't know how small. Maybe it's 10 to the minus a million, right? And

• 2:32:06Sean Carroll?

  that is certainly unobservable. If, on the other hand, we live in a true vacuum state that came from a bubble of false vacuum state, then you would expect the curvature to be small and negative, right? So we really have no very good idea of what to expect there. That's why we keep doing the measurements. Steve G. says, "In a recent AMA, you said that you saw zero barriers in principle to some kind of computational architecture doing everything that human beings can do, as similar processes could absolutely be replicable in machines. For a machine to extrapolate from knowledge and experience in truly original and creative ways, do you think the machine would need to exhibit awareness and qualia? My feeling is that these would be necessary to generate a truly novel concept, although I struggle to explain why I think this."

• 2:32:51Sean Carroll?

  For one thing, I think the word "qualia" is just bad. I don't like that word. You notice I never use that word, except when I'm reading somebody else. It's very ill-defined what a qualia is supposed to be. I don't know it when I see it. So, consciousness, I think, is slightly better defined. Sentience is certainly better defined. Awareness is somewhat better defined. So I'm willing to use those words. "To do things in truly original and creative ways?" I think the barrier to that is much lower than you might think. I see no barrier to a very non-self-aware, non-conscious computer creating what appears to us as an original work of art, an original visual image, or an original piece of musical

• 2:33:34Sean Carroll?

  composition, etc. It might not... it might fall short of what you want for various reasons, but if all you're asking is "original and creative," then no, I see no barrier to that at all. It might fall short in the sense that it lacks a certain history, a certain connection to human feeling and emotion and things like that, and that might be something that is hard to tell from the outside unless you actually know the provenance of the thing that you're thinking about. But no, I don't see any reason why things couldn't do that. So therefore, I cannot explain why you have that feeling. Sorry about that. Anonymous says, "What is your gut credence that general relativity is a continuous smoothing

• 2:34:18Sean Carroll?

  of a statistical thing, like heat and fluids are a continuous smoothing of statistical things?" Well, I truly don't know. That's a good question, except, let me say, it is... my credence is extremely low that it is a continuous smoothing of a statistical thing exactly like heat and fluids are continuous smoothing of statistical things, because I think that there's good reason to think that... Gravity is not local in a perfect sense, namely the holographic principle, ADS-CFT, things like that, things like we've talked about on the podcast before. There are reasons to think, admittedly only at the thought experiment level right now, not at the level of experiments we've actually done, but reasons through thought experiments

• 2:34:59Sean Carroll?

  to think that strict locality breaks down when it comes to quantum gravity. And if you think that the world is made of little things, which you then just average over some region of space to get more emergent, higher-level, kind of fluid description variables, that's a strictly local kind of behavior, okay? So I think that people get a little bit confused, and I think this is why I think that things like causal set theory, loop quantum gravity, et cetera, are just on the wrong track. They're taking the locality of space and spacetime far too literally. I think that everything we've learned from black holes and holography and ADS-CFT points us in a different direction than that.

• 2:35:42Sean Carroll?

  So it might be that spacetime emerges out of some fundamentally discrete thing, but those discrete things are not going to be located locally in space. It's not like this little cubic micrometer in front of me contains a certain number of little atoms of spacetime or anything like that. Humberto Nani says, "I love swimming, and I can watch hours of an open water event that for other persons will be boring, as I can appreciate tactics and strategies being deployed. Watching basketball is the opposite of boring, but not being a player, neither long or nor long-term fan, I feel I could use some tips on what to watch, either in the games or in the drafts. May you please share something that you like to latch a lot of attention into?"

• 2:36:23Sean Carroll?

  Yeah, I think it's a great question, because sports in general, or even many activities, many game-like activities like playing chess or playing poker, these are all examples of things where you can kind of get some appreciation for what's going on without a lot of expert-level knowledge. But if you do have some more detailed knowledge about what's happening, your enjoyment of spectating the activity becomes enormously greater. In the case of basketball... There's moments of action, you know, there's some tough shot making, some dunking, some steals and whatever, that basically anyone can appreciate the athletic prowess being shown. But there's a lot of levels going on.

• 2:37:04Sean Carroll?

  I think that if I were just to give one little pointer for going one level deeper than the most superficial, it's a very obvious thing, but don't just watch where the ball is. What we tend to do when we watch any sport is to watch the ball. If it's soccer, hockey, football, whatever, if we count pucks in hockey as part of balls in sports in general, it's that's the locus of our attention, right? But there's a lot going on elsewhere. In basketball, you know, it's a little distorted since there's emphasis now on the three-point line, but what you want to do is you want to put the ball in the basket when you have the ball on offense and prevent the other team from putting the ball in the basket when you're on defense.

• 2:37:47Sean Carroll?

  And it's a basic fact that goes down to physics, that it is easier to put the ball in the basket accurately if you are close to the basket. Again, the three-pointer gives you a little bit of extra reward for shooting a long-range shot, but still, most teams are going to make some effort to get a shot at the basket as close as they can, right? And the other team, the defensive team, is going to try to stop them. So how do you do that? Famously, there's lots of different ways to do it. One way to do it is to sort of clear out and let a person go one-on-one. So if you have one person who is extremely good at dribbling and moving very, very quickly,

• 2:38:30Sean Carroll?

  then you would like them to, one thing you could do is just move everyone else, all of your rest of the players to one side of the floor and them to the other side, and then let them go to work. The problem with that is that... There certainly are advanced professional basketball players who can win one-on-one battles very frequently, but if that becomes your regular diet, the other team figures it out very quickly and sends more than one defender to stop you, and that's generally easy to do. There's nobody in professional basketball who can regularly score when two people are just defending them and not paying attention to anyone else. So the strategies become, how do you get your good shooters either shots close to the basket, or at least shots

• 2:39:14Sean Carroll?

  where they're not being very closely defended? And that is where the strategy comes in, because you both pass the ball around to try to get it to people who are in one of those situations, or you bump into people. And this is what makes basketball fun. It's the setting of screens and running of plays. So setting a screen is when, and this happens in football and soccer and hockey, I don't know if it happens in soccer, I presume it does, certainly happens in hockey all the time, where a person without the ball, well, let's put it this way, one person has the ball, and they're being defended by another person, and then a person on the offensive team just stands in the way of the defender.

• 2:39:55Sean Carroll?

  So the person with the ball moves to bring their defender to bump into the person that is called setting the screen, and then that gives them either a little bit of moment to shoot a long-range shot, or a little bit of opening to drive toward the basket. And then there can be layers on this, of course, right? You can hand off the basketball to someone who is moving your direction. You can get a little bit of open floor and therefore move toward the basket, and then some other defender comes toward you, and you can pass to the person who was being defended by that other defender. So it's really, the fun is in watching the teamwork of all the different players coordinate. And as far as I can tell, I don't, I'm not really a super expert on this, but...

• 2:40:42Sean Carroll?

  Some fraction, depends a lot on the team and the coach and the players. There are plays, right, that the coach works out ahead of time with the players and either coach can call out a number or one of the players, generally the point guard, the one who brings down the ball, can call out a number that corresponds to a play and then you run a play, okay? You literally have every player on the offensive side in some predetermined location and do a predetermined mo- mo- ah, motion. Or you can sort of run little half plays, so like rather than the coach calling out a specific play, the players can sort of scout the terrain, see what the defenders are doing and they can run what is called an action.

• 2:41:24Sean Carroll?

  An action is less than a full play, it might only involve, you know, two players or maybe three players, and they're sort of spontaneously improvising around some set idea like that pick and roll that we just talked about, but with a special wrinkle that these players have worked out ahead of time. And so, you know, that's what's, that's what's fun for me about watching basketball is this is constant interplay of structured running of plays plus improvisation plus amazing athletic feats of agility and jumping and shooting from a long time, long way away. Darren Vigliotti says, "How and when did you first come across the concept of poetic naturalism? Did you take to it right away or was it something that grew on you over time?

• 2:42:07Sean Carroll?

  How did it change the way you thought about the nature of reality and the universe?" I'm someone who doesn't, you know, well, let's put it this way, I don't remember epiphanies like that very much. I think generally I don't have such epiphanies. I tend to creep up on ideas that I agree with. I mean, people ask me like, "When did you start believing in many worlds?" Or, "When did you blah, blah, blah?" It's always very gradual. And so, no, there's no moment when I said poetic naturalism is the right way to do it, you know, I- Gradually became a naturalist. I gradually learned about emergence and levels. I gradually thought about how to put them together. And eventually, when I was, it was only when I was literally writing the book, The Big Picture, that I had to come up with a label

• 2:42:50Sean Carroll?

  for it. And people generally like the label. The professional philosophers who would be sympathetic to that kind of idea have responded positively. I do think that it would be nice someday to write a, you know, technical philosophy monograph about poetic naturalism. The Big Picture was an enjoyable book to write. I'm very proud of it, but it was a popular level book. It was aimed at a general audience. There's all sorts of details that one might want to flesh out in a more philosophically rigorous way. Maybe someday I will do that. That is a, you know, who knows? It depends on how long I live, how much energy I have. That's one of the things, on the very, very long list of things I would like to someday do.

• 2:43:35Sean Carroll?

  Ross says, "In your excellent conversation with Edward Miguel, you asked whether state policy can positively shape culture," paraphrasing a bit. Miguel pointed to Tanzania's decades-long effort to cultivate a national identity, and you seem supportive of its success. "That struck me, since efforts to engineer patriotism often raise red flags for me. For example, China also frames its policies in Xinjiang as promoting national unity, and reducing conflict ostensibly a similar goal but widely condemned. How do you think about the trade-off between encouraging social cohesion and protecting cultural pluralism? When does a national identity project become ethically problematic? Is it purely a matter of method and severity, or are there more fundamental differences?"

• 2:44:18Sean Carroll?

  Yeah, I think it's an excellent question, actually. And in my view, as not an expert in international relations, or political science, or anything like that, I have no problem with the idea of a national identity or a patriotism. In fact, I think it's a good idea, if anything. Of course, they can be misused, right? For better or for worse, there is a tendency in different countries for patriotic sentiment to sort of be mobilized by right-wing forces, or by more nationalistic forces, let's say, in order to create a barrier between the nation and everyone else. And therefore, when you have a

• 2:45:01Sean Carroll?

  In-group, out-group barrier, it increases the chances that both you will think less well of the out-group and maybe even come into conflict with them, all of which is bad, okay? I do think that nevertheless, there are also benefits to it, especially if you're not at the level of superpowers, okay, or some of the wealthiest countries in the world, where you're well-established, we know who you are, and so forth. In a country which is still in the developing world, maybe even a country that has boundaries that have been kind of arbitrarily drawn during the post-colonial process and so forth, I think the story is different. I think that you do, it does have a more positive connotation to think of generating

• 2:45:44Sean Carroll?

  some social identity within the country. Hopefully, I would argue, I would want that social identity to be open and welcoming and cosmopolitan and allowing all sorts of different ethnic groups or religious groups, et cetera, into the project. How easy is that to pull off? I don't know. This is why I'm not an expert in these things. But I would not, you know, take the experience of having lived in a developed, heavily industrialized, powerful nation and say, "Well, in this context, we have some negative connotations of nationalism, therefore, we're going to worry about it in this very different context." It's okay to worry about it, but I think we have to be very, very open to the idea

• 2:46:29Sean Carroll?

  that when the context is that different, the implications are going to be very different also. Rod Adkerson asks a priority question. "If we were to map human relationships or human behavior, or the behavior of all life, on some set of axes, do you believe it's possible that those points would correlate with some metaphysical biology where we might ultimately find intelligent life?"

• 2:46:56Sean Carroll?

  I really have no idea what is going on with your question, Rob. Sorry about that. Some set of axes is just ill-defined. Human relationships are human behavior. So, one axis could be, um, how many people are in the relationship? How strong is the relationship? Human behavior, human relationships are human behavior. Two very different things. You would need very different axes to describe them. And I presume, uh, you're imagining mapping like empirically the actual relationships of human being and, and, and stuff like that. And so by definition, all the points that show up on the graph are correspond to intelligent life. So if you're asking, do all possible intelligent creatures

• 2:47:41Sean Carroll?

  live at the same places on such a graph? I would be highly skeptical of that. I mean, we know that right here on Earth, um, among species who are not as intelligent as us, but still social species like ants or even different kind of primates have very different kinds of relationships and behaviors. I see no reason why aliens wouldn't be similarly diverse in how they went about such things. Okay, I'm gonna group two questions together. One is from Wonder and says, "If there are a finite number of branches in many worlds, what happens when we reach that number? Is that what happens when we reach maximum entropy?" And Marie Roskew says, "I recently listened to an interview with Stephen Wolfram. He brought up something that has been boggling me,

• 2:48:25Sean Carroll?

  merging of branches of many worlds in the many worlds interpretation. Could those branches possibly be merged? It sounds counterintuitive to me. I would like to know where you stand on that being a many worlds person." Yeah, it's, so, on the one hand, entirely possible in many worlds for two things that you thought of as separate branches to merge again. What does that mean? Well, what it means is you have two different branches of the, of the wave function of the universe. To make things simple, let's imagine we've separated out system and environment, right? So we have a certain set of things in the world, which is basically every macroscopic object. I'm not thinking just like a single spin or a single cat. Literally every macroscopic object in the world is part of the system.

• 2:49:10Sean Carroll?

  And then all the microscopic degrees of freedom that we don't keep track of, the position and velocity of every photon in the world, that's part of the environment, okay? And the thing that separates branches of the wave function is that the environment... Entangled with one branch is perpendicular to the state of the environment entangled with the other branch. Perpendicular orthogonal as vectors, okay? The two environments are just pointing in different directions in the Hilbert space corresponding to the environment. Now, you might say that sounds pretty specific, two vectors being perpendicular. I can easily imagine two vectors that are not perpendicular.

• 2:49:49Sean Carroll?

  In order to merge two vectors, two branches of the wave function, all that has to happen is that their environment states need to line up with each other, so they're parallel rather than perpendicular. And maybe that doesn't sound that hard. But what you have to keep in mind is that the Hilbert space corresponding to the environment is very, very big, very, very high dimensional, right? When we talk about the dimensionality of real world Hilbert spaces for big macroscopic things, you're thinking about numbers that look like 10 to the 10 to some number, okay? 10 to the 10 to the 50th, 10 to the 10 to the 80th, or something like that. Very, very, very high dimension.

• 2:50:31Sean Carroll?

  And what that means is a mathematical fact that in high dimensional vector spaces, any two randomly chosen vectors are incredibly likely to be perpendicular to each other to a very, very high degree of precision. And what is worse, the chances that those two vectors are going to line up and be parallel to each other is very, very, very, very small, you know, more small than you would ever worry about happening. So in the real world, can branches recombine? Yes, in principle. Would it ever actually happen? No, it really, really wouldn't. At least not until, as Wonder's question gets to, we get far, far, far into the future

• 2:51:11Sean Carroll?

  and you approach something like thermal equilibrium, where basically, there's not a lot of distinction between what is going on in one branch or another, and so, everything sort of blurs together in one big thermal equilibrium thing. So that is kind of the maximum entropy, heat death of the universe that we'll reach once every black hole has evaporated, et cetera. Then, there won't be any principled way to divide the wave function of the universe up into different worlds.

• 2:51:41Sean Carroll?

  Mick Norden says, "I worked as a computer programmer for many years, but I was trained as an archaeologist and worked as an archaeologist for the first seven years of my career. I know a lot of archaeologists who would desperately need help with writing programs for data analysis. Surely that is the same for a lot of physicists. Is there a way for people who are good in specific skills such as programming to get in touch with researchers who need help in those skills for their research?" Well, I think it's not quite the same for archaeologists and for physicists. A lot of physicists will learn computer programming, or at least some bit of it, along the way as an undergraduate, graduate student, postdoc. And so those particular physicists who do a kind of research where computer programming

• 2:52:25Sean Carroll?

  is needed, do it themselves, or get their grad students to do it, or their postdocs to do it, or whatever. There will often be people, depending on the situation, if you're an experimental physicist or you're part of a big collaboration or something like that, there absolutely will be people who are specially hired to do computer things, very often hardware things, but sometimes data analysis software things. In smaller areas where, you know, like in my area where it's just me and a grad student and a postdoc or whatever, you don't hire an extra person to do that, you just get it done. So for those big collaborations that do hire data analysts or IT people or whatever,

• 2:53:09Sean Carroll?

  no, I really don't know how to get in touch with them, I'm afraid. I mean, I think you can look at job postings, right? Different universities have job postings very often, and different fields have job postings. So like Physics Today or, you know, different online resources will tell you what departments are kind of hiring different kinds of people. The online ones, the Physics Today ones, et cetera, those are mostly for academic level, you know, postdocs and faculty members, but I think if you look at specific universities, they will generally try to post every available job opportunity. So you could check them out and maybe find one that fits what you're looking for.

• 2:53:48Sean Carroll?

  Pete Newton says, "I'm wondering why there is such controversy over the initial low entropy state of the Big Bang. It was, it was very hot and all the fields were probably unified. It wasn't until it cooled a bit and more degrees of freedom emerged that entropy could start increasing. Please clarify this point for me again." Well, there's two things going on here. One is, the most obvious thing, which is the answer to the question, which is that gravity matters in the early universe. Think of a box of gas, okay, the traditional thing to think of when you're talking about entropy and so forth. You think that a box of gas, if all the gas is squeezed over into one corner or something, that's low entropy. If it's spread uniformly throughout the box, that is high entropy. But in the box of gas you're probably envisioning, gravity among

• 2:54:33Sean Carroll?

  the particles in the box is not important. Maybe you're sitting on the Earth's surface and there's some gravitational field, there's a gradient of density or whatever, but forget about that. The mutual gravitational force in between the particles is irrelevant. It's there, but it's incredibly tiny, and in fact, quantitatively, there's pressure inside the gas in the box, and that pressure is much more important than the gravitational field. So, therefore, uniform, constant temperature is high entropy. But if the box in your thought experiment gets bigger and bigger, gravity becomes more and more important and pressure stays the same. So, James Jeans, long ago, worked out this fact, that there is something

• 2:55:16Sean Carroll?

  called the Jeans length, that basically the size of the box, given the temperature and pressure and so forth, at which gravity will always win. And then gravity starts making things inhomogeneous. It starts contracting things together. That's even true in Newtonian gravity. Of course, in general relativity, you can go all the way to make a black hole. And a black hole is going to be much higher entropy. A black hole surrounded by empty space is much higher entropy than a smooth distribution of particles or fields or whatever. In the early universe, you have a lot of gravity. It's very, very dense, okay? The gravitational field is very strong and the box is as big as you want it to be, in some sense.

• 2:55:57Sean Carroll?

  And so you can just run the numbers and show that the high entropy state would be one where you had a lot of lumpiness, black holes here, empty space there, etc. That is much higher entropy than what you actually have. So that's enough of an answer that is the right answer to the question. But there's another thing which is a little bit more subtle. When you say entropy could start increasing only when things cool down a bit. Well, that seems to imply, and it's not just you who think this, many people think this, but it's a little bit misguided because it seems to imply that someone from outside says it must have a certain temperature. What we're doing here is we're thinking about all the possible configurations of matter

• 2:56:42Sean Carroll?

  and energy in the universe at that time. One of the possible configurations is that everything is much more spread out and dilute and cool, right? The fact that general relativity says space and time can expand means that you can't just fix the size of the universe at any one time and say, "I insist that that be the state I'm looking at," if what you're doing is asking, "What is the highest entropy configuration?" The physical system that is the universe today is the same physical system that was the early universe. It's just in a different configuration. So you should ask yourself, "Why was the universe ever hot and dense, much less why it was smooth?" We don't know the answer to these questions, but this is part of asking why the early

• 2:57:27Sean Carroll?

  universe had low entropy. Mike VR says, "I'm studying systems that maintain identity through complete component turnover, like how ecosystems persist across species changes or institution across personnel changes. These systems seem to constrain their own components while emerging from them. How does poetic naturalism distinguish between statistical regularities and genuine emerging constraints that have real causal influence?" So I'm not sure that poetic naturalism has anything to do with this. I think this is just a question of self-organization and emergence, and I'm not quite sure how to answer it because you want me to distinguish between statistical... ...regularities and genuine emergent constraints that have real causal influence.

• 2:58:11Sean Carroll?

  I'm not exactly sure what counts as a genuine emergent constraint. In some sense, if, again, if you're Laplace's demon and you can just know everything that is out there, even things that appear to be higher-level self-organized things are really just the combined result of all sorts of tinier things doing their thing, behaving according to their dynamical laws. So I'm not completely sure there is such a distinction. I'm not quite sure what counts as a statistical regularity and a genuine emergent constraints. In other words, I think that it's important, when you talk about the emergent level and think about it in terms of the smaller things, to really pick a side.

• 2:58:53Sean Carroll?

  Are you talking about the microscopic dynamics? In which case, it's not really fair to then add in emergent constraints, just as we were talking about the Big Bang just a second ago. You can't have this plasma of early-universe stuff and say, "No, no, no, I only want to look at a very dense, smooth, early condition." You have to ask, "What are all the conditions it could be in?" Likewise, if you want to work at the higher level, at the emergent level, then you can do that, but then you just ask yourself whether there are higher-level emergent laws that give you an adequate description of the system. You don't need to worry about what the microscopic constituents are doing.

• 2:59:33Sean Carroll?

  Yasir Abdul Amir asks a priority question, but then asks actually several questions, which you're not allowed to do, asking several questions. So I'm gonna, I'm gonna try to like, take a couple of them that are on the same theme and answer those. "How much proof-based mathematics do I need to understand in order to understand theoretical physics? How rigorous does my mathematical understanding need to be? When it comes to calculus, is real analysis important if my eventual aim is to understand theoretical physics, or would a sort of plug-and-play calculus suffice?" This is a good question. You know, when, I remember when I was an undergraduate and I was an astronomy major, but I was taking all the physics courses that a physics major would take, and I also decided to take a real analysis.

• 3:00:17Sean Carroll?

  ...course in the math department. And for those of you who don't know, there's calculus as we teach it to scientists, but then there's the calculus as mathematicians, think about it, which is a higher level. It's not that you're better as a mathematician at doing complicated integrals or derivatives or anything like that. In fact, you're probably worse because instead of thinking about that stuff, how do you actually do the integral of some function, you're thinking about the rigor and how you can prove that certain limits exist and, you know, the topology of the real line becomes very important and you're developing techniques for proving things with epsilons and deltas and so forth. Roughly speaking, that stuff is not that useful to the working theoretical physicist,

• 3:01:02Sean Carroll?

  but I can't say that for sure because it depends precisely on what kind of theoretical physics you end up doing. I have mixed feelings about learning mathematics as a physicist. On the one hand, all else being equal, the more math you learn, the better you are. Okay, there's no doubt about that. But on the other hand, not all else is equal. Maybe when you're spending your time learning that math, you're not learning physics that you could learn. Like, maybe you're, you think that, ah, this E and M stuff is not really important, or statistical mechanics, how important could that be? Or whatever. And this remains true as you go past undergraduate to higher level things. So I think it's sort of an optional direction to do. I think it's going to depend on exactly what kind of physics you end up doing.

• 3:01:46Sean Carroll?

  Certainly, many, many physicists don't need much more out of calculus or even mathematics more generally than a bit of complex analysis, the ability to actually do integrals, especially contour integrals in the complex plane, a bunch of stuff about series and linear algebra, maybe a little group theory, right? It's actually, by the standards of super high-level mathematics, not that much math that you really need. You just need to be very facile at it, and however you get that is up to you.

• 3:02:18Sean Carroll?

  Erkan Sertali says, "When talking about measuring the spin of entangled particles, I understand that the axis at which we measure is very important. What I don't understand is how two people at different places can possibly agree on which axis points in which direction." Yeah, this is a very good point that comes up in discussions of curved spacetime and general relativity. When I have in curved spacetime and a set of axes in my laboratory, X, Y, and Z, and someone else has a laboratory far away with some curved spacetime in between us, there is no unique way to say that their axes are set up in the same way as mine.

• 3:02:57Sean Carroll?

  What there is a unique thing to do is to pick a path between my laboratory and theirs, and along that path, I can parallel transport the axes. And then I can ask, what would be the corresponding axes that I would set up by traveling along that path from my laboratory to theirs and bringing the axes along via parallel transport? The reason why that's sufficient for talking about these experiments is that you do indeed need to travel with a particle that has some spin from one laboratory to the other, and therefore, you are basically parallel transporting its spin vector.

• 3:03:39Sean Carroll?

  Now, actually, in practice, spacetime is pretty close to flat, so it doesn't even really matter for this kind of thing. Often, when you're hearing discussions about EPR, entangled particles, and stuff like that, we just ignore the fact that spacetime is curved because we're just doing a thought experiment. There isn't actually a guy named Bob living around Alpha Centauri who is measuring any spins. Lishan Aklog says, "I recently heard Rafael Buzo outline the cosmological many-worlds paradigm that he and Leonard Susskind proposed in 2011. While I still find the clean Everett-Plastik coherence picture more compelling, their reliance on causal diamonds made me wonder how an observer's light cone constrains non-relativistic quantum mechanics."

• 3:04:21Sean Carroll?

  Suppose I measure the position of a single free nonrelativistic electron, collapsing its wave function to something close to a position eigenstate. The wave function then evolves and is spreading wave packet according to the Schrodinger equation. Before I make a second measurement, say one second later, the packet's peak would lie along the classically expected path, but its tails would assign a tiny yet non-zero probability of detecting the electron more than a light second away, i.e. outside my light cone, and seemingly in conflict with special relativity's speed limit. What am I missing? Am I mishandling reference frames, spacetime calculations, or something else? Well, so you've sort of provocatively mentioned this wonderful idea of Raphael Bousso and Leonard Susskind,

• 3:05:04Sean Carroll?

  which also, there was a paper about the same time by Yasunori Nomura, both of whom talked about is the many worlds of quantum mechanics, are the many worlds of quantum mechanics the same as the cosmological multiverse? You will often hear people, including myself, say that they're very different ideas, right? Many worlds of quantum mechanics are, I measure a quantum system, I get different answers, and right here in the room, there are now two different rooms, one where I measured one outcome, one where I measured the other, whereas the cosmological multiverse refers to regions far, far away in space, where the local laws of physics might be very different. But if you think hard about quantum gravity,

• 3:05:46Sean Carroll?

  you might be able to convince yourself of the causal diamond view of how we should think about patches of the universe and what can be observed in them. Namely, if you think that there is a landscape, so you think that there is a possible cosmological multiverse with different metastable vacuum states where local laws of physics look different, then every individual vacuum state will have its own vacuum energy, its own cosmological constant, and that means it will have its own horizon size, because a positive cosmological constant implies a future cosmological horizon, including our cosmological constant that we have right now. So there's a horizon around us, or will be, of a few tens of billions of light years across.

• 3:06:31Sean Carroll?

  And that's all that you can see when you're inside. And there is a view about horizon complementarity, which is a little vague, but basically it says, from the point of view of the inside observer, the whole universe is effectively what they can see inside the horizon around them, and degrees of freedom that they consider to be on the horizon. It's very much like saying if you're outside a black hole, from your perspective, what exists in the universe is the spacetime outside the black hole, and degrees of freedom that are on the horizon of the black hole. This was the beginning of the holographic principle in black hole physics.

• 3:07:13Sean Carroll?

  And complementarity says if you fall into the horizon, it looks like nothing special happens and everything is still three-dimensional, but as long as you're outside, all the information is distributed around the horizon. So if you apply that to cosmology, it's kind of saying that there isn't anything outside the horizon you can see, much less a multiverse of many, many things going on, okay? So does this mean there is no such thing as a multiverse? No, because inside your bubble, you can have a quantum mechanical probability to undergo a phase transition from one vacuum to another. And in the Everreadyian language, this would be saying that the wave function of the universe

• 3:07:55Sean Carroll?

  right around your location is a superposition of different possible vacuum states. It's much like the quantum mechanical, the cosmological multiverse, except it's all right here in your backyard. It's saying that your wave function is a superposition of every possible vacuum state. There's a multiverse right here within a single observable patch. Of course, two different branches of the wave function, the size of the observable patch will look different, but the whole shebang is basically both an Everreadyian many worlds and a cosmological multiverse. Okay, so that's just a fun little discourse. I wrote a blog post about it at the time. It was one of those things, I was kind of sad I hadn't thought of it myself. It was a very good idea.

• 3:08:38Sean Carroll?

  We don't know enough about how to talk about horizons and the cosmological multiverse to make sure that it's correct, but you know, it's an intriguing possibility. Anyway, you're actually asking a question, Li Shang, about light cones and quantum mechanics, right? When you measure a particle, what does it sort of leak outside of its future light cone a little bit? And give you a tiny yet non-zero probability detecting the electron outside of its future light cone? So I think part of the issue is that you're trying to ask these questions in the context of non-relativistic quantum mechanics, but you can't. You actually have to use the relativistic quantum mechanics if you care about what happens outside the light cone.

• 3:09:23Sean Carroll?

  After all, if I do, if I use Newtonian gravity as an approximation here in the solar system, which I'm happy, which I'm allowed to do, it does naively predict that as the Earth moves around the Sun, there is a measurable change in the gravitational field infinitely far away, right away, right? Now, we know that's not true. It's just part, it's outside the approximation. Okay, you actually have to use general relativity to find out what happens. Likewise, here you have to use relativistic quantum field theory to figure out what happens. But what does happen is a little subtle, and I think that it's hard to really wrap our brains around until we make peace with it. There is a non-zero probability that the wave function of a particle, even in relativistic quantum mechanics, if it's a massive particle,

• 3:10:06Sean Carroll?

  there is a non-zero probability of quote-unquote "detecting it outside the future light cone." Why do I say quote-unquote? Well, because you're dealing with quantum fields, and when you detect something, you cannot uniquely say, "I detected this particular particle." The reason why we think that particles are identical particles is because they're really all vibrations in the same underlying quantum fields. And if you do the calculation of, okay, I measure a particle, and I watch its wave function spread out, there will be an exponentially suppressed, but not zero probability to detect it outside the future light cone. But it is so exponentially suppressed, it is so tiny, that you can't really distinguish detecting that electron

• 3:10:49Sean Carroll?

  from just measuring the vacuum and detecting an electron. As we were talking about before with the Unruh effect and the Hawking effect, it is a feature of detecting quantum fields that even in empty space, you have a non-zero chance of detecting something. I guess I didn't say this explicitly in the discussion of the Unruh effect, but if, even if you're not accelerating, okay? Even if you just have a detector in empty space that is just sitting there. The definition of the vacuum state is that there are no particles. So you might think, if I turn on my detector, I will never detect a particle. But when you turn on your detector and let it do some detecting for a while, you're never measuring the whole vacuum state all at once, because the whole vacuum state is infinite in extent, right?

• 3:11:32Sean Carroll?

  You're measuring some finite region of space and time. And what that means is you're not measuring the whole vacuum. You're measuring some little piece of it, and that is not guaranteed to have no particles in it. This is one of the weird things about quantum field theory. The whole universe has no particles, but if you confine an observation to a subsystem, you can see particles, right? This is just weird quantum mechanical stuff that you have to learn to deal with. So anyway, I mean, the short answer is you can't send signals. You cannot reliably translate information by measuring a particle and letting its wave function spread out. But if you run through the calculation and are super duper careful, there is a non-zero chance that you detect it. You're just not exactly sure what it is that you detected.

• 3:12:16Sean Carroll?

  Ryzard Cherminski says, "I started listening to your podcast a few months ago, and I really enjoy it. What is your take on modifying current Byzantine US tax system and moving away from an income-based system toward a Pigouvian-style taxes? In simple terms, do not tax what is beneficial, i.e. incomes. Instead, tax what should be minimized, i.e. energy and finite resource usage. As I see it, the benefits of this change would be too many to list here, one very obvious being radical tax collection simplification." Well, I think I haven't really thought about it enough to give an expert-level opinion, but my feeling is I would mostly be against it. I think that some Pigouvian-style taxes are appropriate in certain circumstances, sometimes called "sin taxes",

• 3:13:01Sean Carroll?

  because you're taxing people who are sinning in some moralistic framework. But it's tempting. It's tempting to say, like, let's figure out what are the bad things that people do and put taxes on those kind of expenditures, rather than just on earning money, which is supposed to be good. The problem is, number one, who decides what's good and what's bad? You know, some people might want to tax drinking wine. Some people might want to tax taking a vacation. Some people might want to tax betting on sports. And of course, all these are worth taxing, you can tax them. But people's feelings about how sinful and bad they are might be very different from each other. Some people are going to think that exploring for new oil fields is a good thing. Some people are going to think it's a bad thing.

• 3:13:44Sean Carroll?

  And it's certainly very subject to abuse, right? If you're under a government that has very different moral priorities than you do, and all of your taxes are based on that kind of thing, you might find yourself in trouble. I don't see why it would be radical tax collection simplification. I think that the level of simplification of a tax system is more or less independent of what kind of taxation it is. Because no matter what kind of taxes you have, you can always complicate them. You know, income taxes, in principle, are the simplest thing there is. You tell me your income, I read off a table what percentage of taxes you have to pay, right? The reason why they're complicated is because there are all sorts of other taxes and ways

• 3:14:28Sean Carroll?

  to get around the taxes, things that are deductible and exemptions and things like that, and you know, you can claim losses if you're a business. That's why it's all very, very complicated. Certainly, the US tax system is far more complicated than it needs to be. And it's almost a principle, it's a folk theorem, that the more complicated the tax system is, the easier it is for rich people to find loopholes and not pay their fair share. That's what often actually happens. So I'm all in favor of simplifying the tax system, but I think that, you know, taxing wealth and income are the most straightforward things to do, and maybe some occasional additional taxes on things we think are really bad.

• 3:15:09Sean Carroll?

  Mark Kumari says, "My question is on a comment you made at the end of the interview on Piers Morgan. You mentioned here and other places that time can potentially be infinite in both directions. If that is true, doesn't that lead to all sorts of problems, like everything that can happen happens an infinite number of times, recurrent universes, and the problem with Boltzmann brains?" Well, I mean, the short answer is, we don't know, okay? But it's certainly not necessary that all of those things happen. You know, so one thing is, it is not true that even if time goes forever, everything you can conceive of will happen. The example I like to use is, the set of all even numbers is infinitely big, and it extends

• 3:15:50Sean Carroll?

  both infinitely far into the bigger numbers and to the more negative numbers. And yet, you will never hit an odd number when you march up through the even numbers, even though there's an infinite number of them. There's no logical connection between an infinite number of things happening and everything happening, okay? In the context of cosmology, this means you have to be a little bit more specific about what actually does happen according to the laws of physics. And the very rough guide here is, are you imagining a theory of everything, if you like, where there's only a finite number of possible things that can happen? Or a theory where there's an infinite number of possible things that could potentially happen?

• 3:16:34Sean Carroll?

  If you have a finite number of things that can happen, and an infinite amount of time for them to happen, then roughly, yes, you do have a certain number of things that happen, and you cycle through them over and over again, and you would indeed have a Boltzmann brain problem. But if there's an infinite number of things that could happen, then even given an infinite amount of time, you might only sample some of them, and you might certainly not sample them repeatedly over and over again. There's no reason for that at all. So it comes down to building an appropriate cosmological model that avoids these problems you're mentioning. I'm going to group two questions together. They're both medium long, so hang in there. One is from Steve, who says, "It seems a philosophical concept, per Ludwig Wittgenstein

• 3:17:19Sean Carroll?

  and others, that language does not have any deeper significance. That words do not stand for something other or more profound, but can only be defined by how they are used, as opposed to the notion, per Plato, that words and language stand for some deeper essence, something beyond themselves. It seems that this idea is what is at the core of the language of AI. Words follow words in customary order due to their customary use. This leads to two conclusions. Number one, language may not be such a magical and intelligence-defining phenomenon separating mankind from the rest of the universe. And two, AI general intelligence may be closer than we think. Is this possible?" And then Armchair Pistemologist says, "To what degree do you think language is fundamental within our brain's internal representation of the world?

• 3:18:03Sean Carroll?

  Do you think that without language we'd not be able to have a full human-level intelligent representation of the world? Or is it more accurate that language is just a system for communicating these representations? The implication here being that if language is more fundamental than we might assume, maybe LLMs can come to some degree of a real model of the world, contained within their parameter weights somehow." So both questions are about, you know, how fundamental the idea of having a language is, and roughly asking, you know, "Once we have a language, isn't that halfway to being intelligent in some way?" I guess the first thing I would say is, I'm not quite sure what the threshold would be to say that you have a language, right?

• 3:18:44Sean Carroll?

  Certainly, other species of animals communicate with each other. They even communicate symbolically in some way. What they don't seem to do, and I'm not an expert here, but what they don't seem to do, as far as I know, is have sort of modularity and the kind of subject-verb grammar that we human beings have. So it's one thing to communicate symbolically, that is to say, like, birds can have different kinds of chirps that convey different meanings to other birds, even though they're not directly representing, they're not directly... Picturing what it is they're trying to say, but they just have different sounds that convey different meanings, okay? That's a step toward language, but it doesn't have the general purpose power that a true

• 3:19:28Sean Carroll?

  grammar can have. They don't have an infinite number of sentences that they can intelligibly make out of their chirps, again, as far as I understand. But I can imagine a continuum from just having some chirps to having a full-blown grammatical language, so I'm not exactly sure what counts as having a language in this context. As far as the LLMs are concerned, though, sure, even if you believe, and I'm not sure it's completely believable, but even if you believe that, well, let's back up a little bit. Think about what Wittgenstein is trying to say, okay? Where he says that words are, you know, the meaning of a word is its use, right?

• 3:20:09Sean Carroll?

  Rather than some abstract connection to some fact of the matter outside. You can't exclude the fact that there are facts of the matter outside. When you talk about the meaning of a word being its use, you can't just look at words and how frequently they appear next to each other, you have to look at the context in which they are used. When certain things happen, certain words are used to describe those things, right? You can't completely divorce yourself from the outside world, even if you think that the meaning of words is not found in one one-to-one representational map from those words to the world. And indeed, I think, you know, that latter point of view is very compelling, especially

• 3:20:49Sean Carroll?

  because lots of words are just sort of grammatical words that don't map on right to the world. Like, what does the word "the" map on to in the world, right? It's sentences, maybe, that map on to the world, and even those often are just sort of interacting with other sentences. But still, there's an interaction with the world at some point, okay? And that does really matter. So I think that both things can be true, that words get their meaning from use, but also what you mean by "use" has to include the context in which you are speaking those words. To the other issue of whether this means that AIs or large language models might be closer to being intelligent than maybe we thought, I think that it's absolutely crucial to remember

• 3:21:34Sean Carroll?

  that large language models are trained on the corpus of texts written by human beings who have interacted with the world, right? The fact that large language models can manipulate sentences and sound convincing is completely parasitic on the fact that they are trained from human beings who have been in the world and been interacting with it. And so it does not at all make me think that one fact all by itself provides to me essentially zero evidence that large language models are close to general intelligence. They're just remixing things that human beings have done. Of course, you might want to dig into the fact that, you know, once you have had enough training on enough things that human beings have done, does that count as general intelligence?

• 3:22:19Sean Carroll?

  That's worth thinking about, but just the fact that words are get meaning from their uses, I don't think nearly enough to get you there. David Donald Hawk, sorry Donald, says, "Are you using ChatGPT or other such AI tools? And if so, can you share your use case and experience?" I don't want to mix this up with the previous questions, but it follows naturally on them. Yeah, I use I use LLMs, AI things all the time. I think it's super duper useful. I think there's an obvious use case, which is the LLMs are sort of like a dynamic Wikipedia, right? Wikipedia is great because it's crowdsourced. It's not just from the mind of one person or the mind of some group of people.

• 3:23:03Sean Carroll?

  It's constantly growing and evolving, and you just look things up and get a lot of knowledge. I love Wikipedia and I've always defended it. It's not 100% reliable. It's much more reliable Wikipedia than it used to be, but it's still never going to be 100% reliable. So it kind of teaches you to be careful when you are using this particular resource, even though it's very useful. I think that LLMs are very, very similar to that. The great thing about LLMs, though, is they're trained on everything, and they can sort of Mix and match in ways that Wikipedia doesn't do because Wikipedia is basically just static. So maybe you're interested in, I don't know, what was um the view of Queen Elizabeth the first about the French?

• 3:23:46Sean Carroll?

  What did she think about the French? I have no idea whether she had any actual thoughts about the French, but the point is, I could go to Wikipedia and look at, you know, Queen Elizabeth's entry and and hope that there's some mention of that in there. But maybe there is, maybe there isn't. Wikipedia can't anticipate that that is precisely the question I want to ask. Whereas if I just ask ChatGPT, it can give me the answer. And the reliability of that answer is very, very closely correlated with how common this set of thoughts is within the corpus on which the LLM has been trained. So if you're not trying to trick it and you're trying to talk about something, you're asking about things that other people

• 3:24:28Sean Carroll?

  certainly know and have talked about, then the reliability is relatively high. At the edges of knowledge, you know, if you're doing research as a as an academic, pushing things beyond what people have frequently talked about before, that is precisely where the LLMs become much less reliable and you have to actually use your brains. Okay? So I use ChatGPT for um learning about things that other people know very well. You know, learning new math things or physics things that I might have forgotten. It's useful for things like what is the variable that we use to denote this quantity? You know, when you're writing a book, that's a super useful thing to do. Like, I remember that resistivity has a letter associated with it, but I forget what it is.

• 3:25:14Sean Carroll?

  ChatGPT knows that right away. It's easy to find. Not to mention, you know, sort of more elaborate versions of this remixing ability. You can say, you know, here are the ingredients I have in my refrigerator. What is a good meal I can cook out of them? And you know, Wikipedia will never tell you that. But again, it's if if a meal like that is not necessarily directly in the corpus of training data, but at least things like it are in there and then the LLMs don't have to extrapolate too much. That's when it's on firm ground. That's super duper useful. James asks a hypothetical question. There is a gigantic existential geopolitical threat to the free world, something on the order of World War II.

• 3:25:56Sean Carroll?

  If you got drafted, maybe you'd go fight as a grunt, or recruited to help work on a modern Manhattan project. But would you participate in a large physics disinformation campaign, well-funded, lots of professors, with the goal of making our enemies misunderstand important experiments and theories, and waste time, energy, and money? The plan would aim to really muck up the academic pursuit of knowledge. It would leverage your trustworthiness as an academic and public intellectual, and exploit science as a social form. So I think there's a couple of things going on in this question. It's an intriguing question, it's a good one to ask. One thing is, I can't actually imagine the scenario, right? I can't actually imagine a scenario where I, as a scientist, could try to mislead the

• 3:26:41Sean Carroll?

  enemy by making incorrect statements about science. I mean, if the enemy, whoever they are, is worth their salt, they're gonna do their own physics experiments, and they're gonna try to learn things as much as they can. They're not gonna be fooled by propaganda from the other side, trying to tell them that E equals mc cubed, or whatever, right? So I don't quite imagine that there is a scenario in which this particular dilemma becomes real. Of course, you can nevertheless say that thought experiments like this are useful for stress-testing our beliefs, right? How much would you be willing to sacrifice your kind of honor and devotion to the truth

• 3:27:23Sean Carroll?

  and things like that, for the case of winning this battle in this hypothetical existential threat? In principle, yes, I see the worth of this kind of thought experiment. In practice, I think that they're... they do as much harm as good. Because... You think that you are stress testing your beliefs under, you know, extreme conditions and that might help you clarify what those beliefs are. But the fact is, it is nearly impossible to realistically assess what the conditions would be. There's a lot of variables that are left out of the statement of the problem that really, really do matter, right?

• 3:28:04Sean Carroll?

  I mean, there are questions like this of the form, if you knew for a fact that there was an atomic bomb that was going to go off and destroy New York City and only one person knew it, would you be willing to torture them to get the information about how to defuse the bomb? So that's just assuming a whole bunch of things which I don't think are true. Like, how do you know for sure that there is a bomb under New York City? Do you really think that torturing them would increase the chances of getting a correct answer from them, which I'm skeptical about? So, I see the temptation to lean into these thought experiments, but I'm also very, very skeptical that we should take them too seriously. Schleyer asks a priority question.

• 3:28:48Sean Carroll?

  It's puzzling that humanity as a whole is acting in ways most humans wouldn't want: ecocide, genocide, gross inequality, et cetera. I've started to view our whole civilization as a teleological organism with a goal of growth and with us as its components. While resources were plentiful, the best growth strategy was to provide everyone access to resources. But once resources grow scarce, the best growth strategy is to concentrate power in fewer and fewer individuals who benefit from growth while disempowering everything else. This is the stage we are at today: growing inequality and authoritarianism. Eventually, resources will dwindle and the organism will die and maybe something else will emerge if anyone is left. Does this resonate with you at all? Um, not really, no, I don't, I don't think it does.

• 3:29:31Sean Carroll?

  I mean, a little bit of resonance, because, you know, we're trying to understand why the world is headed in a certain direction and I'm all in favor of thinking through different hypotheses. But there's a couple of things that, that don't quite convince me. You know, one is, if you think about, um, you know, gross inequality, genocide, ecocide,

• 3:29:52Sean Carroll?

  Would we have been equally worried about this 25 or 50 years ago? You know, that's a very short time scale historically, but there were people who thought just a short time ago that we were getting rid of all those things, right? That history was going to end and we're going to just be happy liberal democracies from now on. So these short-term fluctuations are things I'm a little bit reluctant to read too much into. A second thing is, as you say, Schleier, the whole world is acting in ways most people wouldn't want. Well, I'm not sure what most people would want. I don't think that most people's wants are indeed very coherent. Like, maybe they don't want ecocide or gross inequality, but maybe they do want cheap

• 3:30:36Sean Carroll?

  gas and maybe they do want, you know, the ability to get rich quick through some scheme, even though those sort of go hand in hand with ecocide and gross inequality or whatever. So I'm not sure that there is such a thing as what humans would want in any coherent way. Or at least I think that you should think much more carefully about what that might mean. Finally, I think it's very wrong-headed to assign any teleological notions to the global civilization. You know, I think that on the one hand, people around the world are very different from each other, societies around the world are very different from each other, they have different values, different goals.

• 3:31:16Sean Carroll?

  And also, this teleological thing is, it seems to me as wrong as saying that gas in a box has a teleological goal of going to its highest entropy state. The gas does go to its highest entropy state through the random motions of the molecules inside. As we talked about in the intro, talking about the wire in Baltimore, the system is made of people, you know? And people are going to do what they do, given the constraints and the opportunities they have, and then there is some collective behavior that comes out of all of that. It's a little bit too anthropomorphic to assign goals to the system as a whole.

• 3:32:01Sean Carroll?

  Sometimes that can be a useful ...way of talking, and maybe you can establish that in a particular case, that's a useful way of talking. I'm just skeptical. I'm resistant to talking that way unless you really, really establish that that is the best thing to do. I don't think that the global civilization is thoughtful enough to have a goal of something like growth or something like that. I can easily see why individual parts of the global civilization would be growth oriented in practice, because individual people within the societies want certain things and they're driven to do that. But they can also, in practice, decide

• 3:32:39Sean Carroll?

  to step back a little bit, right? Sometimes they do. So I think that it's a little bit of a cop out to assign that level of agency to civilization as a whole. OA asks a question with minor spoilers from the TV series The Expanse, and likely the books, although I haven't read them. OA hasn't read them. I've actually read them, so I'll ask you, I'll answer this question from a rare position of knowledge about literary work. "In The Expanse, a sufficiently advanced alien race disables nuclear fusion, causing issues for the characters whose ships use fusion-powered reactions for thrust. Putting

• 3:33:21Sean Carroll?

  aside the realism, would such a no-fusion field have any effect on anything at the planetary scale, such as for living creatures?" Short answer is no. The only effect it could have is if you aimed it at the sun. If you didn't allow nuclear fusion in the sun, that would be very bad. That indeed would have dramatic effects. But here on Earth, there's no nuclear fusion going on to a very good approximation. There might be some extremely rare things happening that are almost completely ignorable, but there's no part of biology or physics here on Earth that reaches high enough energies for fusion to happen. As you can probably tell from the fact that it's really, really difficult for us to build fusion reactors, you know, I still hold out

• 3:34:03Sean Carroll?

  hope that maybe that will happen, but that electrostatic repulsion of two protons makes it very, very hard to aim two protons at each other and have them fuse together.

• 3:34:16Sean Carroll?

  Jeffrey Segal says, "I enjoyed the talk with Brian Van Norden. The discussion of language and meaning of life is particularly stimulating. As a biologist, I believe that any analysis of the meaning of life should account for the role of natural selection in shaping both our species and all forms of life. Natural selection tends to favor organisms that efficiently detect and exploit salient features of their environment, essentially those that can identify emergent environmental properties relevant to fitness. From this perspective, the search for meaning might be understood as the search for environmental regularities that inform adaptive decision-making and enhance survival and reproductive success. In that sense, one could argue that the meaning of life resides in the emergent properties that exert selective pressure on organisms.

• 3:34:59Sean Carroll?

  Admittedly, this interpretation lacks emotional resonance, but it may offer a biologically-grounded framework for thinking about the meaning of life. I'd welcome your perspective on this view." Yeah, I think that this perspective is not the one that I would share. I think that it is mixing together descriptive and prescriptive ways of talking about the world. When you talk about selection pressures and evolution and how that has guided organisms to have certain goals and certain ways of behaving, that's a description. That tells you what is happening here on Earth. But meaning in life, or mattering, or morality and ethics, all of these things are prescriptive. They are a different level of description.

• 3:35:42Sean Carroll?

  They're saying, "Okay, maybe evolution has guided me to want certain things. Should I want those things? Can I use my capacity for cognition and reflection to say, 'Yeah, you know, I'm sort of pushed in the direction of acting this way, but I'm going to think about it and decide that acting that way is not the right thing to do, okay? Or there's another way of acting that evolution doesn't want me to do, but I want to do it anyway because I think it's the right thing.'" Now, as I said before, I don't think that there are any objective moral standards out there that the universe gives us. Likewise, I don't think that there is any objective thing called the meaning of life or what it means for a life to matter.

• 3:36:25Sean Carroll?

  These are subjective things that human beings construct. But this temptation to go from what is to what ought to be is not only something that David Hume warned us about, it's something that the philosopher G. E. Moore called the naturalistic fallacy, to imagine that because something happens in nature, that it is the right thing to happen. I think that it's sort of intellectually okay to completely deny that there is any such thing as quote unquote "the right thing to happen" or "the meaning of life." That would at least, I don't agree with that perspective, but I think it's a consistent, defensible perspective.

• 3:37:05Sean Carroll?

  What I don't think you can do is just say, "Well, evolution has given me these particular capacities for decision-making that enhance my survival and reproductive success, and therefore, that is how I'm going to find the meaning of life." If you choose to find a meaning of life and things like that, that's fine, but there's no logical necessity that that is how you should define meaning of life. There's a little bit more freedom in that for you individually. Devon Proctor asks a priority question. "I've heard you articulate the Laplacian-Newtonian worldview as being that physics gives you rules to evolve a precisely-defined system-state deterministically forward or backward in time. The determinism of the forward evolution is intuitive to me, but I don't have the

• 3:37:50Sean Carroll?

  same intuition for why that necessarily holds backward in time. What are the conditions that preclude useful physics from permitting non-injective forward evolution rules? Is there anything that would be interestingly different in such a world with respect to entropy, arrow time, etc., resulting from this sort of time asymmetry?" Yeah, I think that things would be interestingly different. Someone like Tim Maudlin would actually be sympathetic to a point of view like that. But his view is very, very, very tiny minority among people who thought about these things. For the simple reason that doesn't seem as a matter of fact to be how the laws of physics work. There's nothing at all stopping me from imagining alternative laws of physics where I can go

• 3:38:33Sean Carroll?

  from the present state and evolve it forward in time, but I can't go backwards. Irreversible laws of physics. Indeed, that's what we have in the macroscopic level of the world where entropy increases. But at the microscopic fundamental level, all of the evidence that we have is in favor of completely reversible laws of physics. Stephen Wolfram is another one who thinks that maybe the fundamental laws of physics are not reversible, and therefore maybe that helps us understand the arrow of time and things like that. But here's the thing. It's very tempting. I get the temptation to say that maybe there is something built into the fundamental laws of physics that picks out a direction of time, okay?

• 3:39:14Sean Carroll?

  But ask yourself, what would be the case if that were not true, okay? What if you had completely reversible fundamental laws that did not have an arrow of time? What would you need in order to have an arrow of time in the macroscopic visible world? The answer is you basically need two things. One is you need some kind of coarse graining. That is to say, you need some fact of the matter that what we can observe about the universe directly as agents within it is not the microscopic state that suffices to be determining what happens both in the future and the past. And secondly, you need a boundary condition at one end of time that entropy was low, okay?

• 3:39:58Sean Carroll?

  Then, given both of those things, even with totally reversible fundamental physics, you will get an arrow of time. Guess what? Both of those conditions exist. They are correct. So there's no more, there's no empirical reason why you need to think harder about where the arrow of time comes from. We know where it comes from, or at least we have a absolutely plausible picture of how we have an arrow of time, even though the fundamental laws of physics are reversible. And they seem to be, like I said, that seems to be what the best models of physics are telling us. So unless you have a complete overhaul of the

• 3:40:39Sean Carroll?

  Schrodinger equation and quantum mechanics and stuff like that, it's a much simpler view to have that it's really the boundary conditions and the coarse graining that give us time directionality, not the fundamental laws of physics. Jonathan Soraco says, "How do you think philosophically about your sports fandom? When people say that they totally switched their favorite team in a league, I find it impossible to relate to. If I've ever tried to do it, my real allegiance would come back whenever my new team faced my old team and the facade would be gone. Have you ever had a favorite NBA team other than the 76ers? If not, do you think you could ever switch?" I think this is a good question because there's absolutely a certain amount of arbitrariness

• 3:41:19Sean Carroll?

  about how one becomes invested in a certain sports team to root for, usually having to do with accidents of geography and history about where you're born. Although, very often, people, when they're young, there's some team that was doing really well and they started following them, even though they're not geographically where they are. You know, if you grew up in the early 90s, no matter where you lived, there was a non-zero chance you'd be a fan of the Chicago Bulls because Michael Jordan was there and romping through the league. Likewise, if you grew up in the 80s, it might have been the Celtics or the Lakers or whatever. But there is absolutely arbitrariness about it. It's not like there

• 3:41:58Sean Carroll?

  is some metaphysically correct team to root for. And I do think, one thing I think, which not every sports fan agrees with me about this, but I think it matters that the people on the team that you're rooting for are good people to root for, you know? I think that I would not root for the 76ers if they started having no people on there but real jerks in one way or another. And indeed, this comes up in a very tangible way when you think about trading away fan favorite players, right? You can take this sort of bloodless perspective on managing a sports team that says, "We

• 3:42:39Sean Carroll?

  just want to win games, so it doesn't matter that someone, you know, has given some player who's been on our team for 15 years and is beloved by the fans, if we can trade them away and get some slightly better player for it, we should do it," right? I actually don't think that. I think that there is value in getting some kind of connection to the human beings that are the athletes playing on these teams and rooting for them. So I could imagine in principle situations where I switched my allegiances, but in practice, I don't think it's going to happen. The other thing, of course, is that I do temporarily root for other teams if they're not coming right up against the 76ers, exactly because they seem plucky and root-forable.

• 3:43:23Sean Carroll?

  I thought that the recent NBA Finals between the Indiana Pacers and the Oklahoma City Thunder was a great example of two teams that are both quite charming in their own way. I was happy to root for both of them. Slight preference, I was rooting for Indiana just because they're underdogs and one of their heroes was TJ McConnell, who got his start with the 76ers and is a great player, easy to root for, but Oklahoma City also easy to root for. So it's not like, well, let's put it this way. It might be that three years from now, if OKC wins three championships in a row and starts getting a little arrogant about it, they will turn heel and people will start rooting against them and hope that someone overthrows them. But for right now, it's their first championship, the first championship for Oklahoma City

• 3:44:07Sean Carroll?

  as a city in, I don't know, forever in terms of professional sports leagues. So I was totally happy that they won.

• 3:44:17Sean Carroll?

  Scipio says, "My girlfriend's family all voted for Trump, but they are lovely and kind people in all the interactions I've had with them. The same is true for several of my neighbors. As Trump's presidency continues to become, in my opinion, more and more disastrous, I am having a harder time understanding seemingly good, kind, well-meaning people who voted for and support this administration. How do you, in a philosophical sense, square this seeming contradiction?" I don't think it's that hard to square the seeming contradiction. You know, I have plenty of people who I know who are very good people who vote for Donald Trump. And why do they do that, despite the fact that he is definitely doing bad things, which I think is objectively, obviously, true?

• 3:45:00Sean Carroll?

  Well, you know, one thing is, people don't always see everything that politicians do. Especially in this particular information ecosystem we have right now, it's become very, very easy to only get a certain perspective on what is going on in the world. And that perspective can be very flattering to the people in power, if that's what the channels and news sources that you follow want to give you, or very negative about them, if that's what they want to give you too. So, they might not know exactly how bad things are in certain places. The other thing is, forget about politics, or information, or news, or anything like that, people are complicated.

• 3:45:42Sean Carroll?

  People can be simultaneously very personally kind, and pleasant, and charming, and in favor of really terrible things in the world. So maybe they do know about the terrible things that are going on and support them. That is entirely plausible. I don't want to say consistent, because it's kind of inconsistent. It's kind of an example of not living the values that you would profess, but it absolutely happens all the time. And there's different meanings to what it means to be good, kind, and well-meaning, right? You know, if you're charming with people you know and are having over to dinner, but you're in favor of throwing other people who you don't know into concentration camps

• 3:46:26Sean Carroll?

  and deporting them without due process, then how well-meaning and kind are you really? You know, I think that all of those things count. The answer is, you are well-meaning and kind to a certain extent, but not to another extent. Those are both possible at the same time. People are complicated, and that's never going to go away. Christian, I'm sorry, Christian, your last name is beyond me to pronounce. I'm gonna try it. Svitoljansky, but I'm, I think that those are the letters, but I'm not sure that I'm attaching the right pronunciations to them. Sorry about that. Says, "Light can be bent by the gravity of a black hole, so it travels in a straight line through curved spacetime. Does Earth go around the sun in a straight line through curved spacetime also?

• 3:47:11Sean Carroll?

  Surely not, because if I would shine a flashlight, it would not follow the same path as the Earth. Please help me understand the difference between massless and massive objects in such a scenario." Well, a tiny edit, I would say that it is not true that the light going around a black hole travels in a straight line, or that the Earth travels in a straight line. They travel on geodesics, which can be thought of as trying their best to move on a straight line, but they don't move on a straight line, because there are no straight lines in the curved geometry of spacetime. On the surface of a globe, a classic example, you try your best to move on a straight line, you take a shortest distance path, but you end up moving in a great circle around the

• 3:47:55Sean Carroll?

  equator, or a line of longitude, or whatever. So, it's not exactly the same, so your intuition is a little bit different. But more importantly, to the question of, so, yes, the Earth does go in a geodesic around the sun, so it's doing its best, just like the beam of light is. The fact that this is not a contradiction to if you shine a flashlight, it would not follow the same path as the Earth, is just because you have different initial conditions, right? You have, you're pointing your straight line in different directions. I can have two straight lines, that are not the same straight line, even though they start at the same point, right? They can intersect each other at a point, but move off in different directions. Something that is moving at the speed of light, and something that is moving slower

• 3:48:37Sean Carroll?

  than the speed of light, are necessarily moving in different directions in spacetime. So it is completely unsurprising that they follow different paths, even though they're both following geodesics.

• 3:48:50Sean Carroll?

  P value skeptic says, I heard you mention a story about a PhD student who used a large language model to generate fake data for a paper that supported the conclusion that AI boosts productivity. What is your opinion about the open science movement and data sleuths trying to expose fraudulent academic research? Have you ever discovered fraudulent academic research yourself? I'm all in favor of the open science movement and data sleuths trying to expose fraudulent academic research. It's a little strange to me because in my specific subfield, number one, as a theorist, I'm not collecting data, so it is much, much harder to even imagine faking a paper, right? How do you have fraudulent ideas?

• 3:49:34Sean Carroll?

  You can have fraudulent data much more straightforwardly than you can have fraudulent ideas. And number two, my field is sufficiently small that, you know, people read each other's papers and think about them. And it's not just, you don't rate people on how many papers they've had, but also if they're good, you know? And I kind of think that that's what science should be about, not just quantifying how many publications you have, but actually reading them and seeing if they're any good. And therefore, there's not a lot of incentive to just crank out papers that are fraudulent in my world. I recognize that my world is not the whole world, especially, you know, if you're a faculty, if you're a physics department at a smaller university that doesn't have enough

• 3:50:19Sean Carroll?

  faculty members to do every specialty, and you're trying to hire someone in an area that none of the current faculty members are experts in, which is a good thing, I'm very much in favor of that, you might have a tougher time judging whether or not a certain person is good or not, and maybe the sort of more quantitative but less judgmental criteria come into play. So there absolutely is an issue here. For me, it's just very weird to be so...

• 3:50:49Sean Carroll?

  Invested in trying to make a splash in science that you fake your data. Because the thing about data in science is that someone else is gonna do it. It's gonna be reproduced, right? If you say, "Oh, look, I have a new room temperature superconductor," as an example that came up not too long ago, it's not gonna give you any credit unless someone else can make it. So if you have just faked it, that someone else is gonna try to do it and fail, and then you look much, much worse than you would when you started. So the rewards of faking data seem to me to be very, very short term in nature, and downside seems to be very, very long term and permanent and bad.

• 3:51:31Sean Carroll?

  So I'm not quite sure what is going through the minds of people who go this way. Having said that, have I ever discovered fraudulent academic research myself? Not directly fraudulent research. I have found on one or two occasions paragraphs in other papers that seemed eerily familiar to me because I realized that I had written them, and they previously appeared in my papers, and people just cut and pasted them into their papers. And I think that that's bad. Usually, like, it's a student, like, a PhD thesis, and they're just trying to pad things out, and so they can cut and paste. Everyone has the LaTeX file for everyone's papers now, so it's not that hard to do.

• 3:52:12Sean Carroll?

  I recently found a paper, I mentioned it on Blue Sky. It looked like a good paper, at least a relevant paper to me. It was about, you know, something like about Boltzmann brains and finding yourself in the multiverse and things like that. So I started reading it, and it didn't quite make sense. And then I said, so I'm looking at, like, who wrote this paper, anyway? And I didn't recognize the name of the author, and there wasn't any affiliation listed for the author. I Googled the author's name, and there was nothing. And then I became suspicious. I'm like, wait a minute, you know, there's not that many people who care about these issues. I at least either know most of them, or could locate their institution, usually.

• 3:52:56Sean Carroll?

  They did, because the paper was on the archive, have An email address, and so I could look up the email address, and it was a Korean AI company. So I'm basically 100% sure now that that paper was written by AI and put on the archive, and it was just plausible enough that, you know, I don't blame the archive moderators for not flagging it. It, you know, seemed pretty innocuous. The there, the archive is someplace where you're supposed to be able to upload your papers more or less without too much of a barrier to entry, but they do ask that there's there's some barrier to entry. If the paper is obviously fake or obviously crackpotty, they will bounce it, but there's

• 3:53:40Sean Carroll?

  just too many papers for them to read them all carefully. And basically, they rely on the idea that if the paper is bad, it will just fall into oblivion. Okay, there's not a lot of harm done by a bad paper appearing on the archive. But the weird thing about this paper was it was clearly written by AI, but I don't know why. Like, I don't know what the benefit is. I presume someone is doing some experiment, right? Someone is wondering whether or not, obviously, AI papers can get attention on the archive and things like that. But the paper wasn't even that interesting, you know? It wasn't like a dramatic new result, so I'm not quite sure what the motivation there was. Who knows? I, I, I think that we're gonna see a lot more of things like that down the pike.

• 3:54:22Sean Carroll?

  Paul Gauthier says, "In the May 2023 AMA, Trevor Morrissey asked if you would consider running a quantum universe splitter bracket to pick one more question to answer. That way, every question would get answered in at least one branch of the multiverse." Here's a link to all the questions submitted in this AMA, sorted based on true quantum randomness, so every permutation of the list will exist in some branch. You can simply find the first question you weren't planning to answer and then answer it too. This will ensure that you answer every question in some branch of the Everreadyan multiverse. Okay, I have done this, and if I understand correctly, I think that I did answer the first question listed in Boles GitHub list, and I did not answer the second one,

• 3:55:07Sean Carroll?

  which I will try to do now. It's from Ben Lloyd, and it says, let me read it correctly. "I'm a layman, and I'm very puzzled about this Boltzmann brain situation. I think I heard you say recently that models which predict Boltzmann brains to be the typical observers are generally somewhat unappealing to physicists. Is this true? Also, does eternal inflation or some other self-reproducing theory like your baby universe one help solve this problem, compared to if there is no eternal inflation or reproducibility? If so, how?" So, the answer is that it depends a lot, as we mentioned a little bit before, on whether everything happens, and there's only a finite number of things that can happen, in which case Boltzmann brains are almost certainly going to be part of those, and those would

• 3:55:49Sean Carroll?

  be bad. But you can avoid them, or at least avoid them dominating, if you live in a universe that goes on, either a universe that doesn't go on forever, or a universe that goes on forever but in which not everything happens, because an infinite number of things could imaginably happen, and you only pick out a subset of them. Remember, you can have an infinitely big subset of an infinitely big set that is still sort of of measure zero, right? I mean, the integers are infinitely big, and the real numbers are infinitely big, and the integers are a subset of them, but it's sort of infinitely small subset. So if you have a cosmology like eternal inflation, for example, that under the right circumstances

• 3:56:31Sean Carroll?

  will keep going forever, but not just repeat itself over and over again, then at least you have the possibility that most observers who arise in that universe are what we call ordinary observers, ones who live in the aftermath of a nice, warm, hot, big bang beginning, as opposed to being freak observers or Boltzmann brains. So if you are a Boltzmann brain, I'm not going to repeat the whole thing, but I wrote a paper on why Boltzmann brains are bad. If you are in a cosmology dominated by Boltzmann brains, you can't say anything at all, because you have no reliable knowledge about the universe whatsoever. Therefore, scientists like to ask, can we come up with theories where Boltzmann

• 3:57:14Sean Carroll?

  brains do not dominate, and that also fit the data? That is what people like me are trying to do. So I hope that worked. I hope I played by Paul's rules correctly. I found the first question I wasn't planning to answer, then I answered it. So in this universe, that one got answered, but somewhere else in the branches of the ever-ready multiverse, different questions got answered. If it's all kosher, I'm not exactly sure. Stuart Hayne says, "It is often stated that the universe is homogeneous and isotropic. I understand that homogeneous means the same throughout and that isotropic may mean the same in all directions, looking out from a point. For example, something with various layers, like an onion, may be isotropic but not homogeneous. However, something that is homogeneous must inherently be isotropic."

• 3:57:58Sean Carroll?

  Am I missing something? Yes, yeah, it is not true that something can be homogeneous, sorry, that something that is homogeneous must inherently be isotropic. You can have situations where there is, for example, an electric field filling the universe, and the electric field points in the same direction, okay, at every point in the universe, and that picks out a direction. At every point, you can say, "Am I pointing in the direction of the electric field or perpendicular to it?" or whatever. But as long as it's the same electric field everywhere, it would still count as homogeneous, but not isotropic. Indeed, I wrote a paper with Mark Wise and Lottie Ackerman some years ago on what would

• 3:58:40Sean Carroll?

  happen if inflation were not isotropic. So you can imagine inflation happening at one rate in one direction of space and a different rate in the two perpendicular directions, and that would still give you a homogeneous universe at the end of it, but it would not be isotropic. And so we made predictions for the cosmic microwave background in that scenario. People have tested them, no evidence that that's how the universe actually is. Big Tuna says, "If we compare how much matter was present in the early universe compared to how much we can observe now, do we have a rough figure for how much of the matter in the universe is already lost to our observation forever now due to expansion?" I tried to ask ChatGPT, it gave a long, complicated answer. I didn't really understand.

• 3:59:23Sean Carroll?

  But it seemed to be saying that most of the matter in the universe is already expanded beyond our cosmological horizon. I think you have to be very careful about this kind of question and understanding what it means to be lost beyond our cosmological horizon. Something that, you know, are you thinking about a particle that exists forever? Or are you thinking about an event that is in spacetime? If you're thinking about an event in spacetime, you don't need to worry about our cosmological horizon. An event, which is something that has a location in both time and space, it's not extended through time, it just has a moment of time. Everything that is further away outside our light cone, even if it's just, you know, at the orbit of Neptune or whatever,

• 4:00:06Sean Carroll?

  that is undetectable by us now because it's further away than we can get to by the speed of light. So that kind of stuff includes almost all of the matter in the universe right now. But I think that what you mean is imagining that particles exist forever, right? That they persist over time, and then we're asking, are there world lines detectable by us? So again, there's still two things to distinguish between. One is, can you see evidence of the world lines of these particles by looking into the past? And then, the amount of stuff that you can see in the universe only grows with time.

• 4:00:50Sean Carroll?

  It never shrinks, right? Because as we get older and older in the universe, our past light cone gets bigger and bigger. And every world line of a particle that used to be intersecting our past light cone still is intersecting our past light cone in the future, okay? So nothing is ever lost in that sense. When we're talking about being lost in the cosmological horizon, it's the second sense that we actually mean, which is that are there particles who, right now, we could decide, can we go chase them and meet up with them? And there are particles that have the property that if we had decided earlier, we could go meet up with them.

• 4:01:33Sean Carroll?

  But by now, the universe has expanded and they're further away and the expansion rate is accelerating, so now we can't reach them anymore. Okay? That is what most people mean by sort of being lost outside our cosmological horizon. Very, very little of the matter in the universe that is observable to us now is in that category. Okay? We haven't lost that much because expansion has just started. But, uh, sorry, accelerated expansion has just started relatively recently cosmologically speaking. As time goes on, that's going to be a bigger and bigger fraction. But these are all cosmological timescales. I wouldn't worry about it as a personal matter. Thomas Henry says, "If all motion is relative, in what meaningful sense can we say that

• 4:02:17Sean Carroll?

  a ball is spinning in an otherwise empty universe? And what's the difference between a spinning ball in the universe and the universe spinning about a static ball?" So it's not true that all motion is relative. It's, that's sort of a paraphrase that maybe gives you an idea of what's going on in relativity, but it doesn't give you the correct idea or the whole idea. The correct thing to say in relativity is there are no preferred locations in space and there are no preferred velocities in space. There's no rest frame with respect to which a particle can move, so you can say its absolute velocity is a certain thing. There is something that is not relative, such as acceleration.

• 4:03:01Sean Carroll?

  There is a standard of acceleration. You can tell the difference between not accelerating and accelerating, right? You can feel that if you're in a rocket ship. It's something you can measure, unlike your velocity. If you're, if you're sealed inside a rocket ship, you can't measure your velocity with respect to the universe, but you can absolutely measure your acceleration. And the thing about a spinning ball is, it feels acceleration, right? There is the motion of the ball at the edge as it's moving in a circle. It's not moving in a straight line, right? In the spinning ball, so it has acceleration and it can feel it. Therefore, it's just an immediate consequence of how relativity works. That spin or rotation is not relative.

• 4:03:44Sean Carroll?

  There is something called not spinning and there is something called spinning. So, does it matter if you want to call the ball spinning or the universe spinning? That so much doesn't matter, but the fact that they are spinning relative to each other is an objectively true fact. Final question, we're gonna have a lighthearted final question today. Roland Weber says, "How did Ariel and Caliban respond to the departure of Puck? Have they reclaimed the territory and attention that was previously dedicated to the house guest?" They basically have, yeah. I mean, it's an interesting thing. You know, Puck had a room in the house. And Ariel and Caliban basically didn't even want to go onto the floor of the house that contained that room, even if Puck was behind the door.

• 4:04:28Sean Carroll?

  Like, they didn't, they didn't want to deal with it. And they didn't, I don't think that they, you know, made a lot of fuss about it while he was here. Caliban has been on something of a hunger strike since Puck came. He's lost some weight, so we're trying to like, get him to eat more. But, I mean, maybe that has something to do with Puck, maybe it doesn't. Anyway, Puck's been elsewhere for a couple of months now, so we're returning to normal. And indeed, the room where Puck was in, you know, we cleaned it out, did our best to remove any traces of the intruder kitty, so that Ariel and Caliban can reclaim it. And they more or less have. By design, it was not a room where they spent a lot of time in, anyway. So, they don't spend a lot of time in there. But if Jennifer and I are here in this room, and...

• 4:05:13Sean Carroll?

  Ariel and Caliban want attention because they want attention all the time. They will by now be perfectly happy to come in. It took a little while, took a few days. Like Caliban was there a couple of days later. For Ariel, she's just happy in her spots. So she doesn't feel the need to explore in the same way that Caliban does. But eventually, you know, she does feel the need to get Pat's in attention. So if both of us are up here, she will definitely join us these days. So we're returning to normal here in Baltimore at the domicile of Ariel and Caliban. And they're very happy for that. We miss the Puckster. He was a very, very charming cat, but I think that he's much, much happier now to have a human who can pay attention to him all the time, which is what, which is what he wants.

• 4:05:58Sean Carroll?

  We never thought that like the stray cat we picked up off of the mean streets of Baltimore would be so devoted to, you know, being glomming on to a human being and wanting attention all the time. But that was Puck. And so we're glad that he were, we were able to find that for him. Thanks as always for supporting the Mindscape Podcast. We're here at the end of this month's AMA. Greatly support, all the Patreon-age support. It keeps us going and hope that this AMA was good for you and we'll talk to you next time. Bye bye.

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