A simple blackhole question I can't find an answer for

[davel18]

I apologize if I'm posting in the wrong part of the forum, I'm new. I have started find cosmology interesting. I have started to learn about black holes but I don't understand something that might obvious and I'm not getting it.

Why is/was there an assumption that things that enter a blackhole are destroyed? I totally get that things that pass the event horizon will never be able to escape but how did we get from never coming out again to destroyed? I get the whole spaghettification thing and while that would seriously warp anything that entered a black hole and hit the singularity, however warping isn't destroying though. It seems like things like Hawking Radiation are based on something we have no evidence for. Displaced, warped stripped down to atoms aren't 'destroyed" which is something we know can't happen in the first place. So how to we get from one point to the next?

Thank you for your time.

 

[Ibix]

In a classical black hole, singularities are the edge of the map we can draw of spacetime. Things that reach there fall off the edge of the map. Presumably that doesn't happen in reality, but we don't know what does happen.

In our models, singularities are usually (always?) surrounded by regions of spacetime where the curvature increases without bound. That means that eventually anything will be torn apart before it reaches the singularity because it would need to be infinitely strong to hold together against the curved spacetime inducing different parts of it to move in different directions. Again, note that we suspect the "edge of the map" is probably a result of our models failing somewhere just short of there, so the curvature rising without bound may be part of the model failing, so something else may happen in reality.

Hawking radiation is completely separate topic. It's related to quantum field theory at the horizon, not near the singularity. As far as I understand it it's related to thermodynamic concerns.

 

[PeroK]

The singularity is where the mathematical model breaks down. It isn't a physical thing. Anything that crosses the event horizon only has a finite time before the singularity, where its time (according to GR) runs out.

I guess at some stage such an object would be considered to have been destroyed.

A theory of quantum gravity is expected to supersede GR and fix the mathematical model, I.e. get rid of the singularity.

 

[PeterDonis]

Why is/was there an assumption that things that enter a blackhole are destroyed?

It's not an assumption; it's a conclusion based on what the mathematical model tells you. The mathematical model tells you that once something falls inside the horizon, it only has a finite amount of time left to exist. That's not something that was put into the model; it's what comes out of it after it's constructed.

As @Ibix and @PeroK have said, we expect that the mathematical model that tells us that is not actually correct--that at some point before the singularity, that model breaks down, and we'll have to replace it by a different model. But we don't have any other model to replace it with at this time.

 

[javisot]

Perhaps the first thing you need to know is why you haven't asked your question in the right place. A black hole isn't cosmology; it's astrophysics. The difference between cosmology and astrophysics, both branches of physics, is that astrophysics studies astrophysical objects (for example, black holes, planets, etc.), while cosmology studies cosmological objects.

 

[PeterDonis]

A black hole isn't cosmology; it's astrophysics.

At PF, we've had questions about black holes in a number of forums.

This particular question, since it's really about the basic mathematical model of a black hole, best fits in the relativity forum, and I have now moved it there.

 

[Dale]

I get the whole spaghettification thing

Umm, that is what destroys things. The tidal forces grow without bound.

Any object composed of multiple fundamental particles will have some finite maximum tidal force that it can withstand before being destroyed. That structural limit will be reached before the singularity.

Yes, we expect the GR model to fail at the singularity, but the tidal forces will exceed any finite threshold before the singularity.

stripped down to atoms aren't 'destroyed"

Well, I certainly would consider myself to be destroyed if I were stripped down to atoms. But even atoms will be pulled apart. It is hard to envision a reasonable definition of “destroyed” under which that wouldn’t qualify.

 

[Matterwave]

In our models, singularities are usually (always?) surrounded by regions of spacetime where the curvature increases without bound.

A few points of clarification here.

"Curvature increases without bound" (assuming you mean some kind of scalar measure of it anyways) is one conception of what happens at a singularity, but it's not the one used in the singularity theorems of Penrose and Hawking. Those theorems only prove that under certain conditions there exists (a) inextendible causal curve(s) which has finite proper time (at least in one direction).

Second, it seems you are talking about the cosmic censorship conjecture with the use of ("always"), please correct me if I'm wrong! In that case, the conjecture states that singularities (except for the big bang one) is hidden behind a horizon. In that case, it requires some strong conditions to hold (and it's still a conjecture). So if by "models" you mean "solutions to EFEs" then "strictly" the cosmic censorship conjecture is false. One can construct fine-tuned counter examples e.g: Violation of cosmic censorship in the gravitational collapse of a dust cloud | Communications in Mathematical Physics | Springer Nature Link https://share.google/eF0fHBCed0A2EwVm4