Sean Carroll: prediction

Transcript context

…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. You stand outside the black hole and I fall in. As I’m falling in, I’m waving to you because I’m going into the black hole, you will see me move more and more slowly. Also, the light for me is redshifted. I kind of look embarrassed, because I’m falling into a black hole. There is a limit. There’s a last moment that light will be emitted from me, from your perspective forever. Okay. Now you don’t literally see it because I’m emitting photons more and more slowly because from your point of view. It’s not like I’m equally bright. I basically fade from view in that picture. Okay. That’s one approximation. The other approximation is I do have a gravitational field of my own, and therefore as I approach the black hole, the black hole doesn’t just sit there and let me pass through. It moves out to eat me up because its net energy mass is going to be mine, plus its. But roughly speaking, yes, I think so. I don’t like to go to the dramatic extremes because that’s where the approximations break down. But if you see something falling into a black hole, you see its clock ticking more and more slowly. How do we know it fell in?…

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