Lex Fridman: belief

Transcript context

…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. 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. 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.…

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