Physics & Philosophy with Sean Carroll

Why is the past different from the future? Neil deGrasse Tyson and Chuck Nice explore the universe’s deepest questions like why is there anything, what is the present, and if there could be a unified theory of physics with theoretical physicist Sean Carroll.

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

• 0:00Neil deGrasse Tyson?

  Sean, Chuck is about to blow a gasket.

• 0:01Chuck Nice?

  No, because you're freaking me out, man! You are freaking me out!

• 0:07Sean Carroll?

  This is not anything that we bump into in our everyday lives. If it doesn't make you a little bit uncomfortable, you're not taking it seriously.

• 0:16Neil deGrasse Tyson?

  Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now.

• 0:38Neil deGrasse Tyson?

  I'm Neil deGrasse Tyson, your personal astrophysicist.

• 0:41Speaker 03

  And I'm Lindsay Nix Walker, senior producer of StarTalk. And Neil and I just co-authored a brand new StarTalk book coming out September 12th.

• 0:49Neil deGrasse Tyson?

  Yeah, this is the third in a series of collaborations with National Geographic Books. And this one is titled "To Infinity and Beyond." And it's available for preorder from the StarTalk website, starTalkMedia.com/books. If you do preorder the book, you have a special access to a conversation that I'll be having with Lindsay Walker. She's been my senior producer for years. She's behind the scenes in practically every StarTalk episode you've ever seen or heard. I look forward to doing a live stream for our audience. And they'd be able to submit questions in advance. So, we'll see you there, we'll see you then.

• 1:32Neil deGrasse Tyson?

  StarTalkMedia.com/books.

• 1:38Neil deGrasse Tyson?

  This is StarTalk. Neil deGrasse Tyson here, your personal astrophysicist. I got with me my co-host, Jeff Nice. Jeff?

• 1:45Chuck Nice?

  Hey, what's happening?

• 1:47Neil deGrasse Tyson?

  All right, always good to have you there, man, especially when we have cosmic queries.

• 1:51Chuck Nice?

  Fan favorite.

• 1:52Neil deGrasse Tyson?

  Oh, yeah, fan favorite. We love them. You get to ask a question with sort of entry-level participation in our Patreon program, just $5 a month.

• 2:02Chuck Nice?

  That's it.

• 2:02Neil deGrasse Tyson?

  And we collected them, and you know in advance what you're asking about, because today's topics involve physics and philosophy.

• 2:12Neil deGrasse Tyson?

  People love just talk about stuff for which there is no answer.

• 2:17Chuck Nice?

  Well, of course, you know why? Because it makes them feel like they have a stake and a say.

• 2:22Neil deGrasse Tyson?

  And a say, because nobody has the answer.

• 2:24Chuck Nice?

  Yeah, nobody has the answer. So I'm just as right as you are, right? Like, no, you're not. You're still an idiot. Okay? See, I have a PhD in philosophy and physics. I have what's called an informed opinion. You, on the other hand, went on the internet, saw somebody on Joe Rogan's show say something about something else, and now you think you're an expert. Shut up. Shut up. All right, sorry.

• 2:51Neil deGrasse Tyson?

  Okay, that's the end of our show.

• 2:55Neil deGrasse Tyson?

  Well, we're bringing into this an old friend and colleague, Sean Carroll. Sean, welcome to StarTalk.

• 3:03Sean Carroll?

  Hey, thanks for having me on, Neil.

• 3:04Neil deGrasse Tyson?

  Welcome back to StarTalk, I should say. This is, you've been on at least four or five times. You are one of the world's experts on not only physics, being a physicist, but you're also that subcategory of physicists who cares about philosophy and have engaged philosophers on the frontier of our understanding of physics. So it's great to have you for this cosmic queries. And dare I say, we crafted this cosmic queries around your expertise. So you are the Homewood Professor of Natural Philosophy at the Johns Hopkins University, right there in Baltimore, and you have a joint appointment at the Santa Fe Institute.

• 3:45Neil deGrasse Tyson?

  Now, that institute, everything I know about it, those are deep thinkers about things that it's like, what? Like, you twist your head like a dog hearing a high pitch. It's like, what? And you're the fractal faculty at the Santa Fe Institute, and your expertise in quantum physics, spacetime, cosmology, love it, emergence, entropy, dark matter, dark energy, symmetry, origins of the universe, and perhaps most people who know you know you through your Mindscape podcast. So always good stuff happening there.

• 4:21Sean Carroll?

  Those things that make you tilt your head and scrunch up your eyes. Like, that's my lane. That's what I do.

• 4:27Neil deGrasse Tyson?

  That's you.

• 4:29Neil deGrasse Tyson?

  Okay, so I say this lovingly and crazily, stay in your lane, okay?

• 4:35Neil deGrasse Tyson?

  So tell me, what is, first, natural philosophy? I last saw that term with, you know, 17th, 18th century philosophers. Natural philosophy was the then term for what today we would call physics. Right, because political philosophy and religious philosophy. Natural philosophy was physics. So what, why isn't that just physics professor?

• 5:00Sean Carroll?

  There's a joke in philosophy circles that once an area of philosophy starts making progress, it gets spun off to a completely different field. So it never looks like philosophy actually makes progress. But you're right, Newton, Galileo, those folks would have called themselves philosophers. The idea of a physicist hadn't been invented yet. But what happens is that academia loves to categorize and silo people. So they invent a physics department and a philosophy department and they examine what questions you could ask and they decide which is which. But what that does is that it means that all the in-between stuff, because it's really a continuum here, not a-

• 5:36Neil deGrasse Tyson?

  The interstitial ideas and things.

• 5:39Sean Carroll?

  Yeah, those just get lost. And we're at a point in physics right now where questions like, you know, what is an observer? What is infinity? Why is the past different from the future? What is emergence? Like, these are physics questions.

• 5:53Chuck Nice?

  What makes this night different from other nights? Sorry.

• 5:59Sean Carroll?

  Philosophy and fraud.

• 6:01Neil deGrasse Tyson?

  Is that a song lyric? Alright, so, so, so you are there to clean up the pieces, is what you're saying?

• 6:09Sean Carroll?

  Well, I'm actually there to understand the universe. I think that the common thing within natural philosophy is, we're not studying the process of science or anything like that. We're studying nature, we're studying reality, but there's a way of doing it that is kind of foundational, that, you know, takes a step back. Look, physicists bless their hearts. You've met physicists, Neil, I know this. And ask them about-

• 6:34Neil deGrasse Tyson?

  By the way, Chuck, anytime someone prefaces something by bless their-

• 6:37Chuck Nice?

  Bless your heart?

• 6:38Neil deGrasse Tyson?

  You're about to insult them in some-

• 6:40Chuck Nice?

  Nothing good ever, you know, it's just like-

• 6:43Neil deGrasse Tyson?

  It's in the same sentence as bless their heart.

• 6:45Chuck Nice?

  Especially if you're an old white lady from the South, you know what I mean? It's, oh, bless her heart, she's just a little whore, that's all. Sarah, you know, that's how it goes.

• 6:56Neil deGrasse Tyson?

  Okay.

• 6:57Sean Carroll?

  I started the book by saying, you do not need a PhD in theoretical physicist in physics to be afraid of quantum mechanics, but it doesn't hurt. And physicists don't want to dig into the deep questions about quantum mechanics. They want to calculate their differential equations.

• 7:13Neil deGrasse Tyson?

  Shut up and calculate.

• 7:14Sean Carroll?

  Shut up and calculate, and that's the opposite of what I like to do myself. So the philosophers humor me on that.

• 7:20Neil deGrasse Tyson?

  So just to be, just to make sure all our audiences are on the same page. Quantum mechanics is amazingly successful at predicting and describing reality. But that reality defies all our common sense. And you can reach a point where you just say, okay, I'll just live with that and continue to calculate. And Sean, you're telling me you're not satisfied with that. You want to understand crazy quantum phenomena on some level where we can sit down and say, okay, now I understand it. Is that what's driving you?

• 8:00Sean Carroll?

  Yeah, absolutely. Just like it drove professors Einstein and Schr ö tiger back in the day. But there was this consensus that developed in the 1920s and 30s by physicists where, like you say, we have some equations, we can solve them, we can calculate them, we can make predictions and book beautiful exquisite agreement between the calculations and the predictions without knowing what's going on, without freeing on what's actually happening in the world. And there's a whole bunch of physicists who will say, oh, no, understanding what is actually happening in the world, that's not my job. I'm just here to make predictions. I strongly feel that's not right. I'm here to help understand what is going on in the world.

• 8:41Chuck Nice?

  Right, so you're the show your work part of this whole equation.

• 8:45Sean Carroll?

  Explain your work. Show it, play it, hide anything, no sweeping the rug.

• 8:49Chuck Nice?

  It's okay to come up with the answers, but we got to know how we got there.

• 8:52Sean Carroll?

  Absolutely.

• 8:53Neil deGrasse Tyson?

  Yeah. So Sean, what's been in all the buzz in recent years is a quantum entanglement. All right. And. Let me offer my best explanation and you correct it and then, but then give me your understanding of it, alright? You can create two particles simultaneously that have sort of complementary properties, quantum properties to them, and they can separate and they know about each other. There I use the word know. They know about each other's existence and, and the moment you make a measurement of one of them, the other particle's properties manifest to whoever is observing them. And they manifest instantly, transcending the speed of light.

• 9:38Neil deGrasse Tyson?

  And so, we know that happens, but Sean, you're gonna tell me that you understand it, or will you not?

• 9:46Sean Carroll?

  Well, I think that we have multiple competing ways to understand it. We have not agreed on the correct way. My way is-

• 9:54Neil deGrasse Tyson?

  Like I said, Chuck, in the bar, you just have the beer and nobody agrees, right? That's the difference.

• 9:59Sean Carroll?

  But like, but like you said, we can make the prediction to exquisite accuracy. People tested the prediction, they won the Nobel Prize for it last year. That was what the Physics Nobel Prize was given for. And by the way, their whole reason we know and care about entanglement was because Einstein in 1935 was trying very hard to figure out what really is going on, right? And he didn't quite succeed, he didn't get the answer there, but it's that drive to understand that led here. And ultimately, what we can say, I think with some confidence, is that what the world is, is not a bunch of separate particles doing their own thing. It looks like a bunch of separate particles under certain very clear circumstances, but

• 10:43Sean Carroll?

  in other circumstances like this entanglement business, it doesn't. It's more holistic than that. So, people like me, who are advocates of the many worlds interpretation of quantum mechanics, we have a very simple, straightforward way of talking about entanglement, but there's other people out there who talk about it differently and, you know, that's great. That's what academia and intellectual curiosity is all about.

• 11:06Neil deGrasse Tyson?

  Remind us of the many worlds hypothesis, because I think that's now 100 years old, right? We're on, we're on this centennial decade- Of the major discoveries of quantum physics, the 1920s. So if you can just remind us what the many worlds, I think it was once explained to me. And I said, what are you smoking? Sure, that's what I said.

• 11:26Sean Carroll?

  When we teach undergraduates quantum mechanics, we say that a quantum system has two different ways of evolving. There's one way it can evolve when you're not looking at it. And that's what Schrodinger and Heisenberg and their friends figured out back in the day. But then

• 11:42Neil deGrasse Tyson?

  Just to be clear, I just, to be precise, I know how you're using the word evolved. But to a biologist, the word evolved means something completely different. So you mean it unfolds, the events unfold.

• 11:53Sean Carroll?

  Yeah, it changes, it has its dynamics, whatever it's doing, whatever its behavior is. But then there's a whole nother way that we need to describe that behavior. When we make a measurement, when we observe the system, famously in quantum mechanics, you can't predict deterministically, precisely with 100% confidence, what answer you're going to get. You can predict a probability distribution over different possible answers.

• 12:18Neil deGrasse Tyson?

  And Einstein said, God does not play dice.

• 12:21Sean Carroll?

  That's what Einstein said, yes.

• 12:23Neil deGrasse Tyson?

  With the universe.

• 12:24Sean Carroll?

  Or

• 12:24Neil deGrasse Tyson?

  It kind of looks like, you know, you are playing dice. I'm just saying, I'm just.

• 12:29Chuck Nice?

  And by the way, what a, what a presumptuous statement on Einstein's behalf. I mean, who knows?

• 12:35Neil deGrasse Tyson?

  Sean, didn't Niels Bohr say Einstein, stop telling God what to do?

• 12:39Chuck Nice?

  Exactly. I mean, who's to say that God's not down on one knee going popping these a new pair of shoes? You don't know that. You don't know that.

• 12:49Neil deGrasse Tyson?

  So wasn't Niels Bohr, somebody said it to Einstein, who was it?

• 12:51Sean Carroll?

  I think, I think it was Niels Bohr. I think the better advice would be like, don't play dice against God. You're not going to win if that's, you know.

• 12:59Chuck Nice?

  There you go. That, that makes sense.

• 13:01Sean Carroll?

  So what is this going on with this weird thing? You don't expect measurements, observations, looking at things to be part of the fundamental nature of reality. Right? You know, it never was before quantum mechanics came along. So you can ask yourself, what if like all of that was unnecessary? This whole idea that we need a separate rule for what happens when we measure something? What if you just erase that from the rules of quantum mechanics? And the answer is that what you find is that every possible measurement outcome comes true. But in a different world, in a different part of the overall quantum universe, you get parallel worlds where different measurement outcomes are true. That's when I asked what the guy was smoking.

• 13:44Chuck Nice?

  Wow. Now, wait a minute. So that these different worlds are in the same realm? Is that the case? Or are we talking about completely different unfoldings that create completely different scenarios that make the whole thing work?

• 14:02Sean Carroll?

  Or another way to ask that, do you have access to these other worlds?

• 14:05Chuck Nice?

  There you go. Thank you.

• 14:07Sean Carroll?

  If you have iPhone, you can download an app called Universe Splitter. And if you're ever stuck on what decision to make, you know, should I have a hamburger? Should I have pizza for dinner tonight or whatever? Ask the Universe Splitter and it will come back with an answer. And you can be guaranteed that there's a whole nother universe, which you can never interact with or talk to, in which you do the opposite thing. It's split in that instant. Right. In that instant, yes. At the decision instant.

• 14:34Chuck Nice?

  Right, okay, okay.

• 14:36Sean Carroll?

  It's a physical quantum process.

• 14:38Chuck Nice?

  And that's because of the entanglement, because they couldn't exist simultaneously. They have to exist in those positions at the time of the decision, so that when one does it, the other does the opposite, or when one does something, so there's an action and there's a reaction. But there can't be action, action. It has to be so that they're existing and then reacting differently. Is that?

• 15:06Sean Carroll?

  Sean, Chuck is about to blow a gasket.

• 15:08Chuck Nice?

  No, because you're freaking me out, man! You are freaking me out!

• 15:13Sean Carroll?

  This is not anything that we bump into in our everyday lives. If it doesn't make you a little bit uncomfortable, you're not taking it seriously.

• 15:21Chuck Nice?

  Okay, all right. Wow.

• 15:33Speaker 04

  Hey, I'm Roy Hill Percival and I support StarTalk on Patreon. Bringing the universe down to earth. This is StarTalk with Neil deGrasse Tyson.

• 15:50Neil deGrasse Tyson?

  Chuck, let's get to the questions.

• 15:51Chuck Nice?

  Okay, man.

• 15:52Neil deGrasse Tyson?

  We've got to digest this.

• 15:54Chuck Nice?

  This is already good stuff, boy. I'll tell you right, I'm so mad I didn't have an edible before this show.

• 16:03Chuck Nice?

  I really should have taken a gummy before this one. Doggone, all right, here we go.

• 16:08Neil deGrasse Tyson?

  Again, these are from our Patreon supporters. Thank you all for what you do for us. All right, Chuck, you got your iPad there? All right.

• 16:15Chuck Nice?

  I do, so here we go. This is from Sai, says, hello, Dr. Tyson, Dr. Carol, Dr. Ha Ha. First time Patreon member. I know, right? First time Patreon member here. Huge, huge fan. I am Sai Anurag Lakajur, Lakaraju from India. Chuck, if you get my name right, I swear to God, I will double my Patreon membership.

• 16:50Chuck Nice?

  Yeah, well, I guess you have no, you're in no danger there, are you?

• 16:54Neil deGrasse Tyson?

  Yeah, I think he's gonna have it after what you just did to his name. But all right.

• 17:00Chuck Nice?

  That was funny. My question pertains to Dr. Carol's research, which says that the universe is infinitely old and Big Bang is just one of many events resulting from quantum fluctuations of a vacuum energy in a cold disliter, disitter space. Please throw some light on what kind of space is this? How can I visualize it better in order to understand it more fully?

• 17:29Sean Carroll?

  Good, very good. I love it. You know, hanging out on the wrong street corners. I don't know where they pick these things up. But yes, this is all driven by the very famous philosophy question. Why is the past different from the future? Why is there an arrow of time? Because the fundamental laws of physics have no arrow of time in them. The answer is entropy and the second law of thermodynamics. The universe used to be more organized, lower in entropy. The whole history of the universe is just entropy increasing, disorder and chaos developing all around.

• 18:04Chuck Nice?

  We're all gonna die, that's the...

• 18:06Sean Carroll?

  It's not your fault necessarily, but you're contributing to the disorder and chaos all around us. And that started about 14 billion years ago, near the Big Bang. At the moment of the beginning of the universe, our universe was exquisitely orderly. Doesn't necessarily look that way, but you run through the numbers and it's true. Why? Why is that true? Why was the early universe so orderly? And so I've long wondered about this. And I wrote a paper years ago now with a woman who was a graduate student of mine at the time, Jennifer Chen, where we proposed that the Big Bang was not the beginning of our universe. Other people proposed that in different contexts, but we made the case that you don't need

• 18:48Sean Carroll?

  a fine-tuned, special, organized, low-entropy beginning of the universe. The universe can be eternal, it can last forever. But what happens is it empties out, just like our universe is doing. A universe can be completely empty, the future of ours will be, but it still won't be perfectly quiet. There are still quantum fluctuations that can lead to whole new universes coming into existence. And as that happens, they all start in low-entropy conditions and the entropy grows and gives that little part of the universe an arrow of time. And the fun part is, the far, far past, the same thing happens, but in the other direction. So there's sort of a symmetric shape to the universe,

• 19:31Sean Carroll?

  where the future is a story of more and more universes being created and the arrow of time pointing in that direction. The past is a story of more and more universes being created with people in them who think that we are in their past.

• 19:47Chuck Nice?

  I'm gonna tell you right now.

• 19:51Chuck Nice?

  If Psy understands what the hell you just said.

• 19:58Chuck Nice?

  Then they need to be the co-host of this show.

• 20:03Chuck Nice?

  Because I am not gonna lie.

• 20:07Neil deGrasse Tyson?

  Wait, wait. So Sean, wait, Sean.

• 20:08Chuck Nice?

  You got, you lost me at the end. I, I need, I need you to do the end again.

• 20:13Neil deGrasse Tyson?

  So Sean, what you're saying is there's some symmetric point among these universes and these time continuum where we're in one direction, where entropy increases. But in principle, there's a whole other realm where entropy decreases.

• 20:38Sean Carroll?

  From our point of view, from the people living in it, they will always see entropy increasing because we always define the past as the direction in which entropy was lower. So it's a big U shape that is perfectly.

• 20:51Chuck Nice?

  I get it now. So we're in their past because they're looking at us and seeing a decreasing entropy, which is, and which is indicative of traveling backwards because we're moving towards a more ordered universe. But we cannot be moving towards a more ordered universe if we're moving into the future because we are always moving towards entropy. So if you are observing that, then you are in my, I am in your past. So, because you're looking at me going towards order.

• 21:24Sean Carroll?

  Chuck gets it. We're done.

• 21:26Chuck Nice?

  Yeah, man.

• 21:35Neil deGrasse Tyson?

  Chuck, you're blowing out the volume level on the microphone.

• 21:38Chuck Nice?

  That's insane. Oh my God.

• 21:41Neil deGrasse Tyson?

  Okay, wait. So, so Sean, before we get to the next question, what, um, these are ideas. Is there any way to experimentally verify any of this?

• 21:52Sean Carroll?

  Well, we're trying, but the short answer is we don't know yet. We don't have.

• 21:55Neil deGrasse Tyson?

  It's not like a no, just say it.

• 21:59Sean Carroll?

  It's very much not a no.

• 22:00Neil deGrasse Tyson?

  Oh, okay.

• 22:02Sean Carroll?

  But all the words, Neil, these are all the words that are here.

• 22:05Neil deGrasse Tyson?

  Okay.

• 22:06Sean Carroll?

  Yeah, and this, but this is a more, a broader idea, right? There are plenty of tentative preliminary scientific ideas which are too ill-formed yet.

• 22:18Neil deGrasse Tyson?

  I'm with you.

• 22:19Sean Carroll?

  Predictions.

• 22:20Neil deGrasse Tyson?

  Einstein's gravitational waves, Einstein's gravitational rings, yeah, I'll give you that.

• 22:27Sean Carroll?

  We'll get there. We will get there. Send money. We'll do it. Just trust us.

• 22:32Neil deGrasse Tyson?

  And we're lifetime?

• 22:35Sean Carroll?

  And my time is getting shorter every year, so I don't know.

• 22:39Neil deGrasse Tyson?

  All right, Chuck, what else you got?

• 22:41Chuck Nice?

  All right, let's go on to Doug Sherman. Doug Sherman says, hi Neil, hi Sean, Lord nice. The Doug, this is Doug from Frisco, Texas.

• 22:51Neil deGrasse Tyson?

  All right, all right, way to go, brother.

• 22:54Chuck Nice?

  Doug says, I thoroughly enjoyed Sean's debate on God and cosmology against William Lane Craig, although I'm still trying to get my head around everything Sean explained. One amongst many arguments I found interesting was Sean's rebuttal against Mr. Craig's technological argument that the finely tuned universe was evidence for the existence of God. I don't recall the specifics, but I believe Sean stated that in some models, the finely tuned universe approaches. Then he says, could Sean, once again, go through the perspective of the fine tuned argument? I also reject the technological argument, but for more simplistic reasons than my flawed brain can rationalize.

• 23:34Neil deGrasse Tyson?

  So just for context, William Lane Craig is a proponent of, basically, a God-created universe, and he's not the Bible-thumping person in such a conversation as others might be. He's trying to stay grounded in the natural world, taking you to a precipice where you say, okay, God must be there. So I'm pretty sure Sean wouldn't.

• 24:01Neil deGrasse Tyson?

  Debate just anybody on that subject. So William Lane Craig has some debating respectability in that regard. Did I characterize your opponent accurately, would you say?

• 24:12Sean Carroll?

  Yeah, that's fine. But the fine tuning argument for the existence of God is what I think is the best argument for the existence of God. I also think it's a terrible argument, but still it's the best of the ones that they have. So I'm glad when they refer to it. And the idea is that you look around the world, the world in which we live, the universe we find ourselves in, and you say, there are features of this universe that need to be the way they are in order for life to exist. If they were different, life couldn't exist. But they easily could have been different, right? The things like the amount of energy in empty space could have been so large that it would rip planets apart before they ever formed. But we seem to have gotten

• 24:57Sean Carroll?

  lucky. We seem to find ourselves in a universe that allows for our existence. And so the argument is, I know why. It's because God did it, because God created a universe in which that's possible. A very common counter argument is, well, it also could just be a multiverse, right?

• 25:15Neil deGrasse Tyson?

  You're not in all the others to have this conversation, right?

• 25:18Sean Carroll?

  Well, they help play an explanatory role in accounting for why our universe looks so fine tuned. So two things going on here. One is the proponents of this argument tend to exaggerate the degree of fine tuning that they need. And that's what the questioner is referring to. There's certain things that William Lane Craig and others say, oh, I just don't get it. That's so fine tuned. But you can raise your hand and say, actually, physics has completely explained that one out. We don't need to go beyond the realm of physics to account for that. So but in other ways, it still looks fine tuned. And my favorite rejoinder is actually, this is a great argument for the nonexistence of God. Because if God existed and God created

• 26:03Sean Carroll?

  life, God is not beholden to the local laws of physics. God can create life however God wants to, because he's God. God can do anything. You don't need the physical conditions to allow for the existence of life unless God does not exist.

• 26:23Neil deGrasse Tyson?

  All right. Now, let me just ask a question here, because just to further clarify.

• 26:29Chuck Nice?

  What if, uh, instead of needing the laws of physics, the laws themselves are a reflection of what God has done? So it's not necessarily that the, that the laws are needed. It's that the laws exist because they just happen to be a byproduct of the creation itself.

• 26:54Sean Carroll?

  That is completely possible and Neil will raise us in here and say, how do we observationally test that hypothesis?

• 27:00Chuck Nice?

  Okay, I'm, and listen, I'm a hundred percent with, with, I'm, I'm on board with that. Okay, good. I just wanted to make sure that that could, that that could be an argument to be had. That's all.

• 27:10Sean Carroll?

  But look, for the last 500 years, as science has done more and more to explain why the universe is the way it is, the role for God as an explanatory move has gone away, has diminished, right? And so you are left-

• 27:25Neil deGrasse Tyson?

  It continues to do so, yes.

• 27:27Sean Carroll?

  And so you're left with, if you want to believe in God, and there's plenty of very, very smart people who do, they, they tend not to rely on God to account for the things that we observe in the natural world.

• 27:39Chuck Nice?

  Gotcha. Wow, fascinating stuff.

• 27:43Neil deGrasse Tyson?

  All right, Chuck, keep going. We've only gone through two or three questions. Let's see if we can speed it up.

• 27:47Chuck Nice?

  We just have to have Sean back, that's all. This stuff is too good. We can't rush, we cannot rush through this. This is too good.

• 27:54Neil deGrasse Tyson?

  At one point, get a double wide episode with him.

• 27:57Chuck Nice?

  Yeah, well, yeah, yeah, absolutely. All right, here we go. Uh, Tom P. Knight says this. Hello, I'm a Patreon supporter of both StarTalk and Mindscape. Great to see Sean on the show. Do voids in the cosmic web, like the Iridanus supervoid, violate the cosmological principle? What could be the cause of these structures? Thank you for your service. Okay.

• 28:27Neil deGrasse Tyson?

  Good one. Eridanus is a constellation? I think it's a waterfall, or it has something to do with water, if my memory serves. But anyhow, Sean, what you got there?

• 28:38Sean Carroll?

  This is a great question, but there's a lot going on here. I'm gonna try to keep it brief here. You know, there's something called the cosmological principle that says that if you squint and look at the universe on very large scales, everything looks the same everywhere, right? The same number of galaxies and whatever. It's a dopey thing to call a principle, because it's not a principle. It's just a fact that you see about the universe. It could easily have been otherwise. Especially because it's not exactly true, and that's what the question is getting at. There are places in the universe where matter is very dense. There's places where it's very empty and so forth. The way that modern cosmologists think about this question is to say, the early universe, it was even smoother than it is now. It was very, very smooth.

• 29:20Sean Carroll?

  There was only a difference in one part in 100,000 as you went from place to place. Number one, why was it not perfectly smooth? But number two, why was it pretty darn smooth? And number three, how did it evolve, using the word evolve again, from that condition 100,000 years after the Big Bang to our conditions now? The last one, how it evolved, is the one we have the best handle on. It was gravity doing the work. Gravity turns up the contrast knob on the universe. So if you have a slightly emptier region, it empties out. You have a slightly heavier region, it collects matter onto it. And so we went from very faint ripples, if you look at the cosmic background radiation,

• 30:03Sean Carroll?

  to these very vivid voids and galaxy clusters that we see today. We still don't know where those first ripples came from. Inflationary cosmology is a favorite thing to talk about, but that's a whole nother episode.

• 30:19Chuck Nice?

  Wow, there you go.

• 30:21Neil deGrasse Tyson?

  So I like the way you said that. You, we, there's, we observe these fluctuations of one part in a hundred thousand. And you say, well, how come it's not perfectly smooth? How come it's one part in a hundred thousand? How come it's not anywhere near that today? That's a fun way to think about that problem. Because it's easy to say, oh, here's the answer and then move on. But wait a minute. Why isn't it something else?

• 30:42Chuck Nice?

  Yeah.

• 30:43Neil deGrasse Tyson?

  Right. And not enough of that goes on, I think.

• 30:45Chuck Nice?

  Well, it's a, it's a, once again, a reflection of the fact that the early universe had low entropy because gravity was so strong in the early universe. A more common generic random configuration would have been wild fluctuations like black holes here and empty space there. And so the fact that it was so smooth does kind of demand an explanation. And we're not sure what the explanation is.

• 31:08Neil deGrasse Tyson?

  Mm hmm. Mm hmm. Okay. All right. Keep this going, Chuck.

• 31:14Neil deGrasse Tyson?

  And that one was from the overlap of the Venn diagram between Mindscape and StarTalk. Love it.

• 31:31Chuck Nice?

  Uh, this is Malcolm Marfan and Malcolm says, hello, Dr. Tyson, Dr. Carol and Chuck, maybe? Okay. Um.

• 31:42Neil deGrasse Tyson?

  Okay.

• 31:42Chuck Nice?

  I love these people, man.

• 31:44Neil deGrasse Tyson?

  They love you, Chuck, no matter what they call you.

• 31:45Chuck Nice?

  I know, no, I know.

• 31:49Chuck Nice?

  Well, Malcolm Marfan here, all the way from Trinidad and Tobago. And he says.

• 31:55Neil deGrasse Tyson?

  Oh, nice.

• 31:56Chuck Nice?

  He says, Dr. Carol, I came across your 2018 paper. Why is there something rather than nothing and thought, wow, this guy's really dedicated to the lot of brain cower to the concept of nothing. Now, since you've clearly become an expert in nothingness, can you shed some light on the various layers of nothing? Specifically, how do these layers of void stand apart? And how are they intertwined with the head spinning realms of cosmology and quantum mechanics? P.S. Can I get a philosophy or physics degree with nothing from, with nothing for my thesis?

• 32:33Sean Carroll?

  I think our questioner missed the point of the title of my paper, which is that there is something, like, you know, we can contemplate that there wouldn't have been anything and there's just nothing. But what I say in the paper is, can we really contemplate that? I mean, I think that we have this informal training from our everyday lives, right? Where we have boxes with things in them and boxes with nothing in them. And so we think that there's an option. There can be things or there could be nothings. But when it comes to the universe, it is not at all obvious that there is an alternative to the universe existing. What does it even mean for nothing to exist? How does nothingness even exist? I mean, that's kind of what I'm getting at in the paper, which is that it's not at

• 33:16Sean Carroll?

  all clear that the reason why the universe exists is the kind of thing that has an answer to a why question. Maybe we just have to accept it as a brute fact and be lucky about it. So I do think this stuff is fun to talk about, but I don't think that it is nearly as down to earth and simple and physical as certain physicists who like to talk about this make it out to be. It's fundamentally a philosophy question.

• 33:47Neil deGrasse Tyson?

  Wait, but Sean, if there were no quantum physics, in principle, you could talk about space as having no particles or none of these virtual particles that quantum physics forces into it. You just say, remove the atoms and all known particles. That's as pretty good a nothing as anyone would hope to describe, isn't it?

• 34:07Sean Carroll?

  No, it's something. It's space. It has...

• 34:11Neil deGrasse Tyson?

  Because you have a word for it, okay? You call the empty space space, and I call the empty space nothing. Aren't we just semantically differing there, not fundamentally differing?

• 34:21Sean Carroll?

  Is empty space three dimensional?

• 34:24Neil deGrasse Tyson?

  I don't know.

• 34:28Sean Carroll?

  You just don't want to answer it because you know.

• 34:29Neil deGrasse Tyson?

  Okay, okay, so let's say, let's say it's three dimensional and, and exists on a time continuum. I'm sure it's.

• 34:33Sean Carroll?

  So it has a property then, it's not nothingness, it, it has, there's a way it could have been different.

• 34:38Neil deGrasse Tyson?

  Well, you invoke a way to measure stuff in it. I wouldn't call that an inherent property of the empty space.

• 34:44Sean Carroll?

  It, it's, it's different than four dimensional empty space.

• 34:47Neil deGrasse Tyson?

  Okay, so how about this? How about this? Okay. Uh, again, one of the reasons why I don't like arguing with philosophers, because ultimately it comes down to how you're defining the words that you're using in your sentence in so many of those arguments. So let me just say, if we define something versus nothing, as something is, there's a thing there. And I take everything out so that as Chuck said, there's, as you said, there is no thing, nothing there, then that's nothing. Okay, now we're, now you're saying there's a grid system there that we can invoke where it's inherent. So that's a thing. Okay, so now I add to my inventory of a thing, particles plus grid system. Okay, fine, then that's there.

• 35:32Neil deGrasse Tyson?

  Now, so now we have to ask, can we take away the grid? What I think is a more interesting question is, do the laws of physics apply in that volume? And therefore it can't be entirely nothing. Because laws of physics apply even though there's nothing there to manifest them.

• 35:49Sean Carroll?

  Ooh, now you're getting deep, so I like it because you're invoking not what the system is, but counterfactual properties of what would happen to the system if you changed it a little bit, which is fine. It's a fine thing to do. Look, you're absolutely right that there is no pre-existing definition of the word nothing to which we're referring here. You could have different ideas. I think that the deepest level of this question is, why is there a universe at all versus complete non-existence? Not even space? I mean, even you had to preface your question by saying like, imagine there was no quantum mechanics. And for that matter, secretly imagine there's no general relativity either because.

• 36:29Neil deGrasse Tyson?

  Okay, yeah.

• 36:31Sean Carroll?

  You're, you're throwing away all of known physics to even ask your question. But of course, within known physics, you can ask plenty of questions about why isn't space empty? Why is space three dimensional? Those are perfectly good physics questions. It's a little bit different than the question of why there's a universe at all. You can ask both of them separately. It's not a right or wrong thing to ask.

• 36:52Neil deGrasse Tyson?

  Right, yeah. All right.

• 36:54Chuck Nice?

  Yeah, well, that's so cool. What a great question. Yeah.

• 36:57Neil deGrasse Tyson?

  Yeah, because when someone says, why is there something rather than nothing? And I look out in the universe, which is mostly nothing. I say, there's a lot of nothing in the universe.

• 37:09Neil deGrasse Tyson?

  So whatever something we have that's side by side with a lot of nothing. Until you start invoking other definitions for what nothing can be. So that's all. That's my only point there.

• 37:20Chuck Nice?

  Well, there you have it, Malcolm. We have discussed your question and we have, we have achieved nothing.

• 37:32Chuck Nice?

  All right.

• 37:33Neil deGrasse Tyson?

  All right. Keep it going.

• 37:33Chuck Nice?

  This is Javier Ortega, who says, hi, you guys are just the best greetings from my artificial limbs development lab in Panama. Just a quick question that will not let me sleep till to the speed of light. We can only see the past while looking into distances of the cosmos. How do we know we are in the quote unquote real present? We sense everything with delay, even short distances. Also, there's a delay between our senses and our brain. Are we sure that we perceive now as now? Maybe it's just something relative to us like movement.

• 38:17Chuck Nice?

  Maybe we are five hours in the past or 200 million light years in the past, according to some other arbitrary timeline or other alien beings that could be that could travel instantly to our place.

• 38:30Neil deGrasse Tyson?

  Everybody's been smoking before.

• 38:32Chuck Nice?

  Let me tell you, man, I told you, I told you. He says, I hope you read this message from the past.

• 38:42Chuck Nice?

  Please keep looking up.

• 38:44Neil deGrasse Tyson?

  He makes an interesting point that.

• 38:48Neil deGrasse Tyson?

  What is, what does the present even mean? If everything we do to interact with the world has some kind of time delay? Like, I, I like that idea of the brain, our understanding, our senses, our-

• 39:01Sean Carroll?

  That's true.

• 39:01Neil deGrasse Tyson?

  Uh, our, uh, you know, my hand is not as it is, it as it was a billionth of a second ago. So what does it even mean to talk about the present?

• 39:10Sean Carroll?

  Yeah, there's a science answer here and a philosophy answer here. I'll give you the science answer, cause it's, it's quite good. Uh, our brain does not perceive the present. Our brain puts together a picture of the world that is on a slight time delay. Like, our brain wants to be able to bleep out things that it doesn't like. If you watch someone dribble a basketball and they're right next to you, you will see the basketball hit the ground and you will hear the thump of the basketball against the ground and they coincide, they go along with each other. If that person walks away, still true. You see and hear the same thing at the same time, even though the light gets to you much quicker than the sound. And what happens is if they keep walking away, suddenly, they will go out of sync.

• 39:55Sean Carroll?

  The vision of the basketball hitting the ground and the sound of it, because-

• 39:59Neil deGrasse Tyson?

  That happens suddenly, so your brain was correcting for it?

• 40:02Sean Carroll?

  Yes, because your brain corrects for it as long as it's near enough. Your brain says, "This is all now." And you can even measure how much it is. It depends on what sense you're talking about and what perception, but roughly think about 40 or 50 milliseconds of time is a little window in which your brain collects things and says, "I'm gonna put this together into a picture of the present."

• 40:22Neil deGrasse Tyson?

  A millisecond is a thousandth of a second. So 40 to 50 thousandths of a second would be like 5 hundredths of a second.

• 40:29Sean Carroll?

  There you go. Good making. Excellent. Yes.

• 40:32Neil deGrasse Tyson?

  Wait, wait. So what you're saying is there's a point where the brain just gives up, uh, recreating the present and said, "I can't compensate for this."

• 40:40Sean Carroll?

  It's too much!

• 40:41Neil deGrasse Tyson?

  It's too much!

• 40:42Sean Carroll?

  I can't do no more!

• 40:44Neil deGrasse Tyson?

  So you do- Deal with it.

• 40:47Sean Carroll?

  Go on without me!

• 40:51Chuck Nice?

  Oh God, it's just, oh, I can't take it.

• 40:59Neil deGrasse Tyson?

  So, so, so this is an experimental result, Sean.

• 41:02Sean Carroll?

  You know, look, as, as I'm sure you are already know, neuroscience, biology, psychology.

• 41:07Neil deGrasse Tyson?

  That's a whole frontier right there.

• 41:09Sean Carroll?

  Be like way harder than physics or astronomy.

• 41:11Neil deGrasse Tyson?

  Way harder, way harder.

• 41:14Sean Carroll?

  Yeah.

• 41:14Neil deGrasse Tyson?

  Okay, I did not know this. So interesting. So the brain constructs a present so that we can help make sense of the world in our own moment that we make decisions.

• 41:25Chuck Nice?

  It's actually doing that with everything all the time. Your brain, because there's just too much input for your brain to actually process in real time. So most of what it's doing is a kind of creating a construct and then painting a picture of what it is and then it looks for changes in that construct.

• 41:46Neil deGrasse Tyson?

  But, but, wait, so Sean, this tells me that if the brain can't complete the picture, it'll make stuff up to fill out the picture.

• 41:53Chuck Nice?

  Absolutely. That's why there are so many black men in prison right now.

• 42:01Sean Carroll?

  I was gonna say optical illusions, but sure, yes, racial incarceration and inequalities, that's another one.

• 42:09Neil deGrasse Tyson?

  Yes.

• 42:09Sean Carroll?

  This is the philosophy part of the answer, which is that there is a real world out there. I mean, there, there is objective physical reality, but there's also the picture, the image, the model of the world that our brain puts together, and they're related, but they're not the same. And our brain is doing a lot of work to take all the many sense inputs that it has and sew them together into coherent picture. So.

• 42:34Neil deGrasse Tyson?

  Yeah, and a clever, a clever optical illusion will hijack that ability.

• 42:40Sean Carroll?

  Hacking the brain.

• 42:41Neil deGrasse Tyson?

  Make you think something that is not, is very far from true as being true.

• 42:44Sean Carroll?

  Right, but it doesn't quite make sense. Yeah.

• 42:47Neil deGrasse Tyson?

  Yeah. All right, we got time for one last question.

• 42:49Chuck Nice?

  Oh my God, this is overall ready?

• 42:51Neil deGrasse Tyson?

  I know, right, right?

• 42:52Chuck Nice?

  Jeez, we gotta do this again. This was fantastic.

• 42:57Neil deGrasse Tyson?

  I think he's got other stuff to do. You know, he's working on books. You know, the man writes books. Okay. What's his latest book you just wrote?

• 43:04Sean Carroll?

  One right now during this episode. Yeah, I'm almost done.

• 43:12Sean Carroll?

  That's excellent.

• 43:13Neil deGrasse Tyson?

  Wait, you've got a book, The Biggest Ideas in the Universe, Volume One, and you're working on Volume Two.

• 43:19Sean Carroll?

  That's right.

• 43:19Neil deGrasse Tyson?

  This is a physics book where you, you, you got equations in there and you, and you are, you are not apologetic about it.

• 43:27Sean Carroll?

  No, I mean, we don't assume that you know any math. We teach you the math. So I teach you calculus and what have you.

• 43:35Neil deGrasse Tyson?

  I love that. Just in case you missed calculus, you know, in your school, it's in the first couple of pages, right?

• 43:42Sean Carroll?

  Couple of chapters, but yes. And then you move on and you learn tensors and differential geometry and Einstein's equation for general relativity.

• 43:49Neil deGrasse Tyson?

  And this is for the public. You just want to boost the public's math literacy a bit.

• 43:53Sean Carroll?

  I, and their self-esteem. I love the public. I want them to feel like they can get, they can get this stuff into their heads. It's not so hard, really.

• 43:58Neil deGrasse Tyson?

  I love what you said to us offline, that when you were 16, you wished someone had written a book like this for you to consume at that time of your life.

• 44:07Sean Carroll?

  It'd be quantum mechanics and quantum field theory and particles and symmetries and gauge theory.

• 44:12Neil deGrasse Tyson?

  Well, had you had such a book, imagine how much smarter you would be.

• 44:19Sean Carroll?

  We'd have figured this all out by now.

• 44:23Neil deGrasse Tyson?

  All right, let, one last quick question. Go.

• 44:25Sean Carroll?

  Okay, this is, uh, Hamed Daoud, who says this, "Hi, StarTalk. Uh, this is Hamed from, uh, Montreal, Canada. My question, are we still chasing the dream of a unified theory of physics, or is that just a dead end?"

• 44:40Neil deGrasse Tyson?

  I love that.

• 44:40Sean Carroll?

  Can it ever be achieved?

• 44:42Neil deGrasse Tyson?

  There you go. Love it.

• 44:43Sean Carroll?

  Yeah, still chasing it because there is the universe. The universe is telling us what it does. It's just up for us to think, up to us to figure it out. So it might-

• 44:54Neil deGrasse Tyson?

  No, Sean, it's a philosophical bias that you presume everything can be explained under one field equation and everything derived. That's a, isn't that a bias? Maybe the universe is fractured in this way with multiple forces that can't talk to one another.

• 45:13Sean Carroll?

  Yeah, that's completely fine. I, I did not say it was just going to be one equation. I was just about to say it might be a terrible mess. We don't have any right to say that the final theory of the whole universe will be simple or elegant or easily understandable by us, but we can shoot for it. We can try and we can give it our best shot and see what happens. That's what we're trying to do.

• 45:31Neil deGrasse Tyson?

  Because we got bad history there in astronomy where Kepler has, he's a mathematician and he knows that geometry is beautiful and perfect and there's certain number of platonic solids, there's like the cube and the pyramid and the dodecahedron and he thinks that relates to the orbits of the planets and he embeds one in the other. He is philosophically driven with some idea that the universe is beautiful and perfect. And he spent 15 years wasting his time until he threw it all away. He said, no, planets just have weird elliptical orbits and sure enough, that's the answer. So we've had to be, we've been through this in my field, Sean, I'm just telling you.

• 46:13Sean Carroll?

  If I feel as hard as Kepler, I'd be very, very happy.

• 46:18Neil deGrasse Tyson?

  All right, we got to call it quits there. Sean, it's been a delight to see you again and chat with you. All right, we out of here. Chuck, always good to have you, man. Neil deGrasse Tyson here, your personal astrophysicist. I'm bidding you to keep looking up.