Sean Carroll on General Relativity, Quantum Mechanics, Black Holes and Aliens

Sean Carroll

The whole point of relativity is to say there's no such thing as right now when you're far away. That is doubly true for what's inside a black hole. You might think, "Well, the galaxy is very big." It's really not. It's some tens of thousands of light years across and billions of years old. You don't need to move at a high fraction of the speed of light to fill the galaxy.

Sean Carroll

Very big.

Sean Carroll

The short answer is the worlds don't exist in space. Space exists separately in each world.

Sean Carroll

Probably the best way to start to try to explain it is special relativity, which came first, 1905. It was the culmination of many decades of people putting things together. But it was Einstein in 1905. In fact, it wasn't even Einstein. I should give more credit to Minkowski in 1907. Einstein in 1905 figured out that you could get rid of the ether, the idea of a rest frame for the universe and all the equations of physics would make sense with the speed of light being a maximum.

But then it was Minkowski who used to be Einstein's professor in 1907 who realized the most elegant way of thinking about this idea of Einstein's was to blend space and time together into spacetime to really imagine that there is no hard and fast division of the four-dimensional world in which we live into space and time separately.

Einstein was at first dismissive of this. He thought it was just like, "Oh, the mathematicians or over-formalizing again." But then he later realized that if spacetime is a thing, it can have properties and in particular it can have a geometry. It can be curved from place to place. That was what let him solve the problem of gravity.

He had previously been trying to fit in what we knew about gravity from Newtonian mechanics, the inverse square law of gravity, to his new relativistic theory. It didn't work. The final leap was to say gravity is the curvature of spacetime, and that statement is basically general relativity.

Sean Carroll

Yes.

Sean Carroll

Yep. I know that's hard. That's a little bit of a joke there, right?

Sean Carroll

Because we all give Einstein a lot of credit. But then we also, partly based on fact, but partly to make ourselves feel better, tell ourselves a story about how later in life, Einstein couldn't keep up. There were younger people doing quantum mechanics and quantum field theory and particle physics, and he was just unable to really philosophically get over his objections to that.

I think that that story about the latter part is completely wrong, almost 180 degrees wrong. I think that Einstein understood quantum mechanics as well as anyone, at least up through the 1930s. I think that his philosophical objections to it are correct. He should actually have been taken much more seriously about that.

What he did, what he achieved in trying to think these problems through is to really basically understand the idea of quantum entanglement, which is important these days when it comes to understanding quantum mechanics. Now, it's true that in the '40s and '50s he placed his efforts in hopes for unifying electricity and magnetism with gravity. That didn't really work out very well.

All of us try things that don't work out. I don't hold that against him. But in terms of IQ points, in terms of trying to be a clear-thinking physicist, he was really, really great.

Sean Carroll

Yeah. That's a great question. I think that if you want to make the case for Einstein's greatness, which is not hard to do, there's two things you point at. One is in 1905, his famous miracle year, he writes three different papers on three wildly different subjects, all of which would make you famous just for writing that one paper.

Special relativity is one of them. Brownian motion is another one, which is just the little vibrations of tiny little dust specks in the air. But who cares about that? What matters is it proves the existence of atoms. He explains Brownian motion by imagining there are molecules in the air and deriving their properties. Brilliant.

Then he basically starts the world on the road to quantum mechanics with his paper on, which again, is given a boring label of the photoelectric effect. What it really was is he invented photons. He showed that light should be thought of as particles as well as waves. He did all three of those very different things in one year.

Okay. But the other thing that gets him genius status is, like you say, general relativity. This takes 10 years from 1905 to 1915. He wasn't only doing general relativity. He was working on other things. He invented refrigerator. He did various interesting things. He wasn't even the only one working on the problem.

There were other people who suggested relativistic theories of gravity. But he really applied himself to it. I think as your question suggests, the solution was not a matter of turning a crank. It was something fundamentally creative. In his own telling of the story, his greatest moment, his happiest moment was when he realized that if the way that we would modern … say it in modern terms, if you were in a rocket ship accelerating at 1G, at acceleration due to gravity, if the rocket ship were very quiet, you wouldn't be able to know the difference between being in a rocket ship and being on the surface of the earth.

Gravity is not detectable or at least not distinguishable from acceleration. Number one, that's a pretty clever thing to think. But number two, if you or I had had that thought, we would've gone, "Huh. We're pretty clever." He reasons from there to say, "Okay. If gravity is not detectable, then it can't be like an ordinary force."

The electromagnetic force is detectable. We can put charged particles around. Positively charged particles and negatively charged particles respond differently to an electric field or to a magnetic field. He realizes that what his thought experiment showed, or at least suggested, is that gravity isn't like that. Everything responds in the same way to gravity. How could that be the case?

Then this other leap he makes is, "Oh, it's because it's the curvature of spacetime." It's a feature of spacetime. It's not a force on top of it. The feature that it is, is curvature. Then finally he says, "Okay. Clearly, I'm going to need the mathematical tools necessary to describe curvature. I don't know them, so I will learn them." They didn't have MOOCs or AI helpers back in those days. He had to sit down and read the math papers, and he taught himself differential geometry and invented general relativity.

Sean Carroll

It's certainly not simple, actually. It's a profound insight. That's why I said I think we should give Minkowski more credit than we do. He's the one who really put the finishing touches on special relativity. Again, many people had talked about how things change when you move close to the speed of light, what Maxwell's equations of electromagnetism predict and so forth, what their symmetries are. People like Lorenz and Fitzgerald and Poincare, there's a story that goes there.

In the usual telling Einstein puts the capstone on it. He's the one who says, "All of this makes much more sense if there just is no ether. It is undetectable. We don't know how fast. Everything is relative." Thus, the name relativity. But he didn't take the actual final step, which was to realize that the underlying structure that he had invented is best thought of as unifying space and time together.

I honestly don't know what was going through Minkowski's mind when he thought that. I'm not sure if he was so mathematically adept that it was just clear to him or he was really struggling it and he did trial and error for a while. I'm not sure.

Sean Carroll

Oh, yeah. All the time. I mean, we, of course, make our lives easy by ignoring two of the dimensions of space. Instead of four-dimensional spacetime, we just draw pictures of one dimension of space, one dimension of time. The so-called spacetime diagram.

I mean, maybe this is lurking underneath your question. But even the best physicists will draw a vertical axis and a horizontal axis and will go space, time. But deep down that's wrong, because you're sort of preferring one direction of space and one direction of time. It's really the whole two-dimensional thing that is spacetime.

The more legitimate thing to draw on that picture are rays of light, are light cones. From every point, there is a fixed direction at which the speed of light would represent. That is actually inherent in the structure. The division into space and time is something that's easy for us human beings.

Sean Carroll

It's the difference between X and Y when you draw axes on a piece of paper.

Sean Carroll

There is almost no difference. There's one difference that is important, which is the following; If you have a curve in space, I'm going to draw it horizontally, because that's usually what we do in spacetime diagrams, if you have a curve in space, you've heard the motto before that the shortest distance between two points is a straight line.

If you have a curve in time, which is by the way, literally all of our lives, we all evolve in time. You can start with one event in spacetime, and another event in spacetime. What Minkowski points out is that the time you measure along your trajectory in the universe is precisely analogous to the distance you travel on a curve through space.

By precisely, I mean it is also true that the actual distance you travel through depends on your path. You can go a straight line, shortest distance and curvy line would be longer. The time you measure in spacetime, the literal time that takes off on your clock also depends on your path, but it depends on it the other way.

That the longest time between two points is a straight line. If you zig back and forth in spacetime, you take less and less time to go from point A to point B.

Sean Carroll

I'm a huge believer in objective reality. I think that objective reality, objectivity …

Sean Carroll

… is real. But I do think that people are a little overly casual about the relationship between what we observe and objective reality in the following sense. Of course, in order to explain the world, our starting point and our ending point is our observations, our experimental input, the phenomena we experience and see around us in the world.

But in between, there's a theory, there's a mathematical formalization of our ideas about what is going on. If a theory fits the data and is very simple and makes sense in its own terms, then we say that the theory is right. That means that we should attribute some reality to the entities that play an important role in that theory, at least provisionally until we can come up with a better theory down the road.

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**Black holes**

Sean Carroll

Yes and no. It depends on exactly how careful we are being. Here is a bunch of things I think are correct. If you imagine there is a black hole, spacetime, the whole solution Einstein's equation, and you treat you and me as what we call test particles. We don't have any gravitational fields ourselves. We just move around in the gravitational field. That's obviously an approximation. Okay. But let's imagine that.

You stand outside the black hole and I fall in. As I'm falling in, I'm waving to you because I'm going into the black hole, you will see me move more and more slowly. Also, the light for me is redshifted. I kind of look embarrassed, because I'm falling into a black hole. There is a limit. There's a last moment that light will be emitted from me, from your perspective forever. Okay.

Now you don't literally see it because I'm emitting photons more and more slowly because from your point of view. It's not like I'm equally bright. I basically fade from view in that picture. Okay. That's one approximation. The other approximation is I do have a gravitational field of my own, and therefore as I approach the black hole, the black hole doesn't just sit there and let me pass through. It moves out to eat me up because its net energy mass is going to be mine, plus its.

But roughly speaking, yes, I think so. I don't like to go to the dramatic extremes because that's where the approximations break down. But if you see something falling into a black hole, you see its clock ticking more and more slowly.

Sean Carroll

We don't. I mean, how would we. Because it's always possible that right at the last minute it had a change of heart and starts accelerating away. If you don't see it passing, you don't know. Let's point out that as smart as Einstein was, he never figured out black holes, and he could have. It's embarrassing. It took decades for people thinking about general relativity to understand that there are such things as black holes.

Because basically Einstein comes up with general relativity in 1915. Two years later, Schwarzschild, Karl Schwarzschild derives the solution to Einstein's equation that represents a black hole, the Schwarzschild solution. No one recognized it for what it was until the '50s, David Finkelstein and other people. That's just one of these examples of physicists not being as clever as they should have been.

Sean Carroll

It is absolutely hard to imagine. A black hole is very different to many ways from what we're used to. On the other hand, I mean the real reason, of course, is that between 1915 and 1955, there's a bunch of other things that are really interesting going on in physics. All of particle physics and quantum field theory. Many of the greatest minds were focused on that.

But still, if the universe hands you a solution to general relativity in terms of curved spacetime and its mysterious certain features of it, I would put some effort in trying to figure it out.

Sean Carroll

It's best to think of a black hole as not an object so much as a region of spacetime. Okay. It's a region with the property, at least in classical general relativity, quantum mechanics makes everything harder. But let's imagine we're being classical for the moment. It's a region of spacetime with the property that if you enter, you can't leave. Literally the equivalent of escaping a black hole would be moving faster than the speed of light. They're both precisely equally difficult. You would have to move faster than the speed of light to escape from the black hole.

Once you're in, that's fine. In principle, you don't even notice when you cross the event horizon, as we call it. The event horizon is that point of no return, where once you're inside, you can't leave. But meanwhile, the spacetime is collapsing around you to ultimately a singularity in your future, which means that the gravitational forces are so strong, they tear your body apart and you will die in a finite amount of time.

The time it takes, if the black hole is about the mass of the sun to go from the event horizon to the singularity takes about 1 millionth of a second.

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**Hawking radiation**

Sean Carroll

Well, you've raised a crucially difficult point. That's why I keep needing to distinguish between black holes according to Einstein's theory, General Relativity, which is book one of Spacetime and Geometry, which is perfectly classical. Then come the 1970s, we start asking about quantum mechanics and what happens in quantum mechanics.

According to classical general relativity, the information that makes up you when you fall into the black hole is lost to the outside world. It's there, it's inside the black hole, but we can't get it anymore. In the 1970s, Stephen Hawking comes along and points out that black holes radiate. They give off photons and other particles to the universe around them. As they radiate, they lose mass, and eventually they evaporate, they disappear.

Once that happens, I can no longer say the information about you or a book that I threw in the black hole or whatever is still there, is hidden behind the black hole because the black hole has gone away. Either that information is destroyed, like you said, or it is somehow transferred to the radiation that is coming out to the Hawking radiation.

The large majority of people who think about this belief that the information is somehow transferred to the radiation and information is conserved. That is a feature both of general relativity by itself and of quantum mechanics by itself. When you put them together, that should still be a feature.

We don't know that for sure. There are people who have doubted it, including Stephen Hawking for a long time. But that's what most people think. What we're trying to do now in a topic which has generated many, many hundreds of papers called the Black Hole Information Loss Puzzle is figure out how to get the information from you or the book into the radiation that is escaping the black hole.

Sean Carroll

Right now, we are nowhere close to observing Hawking radiation. Here's the sad fact. The larger the black hole is, the lower its temperature is. A small black hole, like a microscopically small black hole might be very visible. It's given off light. But something like the black hole, the center of our galaxy, 3 million times the mass of the sun or something like that, Sagittarius A star, that is so cold and low temperature that it's radiation will never be observable.

Black holes are hard to make. We don't have any nearby. The ones we have out there in the universe are very, very faint. There's no immediate hope for detecting Hawking radiation.

Sean Carroll

As far as we know, we don't have any nearby.

Sean Carroll

You don't want them to be too tiny or they're exploding. They're very bright and then they'll be visible. But there's an absolutely regime where black holes are large enough not to be visible because the larger ones are fainter. Not giving off radiation, but small enough to not been detected through their gravitational effect. Yeah.

Sean Carroll

I love them. I love black holes. But the universe weirdly makes it hard to make a black hole, because you really need to squeeze an enormous amount of matter and energy into a very, very small region of space. We know how to make stellar black holes. A supermassive star can collapse to make a black hole.

We know we also have these supermassive black holes, the center of galaxies. We're a little unclear where they came from. I mean, maybe stellar black holes that got together and combined. But that's one of the exciting things about new data from the James Webb Space Telescope is that quite large black holes seem to exist relatively early in the history of the universe. It was already difficult to figure out where they came from. Now it's an even tougher puzzle.

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**Aliens**

Sean Carroll

I think that's fair. Yeah. It's always interesting when something is difficult, but happens anyway. I mean, the probability of making a black hole could have been zero. It could have been one. But it's this interesting number in between, which is fun.

Sean Carroll

Yeah. I have no idea. But I think your intuition is right that it would've been easy for there to be lots of civilizations then we would've noticed them already and we haven't. Absolutely the simplest explanation for why we haven't is that they're not there.

Sean Carroll

How easy is it to make life or eukaryotic life or multicellular life?

Sean Carroll

I mean, I get it. I get exactly what you're thinking. I think is a perfectly reasonable attitude to have before you confront the data. I would not have expected earth to be special in any way. I would've expected there to be plenty of very noticeable extraterrestrial civilizations out there. But even if life finds a way, even if we buy everything you say, how long does it take for life to find a way? What if it typically takes 100 billion years, then we'd be alone.

Sean Carroll

I don't believe that very strongly. Look, I'm not going to place a lot of bets here. I'm both pretty up in the air about whether or not life itself is all over the place. It's possible when we visit other worlds, other solar systems, there's very tiny microscopic life ubiquitous, but none of it has reached some complex form.

It's also possible there isn't any. It's also possible that there are intelligent civilizations that have better things to do than knock on our doors. I think we should be very humble about these things we know so little about.

Sean Carroll

That is absolutely possible. I'm actually putting less credence on that one just because you need to happen every single time. If even one, I mean, this goes back to John von Neumann pointed out that you don't need to send the aliens around the galaxy. You can build self-reproducing probes and send them around the galaxy. You might think, "Well, the galaxy is very big." It's really not. It's some tens of thousands of light years across and billions of years old. You don't need to move at a high fraction of the speed of light to fill the galaxy.

Sean Carroll

100%.

Sean Carroll

Yes. What you should do … If you want the optimistic spin, here's the optimistic spin. People looking for intelligent life elsewhere often tune in with their radio telescopes, at least we did before Arecibo was decommissioned. That's not a very promising way to find intelligent life elsewhere, because why in the world would a super intelligent alien civilization waste all of its energy by beaming it in random directions into the sky?

For one thing, it just passes you by. If we are here on earth, we've only been listening to radio waves for or a couple 100 years. Okay. If an intelligent alien civilization exists for a billion years, they have to pinpoint exactly the right time to send us this signal. It is much, much more efficient to send probes and to park, to go to the other solar systems, just sit there and wait for an intelligent civilization to arise in that solar system.

This is the 2001 monolith hypothesis. I would be less surprised to find a quiescent alien artifact in our solar system than I would to catch a radio signal from an intelligent civilization.

Sean Carroll

I just want to integrate over time. A probe can just sit there and wait, whereas a radio wave goes right by you.

Sean Carroll

Couldn't I be like the elected leader of alien civilization?

Sean Carroll

I think we would figure out that language thing pretty quickly. I mean, maybe not as quickly as we do when different human tribes find each other, because obviously there's a lot of commonalities in humanity. But there is logic in math, and there is the physical world. You can point to a rock and go "rock." I don't think it would take that long.

I know that Arrival, the movie, based on a Ted Chiang story suggested that the way that aliens communicate is going to be fundamentally different. But also, they had recognition and other things I don't believe in. I think that if we actually find aliens, that will not be our long-term problem.

Sean Carroll

Well, again, if it's intelligent and technologically advanced, the more short-term question of if we get some spectroscopic data from an exoplanet, so we know a little bit about what is in its atmosphere, how can we judge whether or not that atmosphere is giving us a signature of life existing? That's a very hard question that people are debating about.

I mean, one very simple-minded, but perhaps interesting approach is to say, "Small molecules don't tell you anything, because even if life could make them something else could also make them. But long molecules, that's the thing that life would produce."

Sean Carroll

I think I'm in favor of this kind of humility, this intellectual humility that we won't know because we should be prepared for surprises. But I do always keep coming back to the idea that we all live in the same physical universe. Well, let's put it this way. The development of our intelligence has certainly been connected to our ability to manipulate the physical world around us.

I would guess, without 100% credence by any means, but my guess would be that any advanced kind of life would also have that capability. Both dolphins and octopuses are potential counterexamples to that. But I think in the details, there would be enough similarities that we would recognize it.

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**Holographic principle**

Sean Carroll

Mm-hmm.

Sean Carroll

Well, it goes back to this question that we were talking about with the information and how it gets out. In quantum mechanics, certainly, arguably, even before quantum mechanics comes along in classical statistical mechanics, there's a relationship between information and entropy. Entropy is my favorite thing to talk about that I've written books about and will continue to write books about. So Hawking tells us that black holes have entropy, and it's a finite amount of entropy. It's not an infinite amount. But the belief is, and now we're already getting quite speculative, the belief is that the entropy of a black hole is the largest amount of entropy that you can have in a region of space-time. It's the most densely packed that entropy can be. What that means is there's a maximum amount of information that you can fit into that region of space, and you call it a black hole.

Iinterestingly, you might expect if I have a box and I'm going to put information in it and I don't tell you how I'm going to put the information in, but I ask, "How does the information I can put in scale with the size of the box?" You might think, "Well, it goes as the volume of the box because the information takes up some volume, and I can only fit in a certain amount." That is what you might guess for the black hole, but it's not what the answer is. The answer is that the maximum information as reflected in the black hole entropy scales as the area of the black hole's event horizon, not the volume inside. So people thought about that in both deep and superficial ways for a long time, and they proposed what we now call the holographic principle, that the way that space-time and quantum gravity convey information or hold information is not different bits or qubits for quantum information at every point in space-time.

It is something holographic, which means it's embedded in or located in or can be thought of as pertaining to one dimension less of the three dimensions of space that we live in. So in the case of the black hole, the event horizon is two-dimensional, embedded in a three-dimensional universe. The holographic principle would say all of the information contained in the black hole can be thought of as living on the event horizon rather than in the interior of the black hole. I need to say one more thing about that, which is that this was an idea, the idea I just told you was the original holographic principle put forward by people like Gerard 't Hooft and Leonard Susskind, the super famous physicist. Leonard Susskind was on my podcast and gave a great talk. He's very good at explaining these things.

Sean Carroll

Mindscape Podcast.

Sean Carroll

That's right, yes.

Sean Carroll

I don't.

Sean Carroll

Oh, thank you very much.

Sean Carroll

Yeah, ideas-

Sean Carroll

Fresh ideas from smart people.

Sean Carroll

Yeah.

Sean Carroll

Yeah, it's a good question. One of the things that quantum field theory indirectly suggests is that there's not that much information in you and me compared to the volume of space-time we take up. As far as quantum field theory is concerned, you and I are mostly empty space, and so we are not information dense. The density of information in us or in a book or a CD or whatever, computer RAM, is indeed encoded by volume. There's different bits located at different points in space, but that density of information is super-duper low. So we are just like the speed of light or just the big bang for the information in a black hole, we are far away in our everyday experience from the regime where these questions become relevant. So it's very far away from our intuition. We don't really know how to think about these things. We can do the math, but we don't feel it in our bones.

Sean Carroll

Well, we'd like to do better, but we're trying. That's why we have an information loss puzzle because we haven't completely solved it. So here, just one thing to keep in mind. Once space-time becomes flexible, which it does according to general relativity and you have quantum mechanics, which has fluctuations in virtual particles and things like that, the very idea of a location in space-time becomes a little bit fuzzy, 'cause it's flexible and quantum mechanics says you can even pin it down. So information can propagate in ways that you might not have expected, and that's easy to say and it's true, but we haven't yet come up with the right way to talk about it that is perfectly rigorous.

Sean Carroll

You do. You do, but I'll point out one other thing. It's information dense, but it's also very, very high entropy. So a black hole is kind of like a very, very, very specific random number. It takes a lot of digits to specify it, but the digits don't tell you anything. They don't give you anything useful to work on, so it takes a lot of information, but it's not of a form that we can learn a lot from.

Sean Carroll

Well, I said that the black hole is the highest density of information, but it's not the highest amount of information because the black hole can evaporate. When it evaporates and people have done the equations for this, when it evaporates, the entropy that it turns into is actually higher than the entropy of the black hole was, which is good because entropy is supposed to go up, but it's much more dilute. It's spread across a huge volume of space-time. So in principle, all that you made the black hole out of, the information that it took is still there, we think, in that information, but it's scattered to the four winds.

Sean Carroll

No one's been there, so-

Sean Carroll

… again, this is a theoretical prediction. But I'll say one super crucial feature of the black holes that we know and love, the kind that Schwarzschild first invented, there's a singularity, but it's not at the middle of the black hole. Remember space and time are parts of one unified space-time, the location of the singularity in the black hole is not the middle of space, but our future. It is a moment of time. It is like a big crunch. The big bang was an expansion from a singularity in the past. Big crunch probably doesn't exist, but if it did, it would be a collapse to a singularity in the future. That's what the interiors of black holes are like. You can be fine in the interior, but things are becoming more and more crowded. Space-time is becoming more and more warped, and eventually you hit a limit, and that's the singularity in your future.

Sean Carroll

Time always ticks by at one second per second. That's all it can ever do. Time can tick by differently for different people, and so you have things like the twin paradox where two people initially are the same age, one goes off in the speed of light and comes back, now they're not. You can even work out that the one who goes out and comes back will be younger because they did not take the shortest distance path. But locally, as far as you and your wristwatch are concerned, time is not funny. Your neurological signals in your brain and your heartbeat and your wristwatch, whatever's happening to them is happening to all of them at the same time. So time always seems to be ticking along at the same rate.

Sean Carroll

You would be converted into photons. You would not be you anymore.

Sean Carroll

It depends on what you might preserve. It's there in the microscopic configuration of the universe. It's exactly as if I took a regular book, made it paper and I burned it. The laws of physics say that all the information in the book is still there in the heat and light and ashes. You're never going to get it. It's a matter of practice, but in principle, it's still there.

Sean Carroll

From outside the black hole, doesn't matter 'cause they're inside the black hole.

Sean Carroll

Right.

Sean Carroll

To let it evaporate.

Sean Carroll

Yeah.

Sean Carroll

As far as we know, nothing dramatic happens. We're not anywhere close to being confident that we know what's going on here yet. So there are good unanswered questions about whether time is fundamental, whether time is emergent, whether it has something to do with quantum entanglement, whether time really exists at all, different theories, different proponents of different things, but there's nothing specifically about holography that would make us change our opinions about time, whatever they happen to be.

Sean Carroll

It really is, yeah.

Sean Carroll

Time just goes along for the ride as far as we know. Yeah.

Sean Carroll

Yeah, that might be a reflection of our ignorance right now, but yes.

Sean Carroll

It does depend on what they would say. I think that there are colleagues of mine who think that we're pretty close to figuring out how information gets out of black holes, how to quantize gravity, things like that. I'm more skeptical that we are pretty close. I think that there's room for a bunch of surprises to come. So in that sense, I suspect I would be surprised. The biggest and most interesting surprise to me would if quantum mechanics itself were somehow superseded by something better. As far as I know, there's no empirical evidence-based reason to think that quantum mechanics is not 100% correct, but it might not be. That's always possible, and there are, again, respectable friends of mine who speculate about it. So that's the first thing I'd want to know.

Sean Carroll

Yeah, that's where it would show up.

Sean Carroll

Maybe. The point is that black holes are mysterious for various reasons. So yeah, if our best theory of the universe is wrong, that might help explain why.

Sean Carroll

Yeah. That would be less surprising to me 'cause I've already written papers about that. We don't have, again, strong reason to think that the interior of a black hole leads to another universe. But it is possible, and it's also very possible that that's true for some black holes and not others. This is stuff, it's easy to ask questions we don't know the answer to. The problem is the questions that are easy to ask that we don't know the answer to are super hard to answer.

Sean Carroll

The regimes are just very far away. So either literally far away in space, but also in energy or mass or time or whatever.

Sean Carroll

Yeah, I'm always interested in, since my first published paper, taking these wild speculative ideas and trying to test them against data. The problem is when you're dealing with wild speculative ideas, they're usually not well-defined enough to make a prediction. It's kind of, "I know what's going to happen in some cases, I don't know what's going to happen in other cases." So we did the following thing: As I've already mentioned, the holographic principle, which is meant to reflect the information contained in black holes seems to be telling us that there's less information, less stuff that can go on than you might naively expect. So let's upgrade naively expect to predict using quantum field theory. Quantum field theory is our best theory of fundamental physics right now. Unlike this holographic black hole stuff, quantum field theory is entirely local. In every point of space, something can go on. Then you add up all the different points in space, okay? Not holographic at all.

So there's a mismatch between the expectation for what is happening even in empty space in quantum field theory versus what the holographic principle would predict. How do you reconcile these two things? So there's one way of doing it that had been suggested previously, which is to say that in the quantum field theory way of talking, it implies there's a whole bunch more states, a whole bunch more ways the system could be than there really are. I'll do a little bit of math just because there might be some people in the audience who like the math. If I draw two axes on a two-dimensional geometry, like the surface of the table, you know that the whole point of it being two-dimensional is I can draw two vectors that are perpendicular to each other. I can't draw three vectors that are all perpendicular to each other. They need to overlap a little bit. That's true for any numbers of dimensions. But I can ask, "Okay, how much do they have to overlap?

If I try to put more vectors into a vector space, then the dimensionality of the vector space, can I make them almost perpendicular to each other?" The mathematical answer is, as the number of dimensions gets very, very large, you can fit a huge extra number of vectors in that are almost perpendicular to each other. So in this case, what we're suggesting is the number of things that can happen in a region of space is correctly described by holography. It is somewhat over-counted by quantum field theory, but that's because the quantum field theory states are not exactly perpendicular to each other. I should have mentioned that in quantum mechanics, states are given by vectors in some huge dimensional vector space; very, very, very, very large dimensional vector space. So maybe the quantum field theory states are not quite perpendicular to each other. If that is true, that's a speculation already. But if that's true, how would you know what is the experimental deviation?

It would've been completely respectable if we had gone through and made some guesses and found that there is no noticeable experimental difference because, again, these things are in regimes very, very far away. We stuck our necks out. We made some very, very specific guesses as to how this weird overlap of states would show up in the equations of motion for particles like neutrinos. Then we made predictions on how the neutrinos would behave on the basis of those wild guesses and then we compared them with data. What we found is we're pretty close but haven't yet reached the detectability of the effect that we are predicting. In other words, well, basically one way of saying what we predict is if a neutrino, and there's reasons why it's neutrinos, we can go into if you want, but it's not that interesting, if a neutrino comes to us from across the universe from some galaxy very, very far away, there is a probability as it's traveling that it will dissolve into other neutrinos because they're not really perpendicular to each other as vectors as they would ordinarily be in quantum field theory.

That means that if you look at neutrinos coming from far enough away with high enough energies, they should disappear. If you see a whole bunch of nearby neutrinos, but then further away you should see fewer. There is an experiment called IceCube, which is this amazing testament to the ingenuity of human beings where they go to Antarctica and they drill holes and they put photodetectors on a string a mile deep in these holes. They basically use all of the ice in a cube, I don't know whether it's a mile or not, but it's like a kilometer or something like that, some big region. That much ice is their detector. They're looking for flashes when a cosmic ray or neutrino or whatever hits a water molecule in the ice

Sean Carroll

Yes-

Sean Carroll

… they're looking for flashes in the ice.

Sean Carroll

It's a bunch of strings, many, many, many strings with 360 degree photodetectors. You will-

Sean Carroll

It's extremely cool. They've done amazing work, and they find neutrinos.

Sean Carroll

Yeah. So the whole point is most cosmic rays are protons because why? Because protons exist, and they're massive enough that you can accelerate them to very high energies. So high-energy cosmic rays tend to be protons. They also tend to hit the Earth's atmosphere and decay into other particles. So neutrinos on the other hand, punch right through, at least usually, to a great extent, so not just Antarctica, but the whole earth. Occasionally, a neutrino will interact with a particle here on earth, and there's neutrinos is going through your body all the time from the sun, from the universe, etc. So if you're patient enough and you have a big enough part of the Antarctic ice sheet to look at, the nice thing about ice is it's transparent, so nature has built you a neutrino detector. That's what IceCube does.

Sean Carroll

It's much more dense than air, but it's transparent.

Sean Carroll

That's all you need. There's not that much ice-

Sean Carroll

… right? Yeah.

Sean Carroll

There's more ice in Antarctic than anywhere else. Right. So anyway, you can go and you can get a plot from the IceCube experiment, how many neutrinos there are that they've detected with very high energies. We predict in our weird little holographic guessing game that there should be a cutoff. You should see neutrinos as you get to higher and higher energies and then they should disappear. If you look at the data, their data gives out exactly where our cutoff is. That doesn't mean that our cutoff is right, it means they lose the ability to do the experiment exactly where we predict the cutoff should be.

Sean Carroll

Oh, just because there are fewer, fewer high-energy neutrinos. So there's a spectrum and it goes down, but what we're plotting here is-

Sean Carroll

… number of neutrinos versus energy, it's fading away, and they just get very, very few.

Sean Carroll

Our effect is a little bit bigger for higher energies, yeah.

Sean Carroll

Let me just mention the name of Oliver Friedrich, who was a post-doc who led this. He deserves the credit for doing this. I was a co-author and a collaborator and I did some work, but he really gets the lion's share.

Sean Carroll

There's very interesting things that happens once you're a theoretical physicist, once you become trained. You're a graduate student, you've written some papers and whatever, suddenly you are the world's expert in a really infinitesimally tiny area of knowledge and you know not that much about other areas. There's an overwhelming temptation to just drill deep, just keep doing basically the thing that you started doing, but maybe that thing you started doing is not the most interesting thing to the world or to you or whatever. So you need to separately develop the capability of stepping back and going, " Okay, now that I can write papers in that area, now that I'm trained enough in the general procedure, what is the best match between my interests, my abilities and what is actually interesting?" Honestly, I've not been very good at that over my career.

My process traditionally was I was working in this general area of particle physics, field theory, general relativity, cosmology, and I would try to take things other people were talking about and ask myself whether or not it really fit together. So I guess I have three papers that I've ever written that have done super well in terms of getting cited and things like that. One was my first ever paper that I get very little credit for, that was my advisor and his collaborator set that up. The other two were basically, my idea. One was right after we discovered that the universe was accelerating. So in 1998 observations showed that not only is the universe expanding, but it's expanding faster and faster. So that's attributed to either Einstein's cosmological constant or some more complicated form of dark energy, some mysterious thing that fills the universe.

---

**Dark energy**

People were throwing around ideas about this dark energy stuff, "What could it be?" And so forth. Most of the people throwing around these ideas were cosmologists. They work on cosmology. They think about the universe all at once. Since I like to talk to people in different areas, I was more familiar than average with what a respectable working particle physicist would think about these things. What I immediately thought was, "You guys are throwing around these theories. These theories are wildly unnatural. They're super finely tuned. Any particle physicist would just be embarrassed to be talking about this." But rather than just scoffing at them, I sat down and asked myself, "Okay, is there a respectable version? Is there a way to keep the particle physicists happy but also make the universe accelerate?" I realized that there is some very specific set of models that is relatively natural, and guess what? You can make a new experimental prediction on the basis of those, and so I did that. People were very happy about that.

Sean Carroll

So the fact that dark energy pervades the whole universe and is slowly changing, that should immediately set off alarm bells because particle physics is a story of length scales and time scales that are generally, guess what? Small, right? Particles are small. They vibrate quickly, and you're telling me now I have a new field and its typical rate of change is once every billion years. That's just not natural. Indeed, you can formalize that and say, look, even if you wrote down a particle that evolved slowly over billions of years, if you let it interact with other particles at all, that would make it move faster, its dynamics would be faster, its mass would be higher, et cetera, et cetera. So there's a whole story. Things need to be robust, and they all talk to each other in quantum field theory.

So how do you stop that from happening? The answer is symmetry. You can impose a symmetry that protects your new field from talking to any other fields, and this is good for two reasons. Number one, it can keep the dynamics slow. So you can't tell me why it's slow. You just made that up, but at least it can protect it from speeding up because it's not talking to any other particles. The other is, it makes it harder to detect. Naively, experiments looking for fifth forces or time changes of fundamental constants of nature like the charge of the electron, these experiments should have been able to detect these dark energy fields, and I was able to propose a way to stop that from happening.

Sean Carroll

The detection, yeah, because a symmetry could stop it from interacting with all these other fields, and therefore, it makes it harder to detect. Just by luck, I realized, 'cause it was actually based on my first-ever paper, there's one loophole. If you impose these symmetries, so you protect the dark energy field from interacting with any other fields, there's one interaction that is still allowed that you can't rule out. It is a very specific interaction between your dark energy field and photons, which are very common, and it has the following effect: As a photon travels through the dark energy, the photon has a polarization, up, down, left, right, whatever it happens to be, and as it travels through the dark energy, that photon will rotate its polarization. This is called birefringence. You can run the numbers and say you can't make a very precise prediction, 'cause we're making up this model.

But if you want to roughly fit the data, you can predict how much polarization, rotation, there should be, a couple of degrees, not that much. So that's very hard to detect. People have been trying to do it. Right now, literally, we're on the edge of either being able to detect it or rule it out using the cosmic microwave background. There is just truth in advertising, there is a claim on the market that it's been detected, that it's there. It's not very statistically significant. If I were to bet, I think it would probably go away. It's very hard thing to observe. But maybe as you get better and better data, cleaner and cleaner analysis, it will persist, and we will have directly detected the dark energy.

Sean Carroll

Well, two ways. One way is, those people would've had to say the same thing when we discovered the planet Neptune, 'cause it's exactly analogous where we have a very good theory, in that case, Newtonian gravity in the solar system. We made predictions. The predictions were slightly off for the motion of the outer planets. You found that you could explain that motion by positing something very simple, one more planet in a very, very particular place, and you went and looked for it, and there it was. That was the first successful example of finding dark matter in the universe.

Sean Carroll

Neptune was dark.

Sean Carroll

There's a difference between dark matter and dark energy. Dark matter as far as we are hypothesizing it is a particle of some sort. It's just a particle that interacts with us very weakly. So we know how much of it there is. We know more or less where it is. We know some of its properties. We don't know specifically what it is. But it's not anything fundamentally mysterious, it's a particle. Dark energy is a different story. So dark energy is indeed uniformly spread throughout space and has this very weird property that it doesn't seem to evolve as far as we can tell. It's the same amount of energy in every cubic centimeter of space from moment to moment in time. That's why far and away the leading candidate for dark energy is Einstein's cosmological constant.

The cosmological constant is strictly constant, 100% constant. The data say it better be 98% constant or better, so 100% constant works, and it's also very robust. It's just there. It's not doing anything. It doesn't interact with any other particles. It makes perfect sense. Probably the dark energy is the cosmological constant. The dark matter, super important to emphasize here. It was hypothesized at first in the '70s and '80s mostly to explain the rotation of galaxies. Today, the evidence for dark matter is both much better than it was in the 1980s and from different sources. It is mostly from observations of the cosmic background radiation or of large scale structure.

From observations of the cosmic background radiation or of large-scale structure. We have multiple independent lines of evidence, also gravitational lensing and things like that, many, many pieces of evidence that say that dark matter is there and also that say that the effects of dark matter are different than if we modified gravity. That was my first answer to your question is dark matter we have a lot of evidence for. But the other one is of course we would love it if it weren't dark matter. Our vested interest is 100% aligned with it being something more cool and interesting than dark matter because dark matter's just a particle. That's the most boring thing in the world.

Sean Carroll

Absolutely. Yeah.

Sean Carroll

You can even see maps of it that we've constructed from gravitational lensing.

Sean Carroll

Bigger than the galaxy, sadly. We think that in the galaxy dark matter is lumpy, but it's weaker, its effects are weaker. But on the scale of large scale structure and clusters of galaxies and things like that, yes, we can show you where the dark matter is.

Sean Carroll

The super cool explanation would be modifying gravity rather than inventing a new particle. Sadly, that doesn't really work. We've tried. I've tried. That's my third paper that was very successful. I tried to unify dark matter and dark energy together. That was my idea. That was my aspiration, not even idea. I tried to do it. It failed even before we wrote the paper. I realized that my idea did not help. It could possibly explain away the dark energy, but it would not explain the way the dark matter, and so I thought it was not that interesting, actually. And then two different collaborators of mine said, "Has anyone thought of this idea?" They thought of exactly the same idea completely independently of me. And I said, "Well, if three different people found the same idea, maybe it is interesting," and so we wrote the paper. And yeah, it was very interesting. People are very interested in it.

Sean Carroll

Here is what we know about dark matter and dark energy: They become important in regimes where gravity is very, very, very weak. That's the opposite from what you would expect if you actually were modifying gravity. There's a rule of thumb in quantum field theory, et cetera that new effects show up when the effects are strong. We understand weak fields, we don't understand strong fields. But okay, maybe this is different.

What do I mean by when gravity is weak? The dark energy shows up late of the universe. Early in the history of the universe, the dark energy is irrelevant, but remember the density of dark energy stays constant. The density of matter and radiation go down. At early times, the dark energy was completely irrelevant compared to matter and radiation. At late times, it becomes important. That's also when the universe is dilute and gravity is relatively weak.

Now think about galaxies. A galaxy is more dense in the middle, less dense on the outside. And there is a phenomenological fact about galaxies that in the interior of galaxies you don't need dark matter. That's not so surprising because the density of stars and gas is very high there and the dark matter is just subdominant. But then there's generally a radius inside of which you don't need dark matter to fit the data, outside of which you do need dark matter to fit the data. That's again when gravity is weak.

I asked myself, "Of course, we know in field theory new effects should show up when fields are strong, not weak, but let's throw that out of the window. Can I write down a theory where gravity alters when it is weak?" And we've already said what gravity is. What is gravity? It's the curvature of space-time. There are mathematical quantities that measure the curvature of space-time. And generally, you would say, "I have an understanding, Einstein's equation," which I explained to the readers in the book, "relates the curvature of space-time to matter and energy. The more matter and energy, the more curvature." I'm saying what if you add a new term in there that says, "The less matter and energy, the more curvature"? No reason to do that except to fit the data. I tried to unify the need for dark matter and the need for dark energy.

Sean Carroll

Super cool. It'd be the best. It'd be great. It didn't work.

Sean Carroll

That was exactly the hope.

Sean Carroll

But the great thing about physics is there are equations. You can come up with the words and you can wave your hands, but then you got to write down the equations; and I did. And I figured out that it could help with the dark energy, the acceleration of the universe; it doesn't help with dark matter at all. Yeah.

Sean Carroll

Yeah, it does. I agree. I tear my hair out when people who are not physicists accuse physicists, like you say, of losing the plot because they need dark matter and dark energy. I don't want dark matter and dark energy; I want something much cooler than that. I've tried. But you got to listen to the equations and to the data.

Sean Carroll

Well, by the way, this is not awesomeness, this is impact.

Sean Carroll

Right?

Sean Carroll

There's no correlation between awesomeness and impact. Some of my best papers fell without a stone and vice versa.

Sean Carroll

Yeah. The first paper was called Limits on the Lorentz and Parity Violating Modification of Electromagnetism… Or Electrodynamics. We figured out how to violate Lorentz invariance, which is the symmetry underlying relativity. And the important thing is we figured out a way to do it that didn't violate anything else and was experimentally testable. People love that. The second paper was called Quintessence and the Rest of the World. Quintessence is this dynamical dark energy field. The rest of the world is because I was talking about how the quintessence field would interact with other particles and fields and how to avoid the interactions you don't want. And the third paper was called Is Cosmic Speed-Up Due to Gravitational Physics? Something like that. You see the common theme. I'm taking what we know, the standard model of particle physics, general relativity, tweaking them in some way, and then trying to fit the data

Sean Carroll

Ideally, yes, that's right. That's the goal.

---

**Quantum mechanics**

Sean Carroll

Quantum mechanics is a harder one. I wrote a textbook on general relativity, and I started it by saying, "General relativity is the most beautiful physical theory ever invented." And I will stand by that. It is less fundamental than quantum mechanics, but quantum mechanics is a little more mysterious. It's a little bit kludgy right now. If you think about how we teach quantum mechanics to our students, the Copenhagen interpretation, it's a God-awful mess. No one's going to accuse that of being very beautiful. I'm a fan of the many-worlds interpretation of quantum mechanics, and that is very beautiful in the sense that fewer ingredients, just one equation, and it could cover everything in the world.

It depends on what you mean by beauty, but I think that the answer to your question is quantum mechanics can start with extraordinarily austere, tiny ingredients and in principle lead to the world. That boggles my mind. It's much more comprehensive. General relativity is about gravity, and that's great. Quantum mechanics is about everything and seems to be up to the task. And so I don't know, is that beauty or not? But it's certainly impressive.

Sean Carroll

It's a monist theory. In classical mechanics, I have a particle here, particle there; I describe them separately. I can tell you what this particle's doing, what that particle's doing. In quantum mechanics, we have entanglement, as Einstein pointed out to us in 1935. And what that means is there is a single state for these two particles. There's not one state for this particle, one state for the other particle. And indeed, there's a single state for the whole universe called the wave function of the universe, if you want to call it that. And it obeys one equation. And is our job then to chop it up, to carve it up, to figure out how to get tables and chairs and things like that out of it.

Sean Carroll

I'm a big boy. I can take it.

Sean Carroll

Sure. The classic experiment to explain quantum mechanics to people is called the Stern-Gerlach experiment. You're measuring the spin of a particle. And in quantum mechanics, the spin is just a spin. It's the rate at which something is rotating around in a very down to earth sense, the difference being is that it's quantized. For something like a single electron or a single neutron, it's either spinning clockwise or counterclockwise. Let's put it this way. Those are the only two measurement outcomes you will ever get. There's no it's spinning faster or slower, it's either spinning one direction or the other. That's it. Two choices. According to the rules of quantum mechanics, I can set up an electron, let's say, in a state where it is neither purely clockwise or counterclockwise but a superposition of both. And that's not just because we don't know the answer, it's because it truly is both until we measure it. And then when we measure it, we see one or the other. This is the fundamental mystery of quantum mechanics is that how we describe the system when we're not looking at it is different from what we see when we look at it.

We teach our students in the Copenhagen way of thinking is that the act of measuring the spin of the electron causes a radical change in the physical state. It spontaneously collapses from being a superposition of clockwise and counterclockwise to being one or the other. And you can tell me the probability that that happens, but that's all you can tell me. And I can't be very specific about when it happens, what caused it to happen, why it's happening, none of that. That's all called the measurement problem of quantum mechanics.

Many-worlds just says, "Look, I just told you a minute ago that there's only one way function for the whole universe, and that means that you can't take too seriously just describing the electron, you have to include everything else in the universe." In particular, you clearly have to interact with the electron in order to measure it. Whatever is interacting with the electron should be included in the wave function that you're describing. And look, maybe it's just you, maybe your eyeballs are able to perceive it, but okay, I'm going to include you in the wave function. Since you have a very sophisticated listenership, I'll be a little bit more careful than average. What does it mean to measure the spin of the electron? We don't need to go into details, but we want the following thing to be true: If the electron were in a state that was 100% spinning clockwise, then we want the measurement to tell us it was spinning clockwise. We want your brain to go, "Yes, the electron was spinning clockwise." Likewise, if it was 100% counterclockwise, we want to see that, to measure that.

The rules of quantum mechanics, the Schrodinger equation of quantum mechanics, is 100% clear that if you want to measure it clockwise when it's clockwise and measure it counterclockwise when it's counterclockwise, then when it starts out in a superposition, what will happen is that you and the electron will entangle with each other. And by that I mean that the state of the universe evolves into part saying, "The electron was spinning clockwise, and I saw it clockwise," and part of the state is it's in a superposition with the part that says, "The electron was spinning counterclockwise, and I saw it counterclockwise." Everyone agrees with this; entirely uncontroversial. Straightforward consequence of the Schrodinger equation.

And then Niels Bohr would say, "And then part of that wave function disappears," and we're in the other part. And you can't predict which part it'll be, only the probability. Hugh Everett, who was a graduate student in the 1950s, was thinking about this, says, "I have a better idea. Part of the wave function does not magically disappear, it stays there." The reason why that idea, Everett's idea that the whole wave function always sticks around and just obeys the Schrodinger equation was not thought of years before is because naively, you look at it and you go, "Okay, this is predicting that I will be in a superposition, that I will be in a superposition of having seen the electron be clockwise and having seen it be counterclockwise." No experimenter has ever felt like they were in a superposition. You always see an outcome.

Everett's move, which was genius, was to say, "The problem is not the Schrodinger equation. The problem is you have misidentified yourself in the Schrodinger equation." You have said, "Oh, look, there's a person who saw counterclockwise, there's a person who saw clockwise; I should be that superposition of both." And Everett says, "No, no, no, you're not," because the part of the wave function in which the spin was clockwise, once that exists, it is completely unaffected by the part of the wave function that says the spin was counterclockwise. They are apart from each other. They are un-interacting. They have no influence. What happens in one part has no influence in the other part. Everett says, "The simple resolution is to identify yourself as either the one who saw spin clockwise or the one who saw spin counterclockwise." There are now two people once you've done that experiment. The Schrodinger equation doesn't have to be messed with, all you have to do is locate yourself correctly in the wave function. That's many-worlds.

Sean Carroll

Very big.

Sean Carroll

The short answer is the worlds don't exist in space, space exists separately in each world. There's a technical answer to your question, which is Hilbert space, the space of all possible quantum mechanical states, but physically, we want to put these worlds somewhere. That's just a wrong intuition that we have. There is no such thing as the physical spatial location of the worlds because space is inside the worlds.

Sean Carroll

That's right.

Sean Carroll

They are existing separately and simultaneously.

Sean Carroll

Without locations in space.

Sean Carroll

It is not. I don't think any human being can visualize four-dimensional spacetime properly either. We just do our best. We have an intuition. We can work with the math. We can say, "Okay, look, in the Hilbert space of all possible quantum mechanical states, there is a particular partition of this Hilbert space into parts that are essentially non-interacting with each other. In principle, you are in one of those parts. You will never see the other parts."

You can talk about it but you can't visualize it. That's okay. The point is, you don't need to be visualizing it. You can understand it because you're using the mathematics as your language of understanding. That's fine. What's crucial is that if you adopt the many-worlds interpretation, the Schrodinger equation applies. You've gotten rid of any weird collapse postulate. You just have one set of laws that governs what happens in the universe, which is the Schrodinger equation. That's a big deal. It gets rid of the measurement problem.

Sean Carroll

Well, it's controversial because a lot of people don't believe in it or think that it's silly or think that it has problems. I think those people are wrong, but you know, that's okay.

Sean Carroll

Oh, there are lots of objections. Some of them are philosophical. "It's unparsimonious. It multiplies entities," say people who haven't read Everett. Everett's whole point was to multiply fewer entities. You have fewer assumptions. You have one equation. You have one wave function evolving according to the Schrodinger equation. That's the opposite of multiplying entities. You're reducing them. The other objection is, "Oh, it's unfalsifiable. We can never test it." Well, that's not quite right. The point is that many-worlds makes exactly the same predictions as other interpretations of quantum mechanics for experiments you can do. That's the whole point. The different interpretations of quantum mechanics all give the same experimental predictions for what you will observe in your laboratory.

Sean Carroll

Because it's true. No, I'm just kidding. Well, the answer is, what you're saying is right. The many-worlds interpretation and other interpretations all make the same predictions. But the question is, "Which one is really true?" And the answer to that is a question of what is real. That goes beyond what you will observe in your laboratory, and I think that's okay. I think that metaphysics and truth questions about reality are legitimate scientific questions.

Sean Carroll

No, I don't think so. In order to do science, you need to make predictions about what you will observe in your laboratory. You do not need, in order to answer metaphysical questions about what is real, you do not need to make falsifiable predictions about those metaphysical questions. That's the job of science, is to answer questions about what we observe. The question of what is real, which interpretation is true, is a different question, and it's a question that you can answer using the tools of science, by finding the simplest and most elegant theory that makes the right predictions.

Sean Carroll

Decoherence is the process by which the different branches of the wave function of the universe become unable to interfere with each other. I mentioned earlier that the branches are non-interacting. They're not actually non-interacting; they're technically still part of the same wave function. But they decohere. Decoherence is a very well understood process in modern quantum mechanics. It's why we can treat open quantum systems classically in practice. It's a very important and useful concept. But the point is, once you have decoherence, the branches don't interact anymore. For all practical purposes they're separate. In the many-worlds interpretation, we say they're literally separate branches. That's it. That's the resolution. The Schrodinger equation applies. And the fact that we experience one branch doesn't mean anything weird is happening. We just experience one branch out of all the branches that exist.

Sean Carroll

The Copenhagen interpretation says that the wave function is just a tool for making predictions about what you will observe. The wave function collapses when you make an observation. There is not a reality to the wave function except insofar as it helps you make predictions. The many-worlds interpretation says the wave function is real. It exists. It's the complete description of reality. The Schrodinger equation applies all the time, everywhere. The wave function evolves deterministically according to the Schrodinger equation, and there's nothing else. It doesn't collapse.

The Copenhagen interpretation has this weird distinction between the observer and the observed system. The moment you measure something, the wave function spontaneously collapses. No one knows how this happens or why it happens. It's magic. In the many-worlds interpretation, there's no magic. It's just the Schrodinger equation. There's no observer-observed distinction. Observer and observed are part of the same universe. They're part of the same wave function.

The cost, so to speak, of the many-worlds interpretation is that you have many worlds. You end up with branching. But the benefit is that you get rid of all the weirdness of wavefunction collapse. You get rid of the measurement problem. You have one equation that governs everything. That's a big deal.

Sean Carroll

Yes. It happens continuously. At every moment in time, decoherence is happening. Different parts of the wave function are branching off.

Sean Carroll

It's not doubling, because branching is not a discrete process. It's continuous. And the number of branches isn't even the right way to think about it. You have a wave function, and in many-worlds, there's the wave function that exists. In that wave function, there are different branches that are separated from each other. You're in one branch and you're not in another branch. The branching is continuous.

Sean Carroll

Exactly. Your experience is determined by which branch of the wave function you're in. And you will never see another branch. And there's no fact of the matter about what's going on in another branch. At least, that's what many-worlds says. Some people debate this, but the standard many-worlds view is that once you're in a branch, you're in that branch, and you don't see the other branches.

Sean Carroll

No. By definition, if the branches are decohered from each other, then they don't interact, so you don't see the effects.

---

**Simulation**

Sean Carroll

I don't have strong beliefs about this one way or the other. The argument for simulation is that if we can build a simulation, and if people in the far future can build many simulations, then statistically most minds would be in simulations rather than base reality. So it might be that we're in a simulation. But there's no evidence for it.

The argument against it is that I don't see how you would tell. What would be a test of whether we're in a simulation? And also, if the simulation is perfect, then it doesn't matter. It's real to us. So I'm just not sure.

Sean Carroll

Well, I would ask them what the evidence is. What is the test of the simulation hypothesis? How would we know? And I think that's a question that no one has a good answer to. In the absence of evidence, I'm an agnostic on the simulation hypothesis.

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Sean Carroll

I think it's a very hard problem. I think that there are people who are very optimistic that we're close to AGI. I think there are other people who think it's much further away. I'm not sure which is right. But I do think that when we build AGI, we'll understand intelligence better. And we might be surprised at what intelligence turns out to be.

The thing about intelligence is that it's deeply connected to information processing. It's about taking information from the world around you and making predictions and making decisions. And that's something that physics and quantum mechanics and complexity theory can say a lot about. But it's also something that philosophy and neuroscience and psychology can say a lot about. So I think building AGI will teach us a lot about ourselves.

Sean Carroll

No, I'm not a pessimist. I think it's going to happen. And I think it's going to be amazing. The question is whether it's 10 years from now or 100 years from now.

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**Complexity**

Sean Carroll

Complexity is a very hard thing to define. If you ask physicists what complexity is, you'll get different answers. But I think that in some sense, complexity is about the amount of information that a system contains that is relevant for prediction or intervention. A random number has a lot of information but very little complexity because you can't predict it or use it to do anything. A crystal has very low complexity because it's very ordered and you can easily describe it. Complexity is somewhere in the middle. It's about systems that are neither completely random nor completely ordered. They have structure. They have patterns. They have information that you can use to make predictions.

Life is complex. Human brains are complex. Economies are complex. Galaxies are complex. The universe is complex. And understanding complexity is one of the great challenges of science.

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**Consciousness**

Sean Carroll

Consciousness is a hard problem. I don't have a complete theory of consciousness. But I think that consciousness is related to information integration. I think that consciousness arises when a system integrates information across many different parts of itself. That's an idea that's been developed by people like Giulio Tononi. I think there's something to it.

But I also think that we need to be careful about making claims about consciousness without evidence. We don't know what consciousness is. We don't know how it arises. We don't even have a good test for consciousness. We think humans are conscious. We think many animals are conscious. We're not sure about some animals. We definitely don't know about plants or computers or AI systems.

I think that the right attitude is to be humble about consciousness and to study it scientifically and try to understand it better.

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**Naturalism**

Sean Carroll

Naturalism is the view that the natural world is all that exists. There's no supernatural realm. There are no gods or spirits or supernatural forces. Everything that happens in the world happens through natural causes and natural laws. We can understand the world through science. We can study the natural world and discover the laws that govern it.

Now, that doesn't mean that naturalism is true. But it's the best working hypothesis we have. It's the basis of modern science. And I think it's correct. I think that the natural world is all that exists, and we can understand it through science.

But there are some people who don't agree with naturalism. There are people who think there are supernatural forces in the world. There are people who think that there are things that science can't explain. And those are legitimate philosophical positions. But I think the evidence is overwhelmingly in favor of naturalism.

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**Limits of science**

Sean Carroll

I think science is incredibly powerful. We can use science to understand the natural world and make predictions and build technology. But there are some things that are outside the scope of science. Science is about understanding natural laws and making predictions about the natural world. But there are philosophical questions about values and meaning and purpose that are outside the scope of science.

For example, the question "What should I do?" is not a question that science can answer. Science can tell you the consequences of your actions. But science can't tell you whether you should do something. That's a question of values. That's a question of philosophy or religion.

Also, science assumes that the natural world exists and that our observations are roughly accurate. But you can imagine a world in which those assumptions don't hold. You can imagine a world in which we're all in a simulation or dreaming. Science doesn't and can't rule out those possibilities.

But within the scope of natural philosophy, within the scope of understanding the natural world, I think science is incredibly powerful.

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**Mindscape podcast**

Sean Carroll

The mission is to have conversations with really smart people about big ideas. I want to talk to physicists and philosophers and biologists and economists and psychologists and artists and musicians and writers. I want to explore ideas that are at the frontier of our understanding. I want to understand what's on people's minds. I want to have long-form conversations where we can really dig into topics and explore them in depth.

And I do it because I think that ideas are important. I think that exploring big ideas is one of the most important things we can do. And I think that by talking to people who are thinking deeply about these topics, I can learn a lot. And I think that the audience can learn a lot too.

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**Einstein**

Sean Carroll

Einstein is great because he was willing to question fundamental assumptions. He was willing to challenge the status quo. He was willing to think deeply about nature and come up with new ideas.

In 1905, he questioned the idea that time is absolute. He questioned the idea of the ether. He came up with special relativity. In 1915, he questioned the idea that gravity is a force. He came up with general relativity. These are the kinds of breakthroughs that happen when someone is willing to challenge fundamental assumptions.

But Einstein is also great because he had a deep physical intuition. He was able to think about problems in a new way. He was able to imagine what the world would look like from a different perspective. He was able to take the mathematics and connect it to physical reality.

And I think that's what makes for greatness in physics. It's not just mathematical ability. It's not just getting the equations right. It's about having deep physical intuition and being willing to challenge fundamental assumptions and imagine new possibilities.

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**End of Transcript**