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I know that if one astronaut falls into a black hole, then a distant observer will see him take an infinite amount of time to reach the event horizon (provided the observer can see light of arbitrarily large wavelengths).

But the falling astronaut will only take a finite amount of time to reach the horizon.

My question is: What will the falling astronaut see if he "looks backwards" while falling. Will he see the distant observer growing old and all the stars dying by the time he reaches the horizon ?

Cosmonut
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  • Also see https://physics.stackexchange.com/questions/82678/does-someone-falling-into-a-black-hole-see-the-end-of-the-universe – ProfRob Mar 31 '18 at 17:04

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No. He would see that only if he stopped falling close to and above the horizon, by thrusting or going into orbit, say. (No stable orbits exist too close.) An observer falling straight in won't see time speed up above him.

A way to visualize this is with the river model of black holes (googleable). An observer who flows with the river, by falling straight inward toward a center of gravity, doesn't notice time speed up above him. To notice that the astronaut must "fight the current" somehow. By fighting the current the astronaut can receive information that is "flowing with the river" faster than normal.

We are able to do that just sitting in our chairs. It just requires fine instruments to measure. If we jump off a cliff (or even a curb) we can no longer notice it.

finbot
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The other answer is incorrect. The correct answer is

Stuff behind you is slowed down, while stuff ahead-ish and to the sides appears sped up.

(see here for more details)

However, from the answers to this question, you wouldn't see the end of the universe as you approach the event horizon. There will be a definite cutoff, meaning the situation is not symmetric with outside observers who see you falling forever.

BlueRaja - Danny Pflughoeft
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Gravitation will cause the time to dilate. Move closer to extreme gravitation, and time will dilate more.

When time is dilated perfectly, completely flat, it ceases to tick - it will get slower and slower until it just stops ticking (technically time ceases). If our time is dilated perfectly and remains so, we will not experience anything within this frame because this frame has no time, relative to everyone else (the outside universe).

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In other words, a falling astronaut will have his time dilate until he gets to the event horizon and it will become totally flat. It means he will see the outside universe speed up, and then nothing - when time is totally dilated flat, his frame of reference will be as frozen in respect to the rest of the universe.

He won't see the end of the universe and the end of time. It will all happen very quickly, and in his frame, the outside will “simply” speed up, but not to the literal end of time because once his time is dilated that much in his frame of reference, he (or anything) will stop experiencing.

What if we could go to the event horizon and come back? What would we see?

We will see the rest of the universe speed up more and more, and when we reach the event horizon, we will stop observing anything. Only when we leave the black hole (somehow magically) our time dilation begins to dilate less, and we will begin to experience something again: The rest of the universe will seem - to us - to have sped up, and then aged instantaneous in this interval of perfect dilation, and then speed less and less until our dilation becomes closer and closer to that of the referenced frame.

How much the universe aged instantaneous, be it ten minutes, or a trillion years depends on how long (from the observer's point of view) we were in the interval of perfect dilation.

My point is: the dilation will speed up the history of the outside universe, but not to the end of time.

If my perspective is wrong, tell me where my logic fails? Cheers :) ——-

Jon Therkildsen
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