Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics | Lex Fridman Podcast #359

A black hole is a mirror. And the way it's a mirror is if light, a photon, bounces off your face towards the black hole, it goes straight to the black hole, just falls in, you never see it again.

Mm-hmm.

But if it just misses the black hole, it'll swing around the back and come back to you.

Yeah.

And you see yourself from the photon that went around the back of the black hole. But not only can that happen, the black hole, the photon can swing around twice and come back. So you actually see an infinite number of copies of yourself.

The following is a conversation with Andrew Strominger, theoretical physicist at Harvard, whose research seeks to shed light on the unification of fundamental laws of nature, the origin of the universe, and the quantum structure of black holes and event horizons. This is the Lex Fridman Podcast. To support it, please check out our sponsors in the description. And now, dear friends, here's Andrew Strominger. You are part of the Harvard Black Hole Initiative, which has theoretical physicists, experimentalists, and even philosophers. So, uh, let me ask the big question. What is a black hole from a theoretical, from an experimental, uh, maybe even from a philosophical perspective?

So a black hole is defined, theoretically, as a region of spacetime from which light can never escape, therefore it's black. Now, that's just the starting point. Many weird things, uh, follow from that basic definition, but that is, that is the basic definition.

What is light-

Well-

... that can't escape from a black hole?

Well, light is, uh, you know, the stuff that comes out of the sun, the stuff that goes into your eyes. Um, light is one of the, the stuff that disappears when the lights go off-

(laughs)

... the stuff that appears when the lights come on.

Yeah.

Um, of course, I could give you a mathematical definition, but, um, or physical mathematical definition, but, uh, I think it's something that we, uh, all understand very intuitively, uh, what is light. Black holes, on the other hand, we don't understand intuitively. They're very weird. And one of the questions is, about black holes, which I think you were alluding to, is, you know, why doesn't light get out, or how is it that there can be a region of spacetime from which light can't escape? It definitely happens. We've seen those regions. We have spectacular pictures, especially in the last several years, of those regions. Um, they're there. In fact, they're up in the sky, thousands or millions of them. We don't yet know how many. Um, but the proper explanation of why li- light doesn't escape from, uh, a black hole is still, uh, a matter of some debate. Um, and one explanation, which perhaps Einstein might have given, is that light carries energy. Um, you know it carries energy because, you know, we have, uh, photo cells and we can take the light from the sun and collect it, turn it into electricity. So there's energy in light. And anything that carries energy is subject to a gravitational pull. Gravity will pull at anything with energy. Now, it turns out that the gravital, gravitational pull exerted by an object, uh, is proportional to its mass. And so if you get enough mass in a small enough region, um, you, you can, uh, prevent light from escaping. And let me flesh that out a little more. Um, if you're on the earth, uh, and you're on a rocket ship leaving the s- the surface of the earth, and if we ignore the friction from the air, um, if your rocket accelerates up to 11 kilometers per second, that's escape velocity. And it can, if there were no friction, it could just continue forever to the next galaxy. On the moon, which has less mass, it's only seven kilometers per second. So, but going in the other direction, if you have enough mass in one place, the escape velocity can become the speed of light.