B Is crossing a black hole's event horizon possible?

[martinbn]

There are many ways to answer the question, neither of course being completely satisfying because we do not have a full theory of quantum gravity.

One answer is because it avoids the information paradox.

Another answer is because general covariance may only be a symmetry of classical theory, not of the full quantum theory, so in dealing with quantum effects you are not allowed to change coordinates from Schwarzschild to Kruskal, so the Schwarzschild singularity becomes a true physical singularity, rather than just a coordinate singularity.

Yet another answer is that quantum effects can be significant even for large objects. Consider, for instance, heat capacity of a big object, it cannot be understood without quantum physics.

Yes, but my point is that all this should also happen in a regime where classical gravity is very accurate. If there is an effect in that regime we should have noticed it by now.

 

[Demystifier]

Yes, but my point is that all this should also happen in a regime where classical gravity is very accurate. If there is an effect in that regime we should have noticed it by now.

Can you give an example of such a regime?

 

[martinbn]

Can you give an example of such a regime?

I did.

For example a very large spherical formation of many galaxies, where we are at the centre, could be collapsing toward us. Then the event horizon will form and grow out to meet the shell. We could be crossing it right now.

 

[srb7677]

The concept of "gravity well" doesn't really make sense at and inside the horizon. Also, there is no invariant way to compare "the rate of time passing" between spatially separated observers in curved spacetime. So this question is not well-defined.


No. As the astronaut falls, he will see light coming to him from the rest of the universe becoming more and more redshifted, and he will therefore see things happening in the rest of the universe more and more slowly.


No. The astronaut's clock continues to run just fine as he crosses the horizon.


This is correct, but not for the reason you give. It isn't that "time stops" at the horizon. Rather, the outgoing light emitted by the astronaut as he falls takes longer and longer to get back out to the distant observer as the astronaut approaches the horizon; and at the horizon, the outgoing light emitted by the astronaut as he falls is stuck at the horizon forever; it never gets back out. That is why we never see it.

Thanks for your clarifications. It is a food job I am here to learn rather than preach. Most of you are far more knowledgeable in this area than I am.

 

[Tomas Vencl]

off topic: Why would the quantum effects be non-negligible for all black holes? For example a very large spherical formation of many galaxies, where we are at the centre, could be collapsing toward us. Then the event horizon will form and grow out to meet the shell. We could be crossing it right now. Why should there be quantum effects preventing that?

While I agree with Demystifier's remark and can imagine a situation where, for example, quantum evaporation generates a different metric (with a firewall), I think that in the given example of a 'galactic collapsing shell with Earth in the middle,' there really can be no observable effects of crossing the horizon. The formation of the horizon inside the shell is a global effect and depends on how the shell will behave in the future (for example, whether it will stop its collapse, which is theoretically possible beyond its horizon). If I were to observe any effect of crossing the horizon on Earth, I would retrocausally gain information about the future behavior of the shell, which would likely lead to some paradoxes.

 

[PeterDonis]

If I were to observe any effect of crossing the horizon on Earth, I would retrocausally gain information about the future behavior of the shell

The word "retrocausal" might be confusing some people. A better way to put this would be that the fact that you can't know in advance the entire future behavior of the shell (and the spacetime in general) is why you can't observe any local effect when you cross the event horizon. Or, to put it another way, the event horizon is not locally defined, it's globally defined; to know where it is, you have to know the entire spacetime, including the entire future.

 

[Eclipse Chaser]

The velocity time-dilation curve as v approaching C resembles the gravitational time dilation curve for r approaching Rsch. If SR predicts time reversal for V>C ("FTL"), does GR predict anything similar for inside the EH (r < Rsch)?

 

[Orodruin]

If SR predicts time reversal for V>C

It doesn’t.

 

[Vanadium 50]

If SR predicts time reversal for V>C

It doesn't.

 

[Vanadium 50]

Beat me by a nose!

 

[Orodruin]

Beat me by a nose!

Those posts may have had spacelike separation!

 

[phinds]

in a process picturesquely dubbed "spaghettification".

@srb7677 just as a minor aside in this thread, that statement is not generally true. It IS true for small and modest sized black holes but it is definitely not true for massive and supermassive black holes, so you need to be more specific, rather than making a general categorical statement.

 

[Vanadium 50]

@srb7677 just as a minor aside

He has been gone for months. The thread got restarted when someone else had a different misconception.

 

[phinds]

He has been gone for months. The thread got restarted when someone else had a different misconception.

What fun, eh?

 

[Eclipse Chaser]

RE: Inside the EH: Well that was convincingly quick! Returning to SR, if the velocity time dilation equation T = T0 x Sqrt ( 1 – V2 / C2), goes imaginary, (but not necessarily negative), can you direct me to the mathematical or other reasoning that predicts time reversal for V>C (FTL)?

 

[Vanadium 50]

can you direct me to the mathematical or other reasoning that predicts time reversal for V>C (FTL)?

No, because it is your claim, and it is wrong.

 

[PeterDonis]

This thread is closed.