Science Friday - Tangling With Entanglement And Other Big Ideas In Physics

Episode Date: December 26, 2025

What have we learned in recent years about black holes? Can entangled quantum particles really communicate faster than light? What’s the story behind Schrödinger’s Cat? And, in this weird liminal space between the holidays, what even IS time, really? Physicist Sean Carroll and Host Ira Flatow tackled those big questions and more at a recent event at WNYC’s Greene Space in New York City. Carroll’s book The Biggest Ideas in the Universe: Space, Time, and Motion is the SciFri Book Club pick for December. Guest: Dr. Sean Carroll is the Homewood Professor of Natural Philosophy at Johns Hopkins University in Baltimore.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hey, it's Flora Lickman, 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. Think back to your high school or college physics class. There were probably a lot of balls rolling down ramps, or maybe imaginary cannon balls 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 is our SciFRI book club pick for this month. Sean, welcome back to Science Friday. Thanks very much, Iris. 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. You think that's actually possible, or is that just a dream? 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 best-selling 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. 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. 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. No, absolutely. And again, people like it at different levels, but I think there's some enthusiasm for science that gets squeezed out a bit in maybe your high school years or 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. 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. Physics does depend on math, right? 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. So he didn't know really how powerful his own equation is. 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. Are these equations any more difficult than the old physics before quantum mechanics? Is physics harder now than it used to be? Yes. You know, we know more, right? Like, it's never going to suddenly get easier because the old physics is still going to be useful. Like, if you want to fly a rocket to the moon, You don't use Einstein's general theory of relativity.

Use Newtonian mechanics, F-Equels 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. It's, yeah, it's hard and it's frustrating. But there's this feeling that you get when you do it,

that it's just amazing. Let's talk about something that we get asked a lot over the years on our show. 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? 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 St. 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 clock measures, and what clocks measures are very personal. Einstein's theory of relativity tells us that two clocks, starting synchronized at the same point, and then zipping out there in the universe and coming back, are not going to 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, it helps you find yourself. If you want to say you're on 6th Avenue or something like that, that helps you find yourself in space. if you say you're at 7 p.m. 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 space time is not necessarily a universal thing. 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 in 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. 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.

There's a lot of physicists out there. They say a lot of things. Don't listen to all of them. Well, I mean, is that true? No. No. I mean, not that. I don't think.

Who said that? Well, I've had physicists talking about there is no such thing as time. That's a different thing than say it goes in all directions. Well, I mean, there are physicists. So the laws of thermodynamics saying entropy, that things can only go. on one direction. 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 space time, 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 that Pierre Simone 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 in 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? But our present thing, our quantum thinking about time, is that it can slow down, depending on who measures it, right? 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 they get slightly more technical definition. So I hate to disappoint you, but it depends on what you mean by time.

Very physicist answer that is. I know, that's what they say. So time is, you know, in one version of it, one way of thinking about it, it's 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. 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. Yeah, one of those precise things that I think physicists love to talk about and people don't really understand is Schrodinger's cat. Yeah. I mean, what was the whole idea by creating that mental picture? What was that about? It is a great story. For those of you who don't know the thought experiment, Schrodinger, who admitted the Schrodinger, 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 mechanic says you can think of as living in a superposition of different possible measurement outcomes. So the electron can be spinning clockwise or counterclockwise. Quantum mechanic 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. Schrodinger didn't like that. despite the fact that he's 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, and you're telling me the Geiger counter goes into a superposition of having clicked and having not clicked. And I hook up a Rube Goldberg-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 Schrodinger'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 a wake and asleep And that's fine for physics purposes But Schrodinger's point of all this Is what you're telling me

Professors Bohr and Heisenberg Etcetre is that Until I opened 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. So that was his way of making

fun of the whole thing. He and Einstein were both on the side of saying, look, quantum mechanics is great. If it's 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. One of my favorite topics on Science Friday that we always, I always find an excuse to talk about 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?

We have some good ideas. We certainly don't know which, if any of them are correct. Yeah. Well, if you don't know what 95% of something is, how do you know anything about it? You know, you asked me that question, I think, the first time I was on, Science Friday. I'm asking it again. It's your favorite question. It is. Can I give the same answer I gave them? Sure.

Go ahead. We understand 5% of the universe. That's amazing. 5% of the universe. What do you want? That's a lot. That's a lot. Well, why is it so hard to understand?

Give me the answer you gave me that time. Why is dark energy, dark matter so mysterious? 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. It just goes right through them. So neutrinos are particles that we know exist.

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. But if the dark energy is changing over time, that's just a universe-shattering discovery. We might be on the verge of it. Let's move from dark energy to the darkest of energy, and I'm talking about 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?

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 disks or being lensed from behind. And you can see that, and those are the images that we're able to look at. But arguably, even more excitingly, we've seen gravitational waves from the in spiral of super, not supermassive, but pretty darn massive black holes 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. Why would that be wild?

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 time. We'll be back in just a moment with more from Sean Carroll about the biggest ideas in the universe Stay with us. 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 million of listeners like you who rely on programs like ours to stay informed. So please go to ScienceFriety.com slash support. Each one of you can make a difference in our work. That's Science Friday.com slash support. And for everyone at Science Friday, I wish you a happy and healthy new year. And thanks. Looks like we have a question from the audience. Go ahead. So I've read a couple

articles recently on the expansion of the universe, and it's been expanding, expanding, expanding, but some papers say, that's incorrect, maybe it's actually shrinking, and I wonder if you could comment on that. Well, some headlines said that. You're completely correct. They're full of nonsense. The universe is not

shrinking. So my question was nonsense. 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. And in 1998, astronomers found what is called the acceleration of the universe.

They look 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 a 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, you want to check that. 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, 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. But it's 5% of a lot of stuff, like you said before. 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 one

and 72 plus or minus one if you include the error bars. So that's an annoyingly large discrepancy that's one of the big challenges in cosmology today. I remember back in, I think it was 98, when we were talking about the discovery of this expansive force,

Stephen 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. Exactly, yes. Right? Yeah.

We haven't found that more part of it yet. Oh, we're not going to 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. It's not empty. It's empty and it has energy. That's the thing. That's why it's called the vacuum energy. It's the energy of empty space. It's not that Trojanger cat thing again. Not the Trottinger 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? 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 one followed by 120. It's a big number. 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? I 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. 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.

about it. 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 than 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 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. Speaking of going far, let's go to this side of it.

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, that you suspect someday we can prove? This is a great question, and it's great because the answer is not as simple as we would like it to be. Never is.

Never is in physics. 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. That's how science works.

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 going to 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 want to 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. 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 in our face a few times, it's the best we can do we're human being. 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. Right? 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.

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 chat GPT 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? 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 got to say. 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? 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 going to be when you measure it. 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 going to be. But you can know, you can arrange the quantum mechanical experiment in such a way that whatever one particles measured to do going clockwise, the other particles going the other way. So it's counterfeit. At the same time. Whenever you're going to measure it, yes. But it's not violating the speed

of life. 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 going to 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 learn 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 in 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. And you're absolutely right. We don't agree on what is truly going on behind the scenes there. 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. 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 Interimates

ferometric 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. 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 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. It's not affected. So basically, the signal will map out the gravitational field around the supermassive. of black holes to exquisite detail. And that, I think, at least gives us a fighting chance 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 axiom. 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, is detecting the interaction of dark energy with photons, which would be fascinating. So you would be able to detect that particle interacting with the dark energy? Yeah.

And that's an experiment coming up? 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 biorefringens, but it's not, you know, people are not quite excited by it yet.

They want an independent confirmation. Would 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, I'm nuts, we found it. I was hoping that chase would go on forever, you know. But Higgs wasn't disappointed.

In this case, I'm the one who predicted this, so no, I will not be disappointed. I want him to hurry up. Right. You know, I give you the blank check question I give a lot of my guests, which is, I have a blank, if I had a blank, oh, come on.

I'm sorry. It's a virtual blank check in my pocket. What instrument, 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? Well, I mean, here's the honest answer that we don't know. We like that. Oh. I'll tell you why that's an honest answer and it's a really interesting one.

The theories we have right now are too good. 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 we'll have 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. 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, like an axiom detector that would really be able to see the axioms 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 birefringens. 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. Well, Sean, we always learn new things when you come on. I want to thank you for taking time to be with us today. Thanks very much. Keep up your writing.

John Carroll is the homework professor of natural philosophy at Johns Hopkins University and his biggest ideas in the universe, space, time, and motion is our SciFRI book club pick for this month. And thanks to all the folks at WNYC's Greenspace for your help. Thank you all for coming out this evening.