Comment by Maro | Hacker News Reader

Comment by Maro | Hacker News Reader

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This is not entirely true.

We can imagine a civilization made up for very sturdy physicists, so tidal forces inside the black hole don't rip them apart immediately. This civilization can enter the event horizon of the black hole, and observe what happens inside, make measurements, new theories, write papers, upload them to pre-prints, go to conferences, etc.

It is true that they cannot communicate with the outside world, but that may not bother them if there's enough of them that they can share their new discoveries with each other and have a society and satisfy their intellectual curiosity.

According to classical general relativity there are no stable orbits inside the black hole, so eventually this society will get arbitrary close to the singularity (or, to extremely large curvatures), so eventually they all would die, irrespective of how sturdy they are.

But this is also true outside the black hole, per our current best theories, we will all either get crushed in a big crunch (reverse of the big bang), or heat death as the Universe expands indefinitely. (Or, sooner, when the sun explodes, or sooner if an asteroid hits us, or global warming, or we nuke each other..)

The weird thing about this is that according to the theory... nothing ever enters the black hole. Everything freezes at the event horizon because time slows down and eventually gets frozen at the horizon. This is what happens relative to the outside observer.

The person entering the black hole does not perceive time freezing. How could he? Relative to himself he will enter the black hole but for him time outside of the black hole will accelerate at infinite speed well past the heat death of the universe forever in an instant. Then once the universe ends if there is an end, he will cross the threshold and move through the event horizon.

No, this is false: "nothing ever enters the black hole".

For the outside observer, it seems like nothing enters the event horizon. It seems like everything just gets infinitely close and redshifts and expands, and forms a giant low-energy redshifted enveloping pancake around the black hole.

But, from the perspective of the object passing through the event horizon, none of this happens, she passes through just fine.

Yes, relativity theory is weird as hell.

> But the object passing the event horizon passes through just fine.

Yes, but due to the extreme time dilation, will they observe/experience the heat death of the universe prior to entering the black hole?

If so, does it make sense to say they actually enter it in any meaning full sense from the perspective of our existence?

As in 'yes', they will enter the black hole, but they will enter it an infinite number of years from now when our universe "functionally" no longer exists.

Isn't this just another form of a particle with constant acceleration, accelerating to the speed of light? Yes, from their perspective, they reach it, but from our perspective, it will take an infinite amount of time and so we say they never can reach it from either our perspective or theirs? I don't see how this can be any different. Gravity is a constant accelerator.

(And note I'm calling out the constant acceleration case above for approaching 'C', not the increasing acceleration requiring 'exponentially increasing energy each second' case).

Consider this argument: if you can see the person falling in for "infinitely long", then you can actually communicate with this person falling in. For example, through light or gestures. Since you can communicate, this means the image you see of the person falling in is not just an illusion or artifact of general relativity, you both must agree on reality, that the person still hasn't crossed the horizon.

Yeah, you're right. It's not an optical illusion. I think I'm wrong about the infinite acceleration part though. We never observe the person hitting the event horizon so time never actually freezes. It just gets really really slow and approaches the point of time freezing.

The person falling into the black hole will have time in the outside universe accelerate relative to him. It will get faster and faster and when he hits the event horizon we don't know what happens as that's the singularity where the math no longer works. We can speculate that time actually freezes but that leads to paradoxes.

Nope, your signals also slow down and never reach them, and they appear to be 'frozen' and cannot respond to you. As they approach the event horizon their "reflexes" slow down in a kind of Zeno's paradox. Your signals never reach the infalling observer, so you cannot communicate. And they have a finite amount of reaction time as they fall through the black hole, but to your perspective that gets smeared over an infinite amount of time in the future. This is only due to the fact that you both disagree over how to measure space and time at the event horizon. To the infalling observer this isn't of any consequence though since all actual physics is local and your ruler and stopwatches aren't relevant to the infalling observer.

If their light reaches you, your light could reach them, this is a reciprocity condition that I believe is valid in GR too (because effectively light follows null geodesics, and it can always go both ways, geodesics don't have a preferred direction).

It's true that infalling observers will experience outside time speeding up, and conversely outside observers will see infalling objects slow down. In fact, as they approach the Event Horizon, they would appear to experience diverging time dilation. So in effect, they become "frozen in time". But remember that you can always maintain two-way communication with light. The very fact that both the outside observer can always see the infalling person and the infalling person can always see the outside world imply there can be two-way communication all the way up to crossing the event horizon (which would take infinitely long for the outside observer). This ought to be enough to conclude the infalling person can in fact never reach the event horizon.

> It's true that infalling observers will experience outside time speeding up, and conversely outside observers will see infalling objects slow down.

And the infinite passage of all external time is observed in a finite time by the infalling observer. All of the communication from the external observer is going to become a high frequency chirp at the event horizon, and the last message the infalling observer could send will take nearly an infinite amount of time to reach the external observer. And in the next instant of subjective time by the infalling observer they will be past the event horizon and unable to communicate any more.

Sure, but it seems this is unphysical if this passage occurs after essentially an infinite time for an outside observer. Before crossing the event horizon, the infalling person sees the whole history of the universe. But the problem is, if crossing the horizon takes infinitely long, how could the horizon form in the first place? That's what I've never seen addressed to a satisfactory degree.

Well elsewhere in this thread I've pointed out that the black hole evaporates before the infalling observer ever crosses the event horizon.

If you have a classical black hole and you don't bother yourself with how it forms, though, there's no paradox, the paradoxical effects arise because the areas of space-time become completely unreachable and you're using coordinate systems which are not local.

No you're wrong. It's not just some optical illusion. It's a seemingly paradoxical statement.

From the perspective of the observer nothing enters the black hole. This is true.

From the perspective of the person entering the black hole he does enter it. It's because of time dilation.

Both of these statements are true according to theory as far as I know.

But then on a long enough timescale the black hole evaporates via hawking radiation in a finite amount of time before the external observer has ever seen the 'frozen' object fall through the event horizon.

Perhaps with that time dilation and the slow evaporation of the black hole, nothing ever enters the event horizon. The horizon would seem to retract away from you as you approached it until it was gone.

Side note, I watched this almost B movie called "time trap" and I feel like falling into a black hole would be just like the end of that movie. It really made me want to jump in a black hole.

Hmm yeah you could be right.

The black hole evaporates at a finite speed relative to the outside observer. So someone falling in will by logic see this evaporation speed accelerate.

Before he touches the event horizon the black hole should have completely evaporated already so time never actually "freezes".

That's an interesting solution and resolves the paradox. When the black hole evaporates the reference frames become consistent again.

I'm hoping for a happy coincidence: that expansion of space due to dark energy increases over time without bound. Then the person falling into a black hole will observe the outside universe passing by faster and faster until the expansion of space overcomes the black hole's gravitational pull. The person is thus rescued from crossing the event horizon ... though still torn apart by the expansion.

Sure, there's no evidence for this (yet?). But it'd be a nice solution to the paradox.

This guy here has a good solution: https://news.ycombinator.com/item?id=38643347

Itm similar to science fiction where time loops resolve through circumstance. If you go back in time to stop your birth you will inevitably instigate your birth instead of creating a paradox.

I'm this case if you try to freeze time by going into the black hole the black hole will inevitably be destroyed before you can get close enough to it to encounter the singularity.

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