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I am by no means an expert in the realm of physics. I do from time to time, try to understand the concepts of modern physics and their applications.

I came across this video that I am currently watching, and in the beginning it explains what would happen to someone if they crossed over the event horizon of a black hole.

From my understanding, the time dilation is so great near a black hole, that if you were to cross over the event horizon someone viewing you from further away would never actually see you cross and you would appear frozen forever at the edge of the event horizon due to this time dilation near great mass.

If this is true, in the sense that the entire existence of the universe outside the black hole would unfold while you were stuck at the edge of the event horizon due to time dilation (Please fact check me here, I would love to know if time does actually stop for you if you cross the event horizon of a black hole), wouldn't it be true that you would actually spend no time at all inside the black hole due to hawking radiation and it's effect on the evaporation of black holes?

What I mean by this is that, apparently from what I've read, a black hole will evaporate and cease to exist at the end of some finite period of time in the universe due to it giving off hawking radiation. If this is true, and if the fact that you appear to be forever frozen at the event horizon due to time dilation is also true, wouldn't it seem from the perspective of the person crossing the event horizon that the black hole ceases to exist?

I am very intrigued by this possible paradox that I was questioning and I would love to know more about it's ramifications, if there are any, from many of you whom are more advanced that I.

Thank you in advance.

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marked as duplicate by John Rennie, Kyle Kanos, ACuriousMind, Ryan Unger, tom Jul 22 '15 at 11:30

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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I will try and answer this in a very simple way. I think you are missing a crucial piece here. It is true that from the perspective of a remote "outside" observer (Bob) the person falling into the black hole (Alice) will never cross the horizon. However from Alice's perspective, nothing unusual will happen (in terms of laws of physics not biology, obviously), she will fall into the hole and continue to record normal coherent laws of physics until she reaches the singularity. So yes, if Bob was to spend all his time observing Alice till the black hole evaporates into Hawking radiation products (10^67 years, which is huge) then throughout that time, he will never see Alice actually crossing over. However remember redshift here. Alice is going to redshift beyond Bob's ability to "see" her even before the time-dilation makes it seem like she is frozen near the horizon.

Beyond this simple explanation Black holes are theorized to exhibit the Holographic principle (in fact the theory is that the entire Universe is holographic, but that's a topic for another thread). In holographic theory, all the information (information here is everything broken down into quantum bits that can be expressed over the Planck's length, the smallest unit of space possible) that is inside a black hole is actually encoded on it's surface. I am not going to get into the details of this, but it is unique and surprising phenomenon (theorized obviously). So Alice on the surface, redshifted and frozen is in a way a scrambled hologram of Alice inside the black hole. Now, the way I have explained this is a little misleading because words are not enough to explain this and I am not sure if you are interested in the mathematics. Don't imagine Alice as looking like Alice and stuck on the surface. Alice on the surface is going to be scrambled beyond recognition but crucially, not lost.

Perhaps you can look up The Holographic Principle. Let me know, though, if you want me to add additional information.

Does that help?

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  • $\begingroup$ An interesting interpretation would be that 1) Yes, Alice is frozen on the surface for infinite time, yet 2) another Alice inside the black hole that was sticked to its surface from infinite past (we assume eternal Schwartzshield solution) unsticks from the surface and falls to the singularity. Something like this. $\endgroup$ – Anixx Sep 30 '15 at 7:11
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If 0celo7 didn't make sense to you: Nothing special happens to the observer that crosses the horizon (according to classical GR), he just falls in and gets crushed.

It appears like someone is "stuck" to someone outside the black hole, but in a sense that are just photons that were massively delayed by the gravitational field. And this processes is very quick, so after a very short time the object that fell into the black hole is already too dim to see.

P.S. In case you discover another paradox soon: xkcd.com/675 (Ask anyway, but perhaps choose a different title ;)

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  • $\begingroup$ Gravitational field cannot delay photons. If you see something, it exists at the time you see it or at least existed short time before, only the time it takes for light to reach you (and its speed is constant). If you see the falling observer to stick forever, this means he sticked forever indeed $\endgroup$ – Anixx Sep 19 '15 at 18:17

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