FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. For the rest of the hour, a special holiday treat. 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.

IRA 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 is our SciFri Book Club Pick for this month. Sean, welcome back to Science Friday.

SEAN CARROLL: Thanks very much, Ira.

IRA 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?

SEAN CARROLL: 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 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 20 pages. They’re big, thick cardboard pages, 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.

IRA FLATOW: 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.

SEAN CARROLL: 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 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.

I hate it, but it’s sometimes true that when people ask a question about physics or whatever, usually you can handwave an explanation, give a metaphor, tell a story. Sometimes you have to say the equations say that. And if you don’t what the equations are, that can be a little unsatisfying.

IRA FLATOW: Because physics does depend on math.

SEAN CARROLL: 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 he got that equation, but then the equation had within it black holes and the big bang and gravitational waves, and he didn’t about any of that stuff.

IRA FLATOW: So he didn’t– he doesn’t really how powerful his own equation is?

SEAN CARROLL: 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.

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

SEAN CARROLL: Yes.

[LAUGHTER]

We more. It’s never going to suddenly get easier because the old physics is still going to be useful. If you want to 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. It’s hard and it’s frustrating, but there’s this feeling that you get when you do it that it’s just amazing.

IRA 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. Einstein said that time is what a clock measures, but that’s it. And we’ve had scientists say that there is no such real thing as time. Why is it so hard for us to define or understand time?

SEAN CARROLL: Yeah, I wrote a whole book just about that, and I could have made it twice as long. There’s all these great quotes. I think St. Augustine had the joke about 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 a different moment, he said what time is perfectly well until you ask me, and then I don’t 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 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 Sixth Avenue or something like that, that helps you find yourself in space. If you say you’re at 7:00 PM, that helps you 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, 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.

IRA FLATOW: Well, you said back in the time when Newton was in charged, time only ran in one direction, and physicists now say it can go any direction.

SEAN CARROLL: There’s a lot of physicists out there. They say a lot of things. Don’t listen to all of that.

IRA FLATOW: Is that true? Is–

SEAN CARROLL: No.

IRA FLATOW: No. OK.

SEAN CARROLL: Not that I don’t think– who said that?

IRA FLATOW: Well, I’ve had physicists–

SEAN CARROLL: I don’t know.

IRA FLATOW: Talking about there is no such thing as time.

SEAN CARROLL: That’s a different thing than saying it goes in all directions.

IRA FLATOW: Well–

SEAN CARROLL: There are physicists–

IRA FLATOW: They said that the laws of thermodynamics saying entropy that things can only go in one direction.

SEAN CARROLL: 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. Time was not a label on different parts of space time. 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 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-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, in 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?

IRA FLATOW: But our present thinking, our quantum thinking about time is that it can slow down depending on who measures it.

SEAN CARROLL: 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.

IRA FLATOW: Very physicist answer that.

SEAN CARROLL: I know. That’s what they say.

IRA FLATOW: Yes.

SEAN CARROLL: So time is 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.

IRA FLATOW: Yeah. One of those precise things that I think physicists love to talk about and people don’t really understand is Schrödinger’s cat.

SEAN CARROLL: Yeah.

IRA FLATOW: What was the whole idea by creating that mental picture? What was that about?

SEAN CARROLL: 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 counterclockwise. 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’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 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, 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.

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

SEAN CARROLL: 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. We want to build some bombs and things like that. We don’t need to think too hard.

IRA FLATOW: One of my favorite topics on Science Friday that 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.

SEAN CARROLL: We have some good ideas. We certainly don’t which, if any of them are correct. Yeah.

IRA FLATOW: Well, if you don’t know what 95% of something is, how do you anything about it?

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

IRA FLATOW: I know. That’s why I’m asking it again.

SEAN CARROLL: It’s your favorite question.

IRA FLATOW: It is.

SEAN CARROLL: Can I give the same answer I gave then.

IRA FLATOW: Sure Go ahead.

SEAN CARROLL: We understand 5% of the universe. That’s amazing.

[LAUGHTER]

5% of the universe. What do you want?

IRA FLATOW: That’s a lot.

[CHUCKLING]

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?

SEAN CARROLL: It’s the darkness is the short answer. 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 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. 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 dark energy is changing over time, that’s just a universe shattering discovery, and we might be on the verge of it.

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

SEAN CARROLL: We know a lot more, but it’s in the world of 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 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 through the LIGO experiment, the Virgo experiment, et cetera. And we’ve definitely learned a lot about 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 there? One possibility is they were formed in the early universe, and that would be wild if that were true.

IRA FLATOW: Why would that be wild?

SEAN CARROLL: Well, it’s not as easy to make a black hole as you might think. 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.

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

This is Science Friday from WNYC studios.

This is Science Friday. I’m Ira Flatow. I’m talking with physicist Sean Carroll about space, time, and some of the biggest ideas in the universe. Looks like we have a question from the audience. Go ahead.

AUDIENCE: 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.

SEAN CARROLL: Well, some headlines said that. You’re completely correct. They are full of nonsense. The universe is not shrinking.

AUDIENCE: So my question was nonsense.

SEAN CARROLL: No, the question was perfectly good. Your question should be 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 looked at distant supernovae, and they were 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 0 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 eighth ERGs of dark energy in every cubic centimeter of the universe over both space and time. But, of course, we want to check that, so we’re looking to see 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 that 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.

But there’s a whole other thing going on called the Hubble tension, which says that the Hubble constant is what we use to measure, characterize the rate of expansion and 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 5%, which is hilarious to me because when I was in graduate school, we were off by 100%. Some people thought it was 50. Some people thought it was 100.

IRA FLATOW: But it’s 5% of a lot of stuff like you said before.

SEAN CARROLL: Well, it’s 5% of a medium sized number. So now we’re arguing is it 67 or 72. 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. That’s one of the big challenges in cosmology today.

IRA 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.

SEAN CARROLL: Exactly, yes.

IRA FLATOW: Right.

SEAN CARROLL: Yeah.

IRA FLATOW: And we haven’t found that more part of it yet.

SEAN CARROLL: 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. 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.

IRA FLATOW: It’s not empty.

SEAN CARROLL: It’s empty, and it has energy. That’s the thing. That’s why it’s called the vacuum energy. It’s the energy.

IRA FLATOW: It’s not that Schrödinger cat thing again.

SEAN CARROLL: Not the Schrödinger cat thing. No, it’s number. You can go out there and measure it. But you could also not only go there and measure. 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 of 120, which is a 1 followed by 120 zeros.

IRA FLATOW: Big number.

SEAN CARROLL: 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 cancelation between things that make the vacuum energy positive and negative and leave us with this really tiny thing close to but not exactly 0?

IRA FLATOW: That keep you up at night thinking about that?

SEAN CARROLL: Other things keep me up at night these days, but, yeah, that– that’s one of them. Weinberg– the thing that Weinberg said back in the 1980s is 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 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. Then in most of the universe’s, 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.

IRA FLATOW: Wow, he was a smart guy.

SEAN CARROLL: I think he’s going to go far.

IRA FLATOW: Yeah.

[LAUGHTER]

Speaking of going far, let’s go to this side of the room.

AUDIENCE: If we only 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?

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

IRA FLATOW: Never is. Never is in physics.

SEAN CARROLL: I hate that.

IRA FLATOW: Yeah.

SEAN CARROLL: 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 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, 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 prove it kind of aspect. 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 a feeling 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.

IRA FLATOW: 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.

SEAN CARROLL: Well–

IRA FLATOW: Right?

SEAN CARROLL: Yeah, it’s– it’s–

IRA 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.

SEAN CARROLL: I think this is a– 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. What does that mean? There’s no– that doesn’t– 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. It really needs to be equations I got to say.

IRA 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?

SEAN CARROLL: Well, that’s why Einstein called it spooky. So–

IRA FLATOW: Smart man.

SEAN CARROLL: 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. 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 going to 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. So it’s a–

IRA FLATOW: At the same time.

SEAN CARROLL: Whenever you’re going to measure it. Yes.

IRA FLATOW: So– but it’s not violating the speed of light.

SEAN CARROLL: 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. 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 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. And you’re absolutely right. We don’t agree on what is truly going on behind the scenes there.

IRA FLATOW: Let’s take a question from over here.

AUDIENCE: So in terms of getting that data to inspire new models for foreign models, are there any particular observational campaigns or research experiments that you’re particularly excited about and want to see the results of that one?

SEAN CARROLL: There’s a bunch. 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 Interferometer 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 4 kilometer long arms, and that’s aimed at a particular 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 like the two medium-sized black holes smashing together, which 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 the supermassive 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 is 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 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 would be fascinating.

IRA FLATOW: So you’d be able to detect that particle interacting with the dark energy?

SEAN CARROLL: Yeah.

IRA FLATOW: And that’s an experiment coming up or–

SEAN CARROLL: 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 a claim that if you look hard enough at our best microwave background data, you can see evidence for birefringence, but it’s not– people are not quite excited by it yet. They want an independent confirmation.

IRA FLATOW: Will you be disappointed when it’s discovered? It 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. And I remember when the Higgs Boson was discovered, people were saying, ahh, nuts. We found it? I was hoping that chase would go on forever.

SEAN CARROLL: 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 him to hurry up.

IRA FLATOW: Right. I’m going to give you the blank check question I give a lot of my guests, which is I have a blank– if I had a blank–

SEAN CARROLL: Oh, come on.

IRA FLATOW: I’m sorry. It’s a virtual blank check in my pocket. What instrument or what would you use it 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?

SEAN CARROLL: Well, here’s the honest answer that we don’t know.

IRA FLATOW: We like that. Oh.

SEAN CARROLL: 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, 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. The best ones are the existence of dark matter and dark energy.

So the more down to Earth answer is, yeah, better dark matter detectors like an axion detector that would really be able to see the axion 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 birefringence, plus 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 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.

IRA 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.

SEAN CARROLL: Thanks very much.

IRA FLATOW: Keep up your writing. Sean Carroll is the Homewood Professor of Natural Philosophy at Johns Hopkins University, and his The 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 Green Space for your help. Thank you all for coming out this evening.

FLORA LICHTMAN: That conversation was recorded at WNYC’s Green Space in New York City.

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