Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens

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. The number of worlds is ...

In book one of the series, The Biggest Ideas in the Universe called Space, Time, Motion, you take on classical mechanics, general relativity by taking on the main equation of general relativity and making it accessibly easy to understand. Maybe at the high level, what is general relativity? What's a good way to start to try to explain it? 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.

I think a nice way to test the difference between objective reality and the observed reality is what happens at the edge of the horizon of a black hole. Technically, as you get closer to that horizon, time stands still? 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.

What happens to you if you fall into the black hole? If we think of an object as information, that information gets destroyed. 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.

These supermassive black holes are formed somewhere early on in the universe. I mean, that's a feature, not a bug, that we don't have too many of them. Otherwise, we wouldn't have the time or the space to form the little pockets of complexity that we'll call humans. 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.

I don't know how we got on this- ... would be enough similarities that we would recognize it.

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. What was the thing that would make physicists happy that would make sense of this fragile thing that people call dark energy?

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.

You wrote the book Something Deeply Hidden on the mysteries of quantum mechanics and a new book coming out soon, part of that, Biggest Ideas in the Universe series we mentioned called Quanta and Fields. That's focusing on quantum mechanics. Big question first, biggest ideas in the universe, what to you is most beautiful or perhaps most mysterious about quantum mechanics? 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.

Do you think it's possible this whole place is simulated? Sure.

Yeah. The practical matter of going from a prototype that's impressive to a thing that governs everything. Similar question on this front is in AGI. You've said that we're very far away from AGI. I want to eliminate the phrase AGI.

Right before we went too far, and that's why we don't see aliens. So you're like I mentioned, associated with Santa Fe Institute. I just would love to take a stroll down the landscape of ideas explored there. Sure.

You've been critical on panpsychism. Yes, you've noticed that.

So, like you said, you are a believer of the mechanistic universe, you're a naturalist and, as you've described, a poetic naturalist. That's right.

Are there limits to science? Sure. We just talked about one, right? Science can't tell you right from wrong. You need science to implement your ideas about right and wrong. If you are functioning on the basis of an incorrect view of how the world works, you might very well think you're doing right but actually be doing wrong but all the science in the world won't tell you which action is right and which action is wrong.

We mentioned Mindscape Podcast, I love it. You interview a huge variety of experts from all kinds of fields so just several questions I want to ask. How do you prepare? How prepare to have a good conversation? How do you prepare in a way that satisfies, makes your own curious mind happy, all that kind of stuff? Yeah, no, these are great questions and I've struggled and changed my techniques over the years, it's a over five-year-old podcast, might be approaching six years old now. I started out over-preparing when I first started, I had a journey that I was going to go down. Many of the people I talk to are academics or thinkers who write books so they have a story to tell, I could just say, "Okay, give me your lecture and then, an hour later, stop." So, the mistake is to anticipate what the lecture would be and to ask the leading questions that would pull it out of them. What I do now is much more here are the points, here are the big questions that I'm interested in and so I have a much sketchier outline to start and then try to make it more of a real conversation.

You said that general relativity is the most beautiful theory ever. So far.