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Help me to envision a man falling towards a black hole with all of the time dilation involved

Physics Asked on February 17, 2021

I’m standing at a safe distance from a black hole and I’m in an Earthlike time frame of reference. Through my telescope I see a man starts to fall towards the black hole. I have looked at this question but it doesn’t quite get to what I’m looking for.

I watch him accelerate towards the black hole until I know that he is getting close to $c$. But, the gravitational time dilation surrounding the black hole would be so great that I would actually see him slow down rather than speed up? And then just a short distance above the event horizon, gravitational time dilation would be so great that I would see him nearly stop, so that even the 14 billion year ago of the universe would not be long enough for him to actually reach the event horizon. Help me to understand this? How can he be falling at near $c$ at the same time that he is nearly stopped?

Of course, in his own frame, he just sees himself falling towards the black hole and never reaching it in the short time involved. He would be facing both kinetic and gravitational time dilation.

Help me to envision this from my frame.

One Answer

Your eyes don't directly perceive the outside world. They only perceive light that enters them through the pupil and strikes the retina. To reach your eyes, the light has to travel from wherever it was emitted.

By the definition of the event horizon, light that's emitted inside the event horizon can't reach any point outside the event horizon, so as long as your eyes are outside the event horizon, you'll never see it. Light that's emitted outside the event horizon, but very close to it, can eventually reach your eyes, but it takes a long time to escape the black hole's gravity. As the point of emission approaches the horizon, the time the light takes to escape approaches infinity. Therefore, it takes forever for all of the light that was released before crossing the horizon to reach your eyes. (Classically, anyway. Quantum mechanically, there are only finitely many photons, and you'll see the last one at some finite time.)

The fact that you see the infalling person "frozen" at the horizon is an optical illusion, like a mirage. Your brain assumes that light travels instantaneously in straight lines from its origin point to your eye, and when that approximation isn't accurate, it gets confused. In reality, falling through the event horizon happens very quickly.

Answered by benrg on February 17, 2021

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