Particles of Thought

HAKEEM: So before we go further, let's define what a black hole is. Let's do black hole 101. Tell the audience how you define a black hole.

JANNA: Yeah, I think there's many different ways to approach it. Most people will say an object so dense that not even light can escape. And there's good things about that description, there's really bad things about that description. So a good thing about the description is that it captures one of the most fundamental attributes of the black hole, and that is that it goes completely dark. Nothing will ever escape, not even light. And so that part is the crux in some sense of what a black hole is. We say that around the black hole. We circumscribe it with the region around which light can no longer escape, we call that the event horizon.

What's bad about that description is that there's nothing there. There's no dense object-

HAKEEM: At the event horizon.

JANNA: ... at the event horizon. Right? So I don't like the image of a thing, a thing that's just really, really dense. Now it's true, often black holes are made by dense things on their way out. But at the event horizon where the light is forced to fall in, there's just empty space. So the black hole is just a space-time.

HAKEEM: A volume of space.

JANNA: Right. It's more of a place in some sense than a thing.

HAKEEM: Than a thing, yeah.

JANNA: Yeah, so I can go up to this event horizon and if I'm trying to shine a flashlight and I'm shining light, the light as I get closer will be trapped in orbit at some point around the black hole. And then eventually as I get closer, it won't even be able to do that anymore.

HAKEEM: It's just going straight in.

JANNA: And it's just going straight in, as am I. But I don't bump into any matter, I don't smack into any surface. I just sail across an empty space. Yeah. So, what are black holes?

HAKEEM: How disappointing.

JANNA: Well, I mean, honestly, you might not even notice anything bad was happening to you. It would be no more dramatic in some sense than stepping into the shadow of a tree. It's just a shadow.

HAKEEM: So suddenly, you're cut off.

JANNA: Yeah. So if you say, what is a black hole? I would say the black hole is really the event horizon. It's this horizon beyond which light cannot escape.

HAKEEM: If you're looking out, you can see light coming in though, right?

JANNA: That's right. So if you fall in, it can be bright on the inside because the light can fall in from all the stars shining, all the galaxies, all of that can fall in behind you, and you can see all of this happening. So it can be right on the inside, it's just dark on the outside.

HAKEEM: So, let's do an experiment.

JANNA: It's a one-way transition.

HAKEEM: So, suppose the light's coming in toward you and you hold up a mirror to reflect the light back out. What happens?

JANNA: Yeah. It's tricky because we are standing on the outside of a black hole, let's say, and we're throwing that light in and your companion has fallen in ahead of you. We are imagining that interior to that event horizon, that that is a spatial direction. I mean, our intuition says it. It says once you cross inside, there's a spatial direction pointing towards the center of a point in space, but space and time are relative.

HAKEEM: Okay, absolutely.

JANNA: And to the observer who has fallen inside, they're very rotated relative to your space and time, what they're calling space and what your calling space are now very misaligned.

HAKEEM: Wow.

JANNA: And so that direction for the observer who fell in that points towards what we sometimes call the singularity, that is a direction in time now. So the person falling in can no more bounce the light that comes in behind them, back out than they can bounce the light backward in time.

HAKEEM: Wow.

JANNA: So there is no such option to do that.

HAKEEM: There is no such option.

JANNA: Nor would they imagine such an option.

HAKEEM: Wow.

JANNA: Because to them, they in the light are continuing to fall forward in time, and that is driving them in the same direction towards the center.

HAKEEM: So, let me create an analogy.

JANNA: So it's tricky, I'm not saying it's-

HAKEEM: Is it like refraction where you see the spoon in the water, and so this was going in that direction, but now it looks like it's somewhere else is disconnected, but it's more extreme?

JANNA: Yeah. I mean-

HAKEEM: So the event horizon would be like the surface of the water.

JANNA: Yeah, I would say there is, you can bounce the light in different spatial directions. Just that direction is no longer space at all. So it really is-

HAKEEM: Oh, wow.

JANNA: It really is in your past, the event horizon. So there's no even turning around anymore. And the light can fall behind you, you can see things, but you can't claw your way back, nor can you send anything back that way. Now, if you could travel back in time, we could get tricky and start to talk about things like that. But then you're doing stuff on the outside that's pretty crazy too.