Tangling With Entanglement And Other Big Ideas In Physics

Physicist Sean Carroll takes on back holes, Schrödinger’s cat, and other big physics concepts that had our audience wondering.

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

• 0:00Ira Flatow?

  Science Friday is supported by MathWorks, creator of MATLAB and Simulink software for technical computing and model-based design. MathWorks, accelerating the pace of discovery in engineering and science. Learn more at mathworks.com.

• 0:19Flora Lichtman?

  Hey, it's Flora Lixman, and you're listening to Science Friday. Today in the podcast, a special holiday treat for you. Ira is talking with physicist Sean Carroll at WNYC's Green Space in New York. There's space, there's time, and head-scratching questions from the audience. Here's Ira.

• 0:39Ira Flatow?

  Think back to your high school or college physics class. There were probably a lot of balls rolling down ramps, or maybe imaginary cannonballs being fired out of cannons in word problems. But probably, there wasn't a lot of thought given to some of the really big, head-scratching ideas in physics, like, how does time work? Dr. Sean Carroll argues that with a bit of math, it is possible for regular people to understand, to think about, and even argue about some of the biggest ideas in the universe. He's the Homewood Professor of Natural Philosophy at Johns Hopkins University in Baltimore. And his book, The Biggest Ideas in the Universe: Space, Time, and Motion,

• 1:22Ira Flatow?

  is our Sci-Fri Book Club pick for this month. Sean, welcome back to Science Friday.

• 1:27Speaker 02

  Thanks very much, Ira.

• 1:29Ira Flatow?

  You say in the introduction to the Space, Time, and Motion book, "My dream is to live in a world where most people have informed ideas and passionate opinions about modern physics." Do you think that's actually possible, or is that just a dream?

• 1:44Speaker 02

  I think that it's possible. I'm a big believer that science is for everybody, and it's going to be at different levels for different people. Let's put it this way, these books, the two that are out and the one I'm supposed to be writing right now, are full of equations. I don't assume that you know any equations. I don't assume that you know anything about math, so I teach you what all the equations are. But nevertheless, my advance was smaller for these books than for my other books. They appeal to a certain audience, and I think that's great. If you go to Amazon and you search for quantum books, the bestselling book is Quantum Physics for Babies. And it's like 20 pages, they're big, thick cardboard pages, you know, and it's like, there are atoms. And I think that's great.

• 2:27Speaker 02

  And everywhere in between, from there up to textbooks, exists. And I thought that there is a missing space for people who didn't want to get a textbook and become a professional physicist, but still wanted a little bit more of the behind-the-scenes, the details.

• 2:41Ira Flatow?

  You know, I found that to be true also. I know in the 34-plus years we've been doing Science Friday, I noticed that people really want to go into the weeds. It's a myth that people don't like science.

• 2:53Speaker 02

  No, absolutely. And again, people like it at different levels, but I think that there's some enthusiasm for science that gets squeezed out a bit in maybe your high school years, your college years, where it's a requirement. You have to take a class, you have to take an exam and whatever. And like you said, there's a lot of balls rolling down hills. There's not a lot of black holes and Big Bang and quantum field theory. And with just a little bit of thinking, you know, you can think about these equations as little poems, little bits of concrete art that have meaning, and you can interpret what the symbols mean. And then you understand things at a deeper level. Like, I hate it, but it's sometimes true that when people ask a question about physics or whatever, like, usually you can hand-wave an explanation, give a metaphor, tell a story.

• 3:38Speaker 02

  Sometimes you have to say, "The equations say that." And if you don't know what the equations are, that can be a little unsatisfying.

• 3:45Ira Flatow?

  Because physics does depend on math, right?

• 3:47Speaker 02

  Absolutely. And the whole motto of the series of books is, "The equations are smarter than we are." And in particular, like, the capstone of this book, Book One, is Einstein's equation for general relativity, for his theory of space and time and gravity. And, you know, he got that equation, but then the equation had within it black holes and the Big Bang and gravitational waves, and he didn't know about any of that stuff.

• 4:12Ira Flatow?

  So he didn't know really how powerful his own equation is?

• 4:15Speaker 02

  Nowhere near, yeah. In fact, he resisted some of the conclusions, as we often do. Paul Dirac famously predicted antimatter but didn't want to admit it because it was there in the equation.

• 4:25Ira Flatow?

  Are these equations any more difficult than the old physics, before quantum mechanics? Is physics harder now than it used to be?

• 4:34Speaker 02

  Yes.

• 4:37Speaker 02

  You know, we know more, right? Like, it's never gonna suddenly get easier because the old physics is still gonna be useful. Like, if you wanna fly a rocket to the moon, you don't use Einstein's General Theory of Relativity. You use Newtonian mechanics, F equals ma. It's easier to discover because it's a little bit easier to wrap your head around it. So, understanding quantum field theory or general relativity is a little bit harder, but it's like mastering anything. It's like learning to be a good chef or play the piano. That, yeah, it's hard and it's frustrating, but there's this feeling that you get when you do it that is just amazing.

• 5:11Ira Flatow?

  Let's talk about something that we get asked a lot over the years on our show, and I know you've written about, the concept of time. You know, Einstein said that time is what a clock measures, but that's it. I mean, and we've had scientists say that there is no such real thing as time, you know? Why is it so hard for us to define or understand time?

• 5:33Speaker 02

  Yeah, I wrote a whole book just about that, and I could have made it twice as long. And, you know, there's all these great quotes. I think Saint Augustine had the joke about how, you know, "What is time? What was God doing before there was time?" And he said, "I don't tell the usual joke." And the usual joke is, "He was creating hell for people who ask questions like that." And in a different moment, he said, "You know, I know what time is perfectly well, until you ask me, and then I don't know anymore." And I think that what Einstein helped us understand is that there's more than one thing going on that we label time. One of the things that time is, is what a clock measures. And what clocks measures are very personal. Einstein's Theory of Relativity tells us that two clocks,

• 6:17Speaker 02

  starting synchronized at the same point and then zipping out there in the universe and coming back, are not gonna agree anymore when they come back. Time is personal in that way. It depends on how you travel through the universe. But there's another aspect of time, which is just, it's a label. It's a coordinate, helps you find yourself. If you want to say, you're on Sixth Avenue, or something like that. That helps you find yourself in space. If you say you're at 7:00 PM, that helps find yourself in time. And we used to think, when Isaac Newton was in charge, that those were the same thing. That clocks told you where you are. And relativity says, actually, no. That's two very different things. How you travel through spacetime is not necessarily a universal thing.

• 7:00Speaker 02

  It is relative, thus the name relativity. And none of that touches the fact that we feel like time passes, right? That time is something that flows around us, and we progress from the past to the future. And all that is a story of the arrow of time. And the distinctions raised by entropy and the second law of thermodynamics. And you know what? We're still working to figure it all out. We know a lot about how time works. We don't truly know what it is, where it comes from, or how to explain every aspect of it.

• 7:34Ira Flatow?

  Well, you said, you know, back in the time when Newton was in charge, time only ran in one direction. And physicists now say it can go any direction.

• 7:46Speaker 02

  There's a lot of physicists out there, they say a lot of things. So you don't, don't listen to all of them.

• 7:50Ira Flatow?

  Is that true? I mean, is...

• 7:52Speaker 02

  No.

• 7:53Ira Flatow?

  No.

• 7:53Speaker 02

  Not that, I don't think, who said that?

• 7:55Ira Flatow?

  Well, I've had physicists talking about there is no such thing as time.

• 7:59Speaker 02

  That's a different thing than saying it goes in all directions. There are physicists...

• 8:04Ira Flatow?

  The laws of thermodynamics, saying entropy, that things can only go in one direction, right?

• 8:09Speaker 02

  It's a great, it's a great story, because in fact, it was before Newton that you wouldn't even have asked whether time can go in different directions. Like, time was not a label on different parts of spacetime, right? Time was just what labeled the moment you're presently existing in and experiencing. But to Aristotle, if you said, Aristotle, why is the past different from the future? He would have looked at you like, what do you mean? They're different things. It's like, you know, why is a carrot different than an elephant? But then Newton's theory comes along. And again, the equations are smarter than we are. And he wrote down his theory, and he was Newton. He was smarter than anybody else. But it wasn't until over a century later

• 8:50Speaker 02

  that Pierre-Simon Laplace invented the thought experiment of Laplace's demon. The idea being that a vast intelligence that knew every position and every velocity of everything in the universe could predict exactly what would happen in the future and the past. And that was the moment where time really started to evaporate. And almost immediately thereafter, they invented thermodynamics and the second law that says entropy increases in one direction of time, but not the other. So suddenly, there was a puzzle that never existed before. Why does time have a direction even though the laws of physics say it doesn't?

• 9:24Ira Flatow?

  But our present thing, our quantum thinking about time, is that it can slow down depending on who measures it, right?

• 9:33Speaker 02

  You know, like many things in science or philosophy, you start with words that make perfect sense in your natural language, everyday use. And then they become specialized. And, you know, they get slightly more technical definitions. So I hate to disappoint you, but it depends on what you mean by time.

• 9:49Ira Flatow?

  Very physicist answer that is.

• 9:51Speaker 02

  I know, that's what they say. So time is, you know, in one version of it, one way of thinking about it, is what clocks measure. There are different kinds of clocks. Some clocks are better than others. So in quantum mechanics, some of the precious ideas about time that you might have tried to cling to in a classical world are no longer available to us. Time is separated now from space in a way it wasn't before because a location of a particle is an observable fact in quantum mechanics. The time when something happens is not an observable fact. So what you try to do is say, well, I'm talking a language of time.

• 10:32Speaker 02

  But what I really mean is a bunch of things are moving around at different moments. And I'm using the language of time to figure out what precise thing I'm talking about.

• 10:42Ira Flatow?

  Yeah, one of those precise things that I think physicists love to talk about and that people don't really understand. Is Schrödinger's cat?

• 10:49Speaker 02

  Yeah.

• 10:50Ira Flatow?

  I mean, what was the whole idea by creating that mental picture? What was that about?

• 10:57Speaker 02

  It is a great story. For those of you who don't know the thought experiment, Schrödinger, who invented the Schrödinger equation, was a giant in the history of quantum mechanics, was trying to illustrate that you could take this idea of a subatomic particle, like an electron or something like that, which quantum mechanics says you can think of as living in a superposition of different possible measurement outcomes. So the electron can be spinning clockwise or counter clockwise. Quantum mechanics says it can do a little bit of both at the same time. And it's not because you don't know which one it is. It really is both. Schrödinger didn't like that, despite the fact that he was one of the founders of quantum mechanics. So he said, "Look, if that's true, then I can put a Geiger counter next to a radioactive source,

• 11:40Speaker 02

  and you're telling me the Geiger counter goes into a superposition of having clicked and having not clicked. And I hook up a Ruben-Golberg gizmo, which if the Geiger counter clicks, a hammer drops, and a vial smashes, and some gas fills a box, and there's a cat in the box." And Schrödinger's daughter once told a friend of mine, "I think my father just didn't like cats." So in the vial was cyanide, and the cat goes into a superposition of alive and dead. In my versions, if you read my books, it's sleeping gas in the vial, and the cat goes into a superposition of awake and asleep. And that's fine for physics purposes. But Schrödinger's point of all this is, what you're telling me, Professors Bohr and Heisenberg, et cetera,

• 12:25Speaker 02

  is that until I open the box, there is no fact about whether the cat's alive or dead, or awake or asleep. There's a superposition of both. And then when I open it, suddenly there's one or the other. Surely you don't believe that, he says. And to this day, physicists do not agree on what is happening inside the box before you open it up.

• 12:46Ira Flatow?

  So that was his way of making fun of the whole thing.

• 12:48Speaker 02

  He and Einstein were both on the side of saying, "Look, quantum mechanics is great. It fits all the data. It's clearly saying something important and true. But it's not finished. It's not sensible. It's not well-defined. It can't possibly be the final answer. We should still think about it." And other physicists said, "Nah, we're good. You know, we want to build some bombs and things like that. We don't need to think too hard."

• 13:10Ira Flatow?

  One of my favorite topics on Science Friday that I always find an excuse to talk about is... Is stuff we don't know. And I'm talking about our universe. Dark matter, dark energy makes up, what, 95% of our universe? We don't know what that stuff is, right?

• 13:27Speaker 02

  We have some good ideas. We certainly don't know which, if any of them are correct, yeah.

• 13:31Ira Flatow?

  Well, if you don't know what 95% of something is, how do you know anything about it?

• 13:36Speaker 02

  You know, you asked me that question, I think, the first time I was on Science Friday.

• 13:40Ira Flatow?

  I know, that's why I'm asking it again.

• 13:41Speaker 02

  It's your favorite question.

• 13:42Ira Flatow?

  It is.

• 13:43Speaker 02

  Can I give the same answer I gave then? We understand 5% of the universe. That's amazing. 5% of the universe. What do you want?

• 13:54Ira Flatow?

  That's a lot. That's a lot. Well, why is it so hard to understand? Give me the answer you gave me at that time. Why is dark energy, dark matter so mysterious?

• 14:04Speaker 02

  It's the darkness, is the short answer. You know, in some sense, there's no reason why the universe has to be readily available to our observation. It can be a little subtle, a little sneaky. We see things, literally seeing things, like the lights in this room, through electromagnetism. Light is electromagnetic waves. All of the light in this room comes from electrons shaking up and down. They're charged particles, they emit light. There's charged particles like electrons in your eyeballs, they detect them. If you have particles or sources of energy that are not electrically charged, then light doesn't interact with them, just goes right through them. So neutrinos are particles that we know exist.

• 14:47Speaker 02

  They're created in the sun, they're going through your body by the thousands every minute. But they don't interact directly with electromagnetism. But there's enough of them that we can do very, very subtle experiments and detect them. Things like dark matter and dark energy just don't interact with the easy images that we get from our telescopes. So we need to be more clever. And we are being more clever. I mean, one of the biggest, exciting frontiers in science today is building very clever ways to detect the dark matter. And one of the big news pieces in cosmology today is that the dark energy might not be strictly constant, which we thought it was. That might go away, it might not, that's just the nature of these things.

• 15:32Speaker 02

  But if the dark energy is changing over time, that's just a universe-shattering discovery. And we might be on the verge of it.

• 15:40Ira Flatow?

  Let's move from dark energy to the darkest of energy, and I'm talking about a black hole. We're what, five or six years now out from seeing the first black hole? Do we know anything more now than we did five or six years ago about black holes?

• 15:55Speaker 02

  We know a lot more, but it's in the world of like, what kind of black holes are there? How many of them are there? Well, there's something called the Event Horizon Telescope, which is, it's not even a telescope. It's a way of grouping together many telescopes to do coordinated observations of black holes, both at the center of our galaxy and the center of other galaxies. And you can't see the black holes, they're black. But there is light near them created by accretion discs or being lensed from behind. And you can see that, and that, those are the images that we're able to look at. But arguably even more excitingly, we've seen gravitational waves from the inspiral of super, not supermassive, but pretty darn massive black holes

• 16:40Speaker 02

  through the LIGO experiment, the Virgo experiment, et cetera. And we've definitely learned a lot about like, the distribution of black holes in the universe. The James Webb Space Telescope seems to be indicating the existence of giant black holes earlier in the history of the universe than we would have guessed. Why are they there? One possibility is they were formed in the early universe. And that would be wild if that were true.

• 17:06Ira Flatow?

  Why would that be wild?

• 17:08Speaker 02

  Well, it's not as easy to make a black hole as you might think, right? The sun doesn't turn into a black hole because you put a bunch of matter together. It heats up, it starts doing nuclear fusion, it puffs up, it makes a star. At the end of the sun's life cycle, it will give out all of its nuclear fuel, but it will collapse to be a white dwarf. There will still be pressure that prevents it from squeezing together to make a black hole. It's actually not so easy to make a black hole. Once you have one, it will keep growing. But even that is harder than you think because matter starts falling in. But then, guess what, it heats up and pushes out again. So to get a really, really big black hole really, really early in the history of the universe requires probably some new, exciting, unanticipated physics at very early times.

• 17:57Ira Flatow?

  We'll be back in just a moment with more from Sean Carroll about the biggest ideas in the universe. Stay with us.

• 18:06Ira Flatow?

  Hey folks, Ira here with a quick reminder that as this year ticks slowly away, this is your last chance to make a tax deductible donation for 2025. We still have a dollar for dollar donation match in effect until December 31st, so take advantage and please make your gift now. Remember, you're not just supporting Science Friday. Your tax deductible donation supports the whole public radio system and the millions of listeners like you who rely on programs like ours to stay informed. So please go to sciencefriday.com/support. Each one of you can make a difference in our work. That's sciencefriday.com/support.

• 18:51Ira Flatow?

  And for everyone at Science Friday, I wish you a happy and healthy new year, and thanks.

• 18:58Speaker 03

  Science Friday is supported by PBS, presenting Going Wild. If you love learning about the natural world and meeting the scientists who help us understand it, be sure to check out the new season of Going Wild with Dr. Ray Wynn Grant. It's the chart-topping and Webby Award-winning podcast from the folks at Nature on PBS, now with a video companion series. In this season of Going Wild, host acclaimed wildlife ecologist Dr. Ray Wynn Grant heads out into the field to explore the animals, people, and stories that make nature worth saving. Stops include a hibernating black bear den in Arkansas and an underwater shipwreck site off the Florida coast. If you're looking for a place to start, check out the season's debut episode where Ray joins beloved biologist and influencer Jordan

• 19:40Speaker 03

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• 20:05Ira Flatow?

  Looks like we have a question from the audience. Go ahead.

• 20:07Speaker 04

  So I've read a couple of articles recently on the expansion of the universe. And it's been expanding, expanding, expanding, but some papers say that's incorrect. That maybe it's actually shrinking. And I wonder if you could comment on that.

• 20:19Speaker 02

  Well, some headlines said that. You're completely correct. They're full of nonsense. The universe is not shrinking.

• 20:25Speaker 04

  So my question was nonsense?

• 20:27Speaker 02

  No, the question was perfectly good. Your question should be, like, why is the universe feeding me nonsense about, in the news media? So what's going on, it's exactly what I already mentioned very briefly, the possibility that the dark energy might not be constant. So the way it works is, the universe has been expanding for about 14 billion years. Zero question that it's still expanding. And what we want to do is check, how does the rate of expansion change over time? What you would have expected, when I was your age, was that it would slow down over time, because the universe expands, but then it's full of matter, and galaxies, and dark matter, and they're all pulling on each other, and it should slow down the rate of expansion.

• 21:09Speaker 02

  And in 1998, astronomers found what is called the acceleration of the universe. They looked at distant supernovae, and they're moving faster than we thought. And we already had the explanation at hand, because Einstein invented it. It's the vacuum energy, or the dark energy, or the cosmological constant. And that's there, too. There's zero question about that. 70% of the universe by energy is the dark energy. The next question is, the characteristic feature of cosmological constant or vacuum energy is, it's completely, exactly, constant. Per cubic centimeter, there are 10 to the minus 8th ergs of dark energy in every cubic centimeter of the universe, over both space and time. But of course, we want to check that.

• 21:52Speaker 02

  So we're looking to see, like, okay, is it changing a little bit? And two things are happening. Number one, there are some experiments that show a little bit of a hint that maybe it is changing over time. So what that means is, the way this got mangled into your headline, is the dark energy we thought was constant, it's actually maybe decreasing a little bit, so the rate of acceleration has slowed. That is not to say that it's shrinking, right? But there's a whole other thing going on called the Hubble tension, which says that, you know, the Hubble constant is what we use to measure, characterize the rate of expansion. And there's the rate of expansion today, forget about how it's changing, just the rate of expansion today. Two completely different, independent ways of measuring it.

• 22:36Speaker 02

  They get incompatible answers. And they're only off by like 5%, and which is hilarious to me, because when I was in graduate school, we were off by 100%. Like, some people thought it was 50, some people thought it was 100.

• 22:47Ira Flatow?

  But it's 5% of a lot of stuff, like you said before.

• 22:50Speaker 02

  Well, it's 5% of a medium-sized number. So now we're arguing, is it 67 or 72? Right? We were arguing, is it 50 or 100? But still, it's 67 plus or minus 1, and 72 plus or minus 1, if you include the error bars. So that's an annoyingly large discrepancy. But that's one of the big challenges in cosmology today.

• 23:11Ira Flatow?

  I remember back in, I think it was 98, when we were talking about the discovery of this expansive force. Steven Weinberg came on our program, and he said, "The real problem with it is not that we discovered the expansive force, but there should be so much more of it."

• 23:28Speaker 02

  Exactly, yes.

• 23:29Ira Flatow?

  Right? And we haven't found that more part of it yet.

• 23:32Speaker 02

  Oh, we're not gonna find it, it is not there. What Weinberg was referring to, and he's the one who came up with a really clever way of explaining this, actually. Empty space, according to Einstein's theory of general relativity, can intrinsically have energy. Like, usually you think of energy as being associated with mass, like E equals mc squared, or radiation if there's photons and whatever. And that's the usual stuff the universe is made of. But Einstein gives us the possibility that space itself can have energy.

• 24:00Ira Flatow?

  Not empty.

• 24:01Speaker 02

  It's empty and it has energy, that's the thing. That's why it's called the vacuum energy.

• 24:08Ira Flatow?

  It's not that Trojaner cat thing again.

• 24:10Speaker 02

  Not the Trojaner cat thing. No, it's a number, you can go out there and measure it. But you could also, not only go out there and measure it, you could be a theorist, and you could say, "Well, what would my guess be? Now, what's a natural value for it to take?" And the answer turns out to be the observed value times a factor of 10 to the power 120, which is a 1 followed by 120, 0.

• 24:32Ira Flatow?

  Big number.

• 24:32Speaker 02

  It's a big number, yes. And so it's not like we didn't find it. If that were real, we wouldn't be here having this conversation. It would rip apart the very nature of atoms and molecules and things like that. So, the question is, why is there some miraculous cancellation between things that make the vacuum energy positive and negative and leave us with this really tiny thing, close to but not exactly zero? Does that keep you up at night thinking about that? Other things keep me up at night these days, but yeah, that's one of them. I mean Weinberg, the thing that Weinberg said back in the 1980s is, you know, let's make lemonade out of these lemons.

• 25:14Speaker 02

  He says, "It's true that as a quantum field theorist, my natural guess about the value of the vacuum energy is way bigger than it possibly is, but if it were that big, I wouldn't be here talking about it." Yeah. So you can see where this is going. How small does it have to be so I can be here talking about it? What if there's a multiverse out there where in fact the vacuum energy doesn't have the same value everywhere, it's a little bit different in every different universe, then in most of the universes, we're not going to be able to live because the vacuum energy is too big? And he actually made a prediction in 1988 for what you should measure the vacuum energy

• 25:56Speaker 02

  to be, and in 1998, they measured it and he was right. Wow, he was a smart guy. I think he's going to go far, yeah. Yeah. Speaking of going far, let's go to this.

• 26:07Speaker 05

  If we only know, say, 5% as you say about the universe, could you speculate either about a law of physics that you think could someday be overturned, that you're a little shaky on, or a theory that intrigues you that you suspect someday we can prove?

• 26:30Speaker 02

  This is a great question, and it's great because the answer is not as simple as we would like it to be.

• 26:36Ira Flatow?

  Never is, never is in physics.

• 26:38Speaker 02

  I hate that. There's a balance going on here. On the one hand, you say, can you imagine that any of the current laws might be overturned? All of the current laws might be overturned. Like, that's how science works. Like, you get closer and closer, you do better and better, you're never sure. You're always willing to say, oh, tomorrow, if there's an experiment, I'm gonna have to change my mind. On the other side, you say, like, well, what about all the new ideas that are rejected? Most new ideas are crap. They should be rejected. And it's absolutely true that you don't wanna have a closed mind and not listen to new ideas and things like that. But a new idea has a certain, you know, prove it kind of aspect.

• 27:21Speaker 02

  Like, should I pay attention to this new idea? And scientists gather a lot of intuition about how the world works and knowledge of what has happened so far. And they build up sort of a feeling of like, what kinds of ideas might pay off and which ones don't. And they're usually right, and sometimes they're spectacularly wrong. But just because an idea is not being given attention by scientists doesn't mean it's probably right. It means it's probably wrong, and we'll be embarrassed, we'll fall on our face a few times. It's the best we can do, we're human beings.

• 27:57Ira Flatow?

  And an idea in science, though, just because people think they have ideas about things, is, I think, not the same kind of thing that we think about as an idea. An idea in science, and tell me if I'm on the wrong track here, is not really a valid science idea unless you can test it out.

• 28:14Speaker 02

  Well, I think, yeah, it's, it's-

• 28:15Ira Flatow?

  And make a prediction that you can test. It's a good idea, but if you can't do experiments, I'm thinking of string theory, for example. It's a great idea, but if you can't really test it out, it really is not a science idea anymore.

• 28:27Speaker 02

  I think this is, again, a subtle philosophy of science question. What counts as science and what doesn't? I will say that now that we're in the ChatGPT era, I get multiple emails per day from people who have new ideas about how the universe works. And it's like, what if time is a vibration?

• 28:49Speaker 02

  What does that mean? There's no, that doesn't, well, I can't do anything with it. So ideally, you're absolutely right. At the end of the day, finally, when we figure everything out and the theory is all well-posed, you need to make predictions, compare them with the data. But that's at the end. Before you get there, you have to turn your idea into something specific and concrete and definite. And it can't just be vibes, right? It really needs to be equations, I gotta say.

• 29:16Ira Flatow?

  Well, what about this concept, this idea, this physical phenomenon of what Einstein called spooky action at a distance? Entanglement, where it seems like you can be on opposite sides of the universe, and when something happens with this entangled particle, this one knows immediately, seeming to violate the speed of light. How is that explained?

• 29:37Speaker 02

  Well, that's why Einstein called it spooky. So, the idea is, you have two particles and they're entangled, but one particle all by itself is already interesting. It has a spin, it could be clockwise or counterclockwise, and quantum mechanics, like we said, says you can't say ahead of time what it's gonna be when you measure it. It's a combination of both, but you only get one answer. It's either clockwise or counterclockwise. So if you have two particles, it can be true for both. Both particles can be spinning either clockwise or counterclockwise, you don't know which one it's gonna be. But you can know, you can arrange the quantum mechanical experiment in such a way that whatever one particle is measured to do, going clockwise, the other particle's going the other way.

• 30:20Ira Flatow?

  At the same time?

• 30:22Speaker 02

  Whenever you're gonna measure it, yes.

• 30:24Ira Flatow?

  So, but it's not violating the speed of light?

• 30:27Speaker 02

  Well, here's the thing. So Alice is here on Earth doing an experiment, and the other particle gets sent to her friend Bob halfway across the universe, okay? And Alice has no idea whether she's gonna get clockwise or counterclockwise, neither does Bob. Alice measures hers, she gets clockwise. Alice knows that when Bob does his experiment, he's going to get counterclockwise. But Bob doesn't know that. Bob has learned nothing. You have not been able to send any message to Bob. So it's exactly maximally frustrating, really. And you can see why Einstein was annoyed by this. It seems like some information traveled faster than the speed of light, but we can't actually use it to signal or communicate. It's like the universe is taunting us a little bit.

• 31:11Speaker 02

  And you're absolutely right. We don't agree on what is truly going on behind the scenes there.

• 31:17Speaker 06

  Let's take a question from over here. So in terms of getting that data to inspire new models for foreign models, are there any particular observational campaigns or recent experiments that you're particularly excited about? I want to see the results of that one.

• 31:30Speaker 02

  There's a bunch, you know. I think that of the ones that I know are probably going to happen, or at least I hope are going to happen. Like we said before, we've detected gravitational waves here on Earth with LIGO and Virgo and other ground-based things. There's a space-based gravitational wave satellite proposal called LISA, Laser Interferometric Space Antenna. And the thing is that when you build a gravitational wave detector, the literal size of the thing is related to the wavelength of the gravitational waves you're looking at, which is related to how big the event is. So LIGO has four-kilometer-long arms, and that's aimed at a particular, you know, if you have 30 solar-mass black holes going into each other.

• 32:14Speaker 02

  And it's been great for astrophysicists and for physicists. LISA will see the following thing. You can have a million solar-mass black hole, and you can have a one-solar-mass black hole orbiting it and very gradually fall in. And this is supposed to happen all the time. It should be very, very visible. And what, unlike like the two medium-sized black holes mushing together, which kind of makes a mess, one little tiny black hole moving around a supermassive black hole, the supermassive black hole doesn't care. It's a million solar masses, and it's not affected. So basically, the signal will map out the gravitational field around these supermassive black holes to exquisite detail. And that, I think, at least gives us a fighting chance

• 32:54Speaker 02

  of discovering a deviation from Einstein's theory of relativity. The other one I'm excited about, but just for personal reasons, is if the dark energy, this stuff that's making the universe accelerate, if it is dynamical, if it is changing over time... My favorite candidate has the property, it's actually very much like an axion. And a little photon with a polarization traveling through space will rotate its polarization. This is called birefringence. And if we could detect that directly, it's not just detecting the dark energy, it's detecting the interaction of dark energy with photons, which should be fascinating.

• 33:33Ira Flatow?

  So you would be able to detect that particle interacting with the dark energy?

• 33:37Speaker 02

  Yeah.

• 33:38Ira Flatow?

  And that's an experiment coming up?

• 33:40Speaker 02

  It's going on. In fact, every time you measure the cosmic microwave background polarization, in principle, you can look for this effect. It's just very, very tiny. So there is already like a claim that if you look hard enough at our best microwave background data, you can see evidence for birefringence. But it's not, you know, people are not quite excited by it yet. They want a independent confirmation.

• 34:02Ira Flatow?

  Will you be disappointed when it's discovered? Sounds like a stupid question, but so many of the physicists I've talked with over the years say, "It's really the chase we like." You know? And I remember when the Higgs boson was discovered, people were saying, "Ah, nuts, we found it." I was hoping that chase would go on forever, you know?

• 34:19Speaker 02

  But Higgs wasn't disappointed. And in this case, I'm the one who predicted this. So no, I will not be disappointed. I want them to hurry up.

• 34:28Ira Flatow?

  Right. You know, I'll give you the blank check question I give a lot of my guests, which is, I have a blank, if I had a blank...

• 34:36Speaker 02

  Oh, come on.

• 34:36Ira Flatow?

  Yeah, I'm sorry. It's a virtual blank check in my pocket. What instrument, or what would you use it for to answer any of the questions we've talked about tonight and things that you worry about? What would you spend it on? What do we need in terms of equipment or ideas solved or what?

• 34:52Speaker 02

  Well, I mean, here's the honest answer, that we don't know.

• 34:56Ira Flatow?

  We like that. Oh.

• 34:58Speaker 02

  I'll tell you why that's an honest answer and is a really interesting one. The theories we have right now are too good.

• 35:08Speaker 02

  We're in an unprecedented era in the history of fundamental physics, where we have theories that fit all the data. At any previous era in the history of fundamental physics, we'd have theories that are pretty darn good, but you could easily point to things we haven't yet made a prediction for that is coming out correct and want to fix that. And sometimes you just fix it and it's pretty elementary. Other times you have to throw out everything, like quantum mechanics or relativity, like get a completely new paradigm. We're in a very strange situation right now, where we have theories that fit the data. We don't have a good experimental clue about what to do next. I mean, the best ones are the existence of dark matter and dark energy. So, you know, the sort of more down-to-earth answer is, yeah, better dark matter detectors,

• 35:53Speaker 02

  like an axion detector that would really be able to see the axions if it were the dark matter. A really super-duper-high-precision, dark-energy observational program that could measure whether it's changing with time or whether it's causing cosmic by-refringements. Plus, you know, various fishing expeditions. Build a giant collider that looks for high-energy particles well beyond what we could possibly observe right now. Look for tiny violations of fundamental principles, like locality and relativity and things like that, energy conservation. We have to sort of like cross our fingers and hope that if we try to do a million different experiments, one of them is going to break what we think the laws of physics are, and then we'll be in an exciting time.

• 36:38Ira Flatow?

  Well, Sean, we always learn new things when you come on. I want to thank you for taking time to be with us today.

• 36:42Speaker 02

  Thanks very much.

• 36:43Ira Flatow?

  Keep up your writing. Sean Carroll is the Homer Professor of Natural Philosophy at Johns Hopkins University, and his "The Biggest Ideas in the Universe: Spacetime in Motion" is our Sci-Fri Book Club pick for this month. And thanks to all the folks at WNYC's Green Space for your help. Thank you all for coming out this evening.