Janna Levin: evaluation

Transcript context

…So in that way, many other ways were kindred spirits. So let’s go in one of the coolest objects in the universe, black holes. What are they? And maybe even a good way to start is to talk about how are they formed. In a way, people often confuse how they’re formed with the concept of the black hole in the first place. So when black holes were first proposed, Einstein was very surprised that such a solution could be found so quickly, but really thought nature would protect us from their formation. And then nature thinks of a way. Nature thinks a way to make these crazy objects, which is to kill off a few stars. But then I think that there’s a confusion that dead stars, these very, very massive stars that die, are synonymous with the phenomenon of black hole. And it’s really not the case. Black holes are more general and more fundamental than just the death state of a star. But even the history of how people realize that stars could form black holes is quite fascinating because the entire idea really just started as a thought experiment. And if you think of it’s 1915, 1916, when Einstein fully describes relativity in a way that’s the canonical formulation. It was a lot of changing back and forth before then. And it’s World War I, And he gets a message from the eastern front from a friend of his, Karl Schwarzschild, who solved Einstein’s equations between sitting in the trenches and cannon fire, it was joked that he was calculating ballistic trajectories. He’s also perusing the proceedings of the Prussian Academy of Sciences, as you do. And he was an astronomer who had enlisted in his forties. And he finds this really remarkable solution to Einstein’s equations. And it’s the first exact solution. He doesn’t call it a black hole, it’s not called a black hole for decades. But what I love about what Schwarzschild did is it’s a thought experiment. It’s not about observations, it’s not about making these things in nature. It’s really just about the idea. He sets up this completely untenable situation. He says, “Imagine I crush all the mass of a star to a point. Don’t ask how that’s done, because that’s really absurd, but let’s just pretend and let’s just imagine that that’s a scenario.” And then he wants to decide what happens to spacetime if I set up this confounding, but somehow very simple scenario. And really what Einstein’s equations were telling everybody at the time was that matter and energy, curved space and time, and then curved spacetime tells matter and energy how to fall once the spacetime is shaped. So he finds this beautiful solution. And the most amazing thing about a solution is he finds this demarcation, which is the event horizon, which is the region beyond which not even light can escape. And if you were to ask me today, all these decade, over a hundred years later, I would say that is the black hole. ich is the event horizon, which is the region beyond which not even light can escape. And if you were to ask me today, all these decade, over a hundred years later, I would say that is the black hole. The black hole is not the mass crushed to a point. The black hole is the event horizon. And the event horizon is really just a point in spacetime or a region at spacetime. It’s actually in this case, a surface in spacetime. And it marks a separation in events, which is why it’s called an event horizon. Everything outside is causally separated from the inside, insofar as what’s inside the event horizon can’t affect events outside. What’s outside can affect events inside. I can throw a probe into a black hole and cause something to happen on the inside. But the opposite isn’t true. Somebody who fell in can’t send a probe out. And this one way aspect really is what’s profound about the black hole. Sometimes we talk about the black holes being nothing because at the event horizon, there’s really nothing there. Sometimes when we think about black holes, we want to imagine a really dense dead star. But if you go up to the event horizon, it’s an empty region of spacetime. It’s more of a place than it is a thing. And Einstein found this fascinating. He helped get the work published, but he really didn’t think these would form in nature. I doubt Karl Schwarzschild did either. I think they thought they were solving theoretical mathematical problems, but not describing what turned out to be the end state of gravitational collapse.…

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