Xenharmonic Forest

There is a conflict between my understanding of what the event horizon of a black hole is vs the way that many theoretical physicists talk about them.

First of all, the assumptions I'm able to identify as the basis for my reasoning are these:
1. Time progresses more slowly in the presence of energy, and at the event horizon of a black hole, time does not progress at all.
2. Black holes eventually evaporate due to Hawking radiation.

Now, since an eternity would have to pass before an object truly reaches the event horizon, the object would inevitably evaporate into radiation first. That means that all observers agree: nobody ever enters a black hole.

Is my reasoning flawed? Are my assumptions wrong? Did I miss any implicit assumptions? If not, why are physicists talking about "what it's like to cross the event horizon" and "what the interior of a black hole is like"? I'm utterly confused by it. Unless I misunderstand, there is no inside.

#physics #askphysics

Dec 29, 2023 at 11:59amWeb
Show threadGeoff 🏴󠁧󠁢󠁳󠁣󠁴󠁿Dec 29, 2023

@sudoreboot Your key misconception is this:

"That means that all observers agree: nobody ever enters a black hole."

Different observers see different things. If I watch you fall towards an EH I see you get slower, and slower...

You, on the other hand, see time further out getting faster and faster. You sail past the EH without noticing much else is different (assuming a fairly big black hole) until the gravitational gradient, the difference in pull across your body, starts spaghettification.

Show threadGeoff 🏴󠁧󠁢󠁳󠁣󠁴󠁿Dec 29, 2023
@sudoreboot (Assuming a fairly standard black hole model - there are other ideas like the firewall hypothesis, where matter is destroyed at the EH.)
Show threadArthur SmithDec 29, 2023
@sudoreboot I’ve had this same question - I think there are some papers out there that argue along these lines that “black holes do not exist”. You do need to factor in that at least at first a black hole is accreting more mass as things approach it, but from the outside those added elements will show the same slowing behavior. Viewing things from the perspective of the infalling matter seems wrong to me - they will only experience a finite amount of time until the end of the universe…
Show threadXenharmonic ForestDec 29, 2023

@apsmith I feel a little validated!

Taking the (theorised) finite life span of black holes into account, it seems to me that the black hole would noticeably shrink the closer I get to the event horizon, since a lot more time is passing around me and such unimaginably large time scales become relevant. Of course, I would first contribute to the black hole's mass by being more or less considered part of it, but then I'd radiate right off of it anywhen after "reaching" (by some measure) the horizon but before actually crossing it.

I'll have to learn more about Hawking radiation because I understand that the "virtual particles" picture is just a story we tell and the mathematical details become sort of relevant to all this; it doesn't make much sense to think about a virtual particle pair being separated by the event horizon if there's no "other side" to it. I'm of the impression that it's more to do with the event horizon restricting certain field configuration modes, which is better.

Show threadArthur SmithDec 29, 2023
@sudoreboot here’s an example paper related to this, from the QM perspective: https://arxiv.org/abs/gr-qc/0609024
Observation of Incipient Black Holes and the Information Loss Problem

We study the formation of black holes by spherical domain wall collapse as seen by an asymptotic observer, using the functional Schrodinger formalism. To explore what signals such observers will see, we study radiation of a scalar quantum field in the collapsing domain wall background. The total energy flux radiated diverges when backreaction of the radiation on the collapsing wall is ignored, and the domain wall is seen by the asymptotic observer to evaporate by non-thermal ``pre-Hawking radiation'' during the collapse process. Evaporation by pre-Hawking radiation implies that an asymptotic observer can never lose objects down a black hole. Together with the non-thermal nature of the radiation, this may resolve the black hole information loss problem.

arXiv.org
Show threadXenharmonic ForestDec 29, 2023
@apsmith Oh, I recognise one of the authors (Lawrence Krauss) as the host of the Origins podcast!
Show threadGeoff 🏴󠁧󠁢󠁳󠁣󠁴󠁿Dec 29, 2023
@apsmith @sudoreboot "The end of the universe" would imply there are no more black holes though, so only somebody falling into the last black hole in existence would experience that? (Assuming a heat death rather than something more exotic.)