Observational Evidence of Black Holes

stevendaryl

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This seems like a question that would be in the Relativity FAQ, but I didn't see it.

Briefly: I've seen the claim made that there is plenty of observational evidence for the existence of black holes. But I don't understand how, from the outside, one can tell the difference between a black hole and a very massive star that has not yet collapsed into a black hole. What are the key differences that are observable from far away?
 

Simon Bridge

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Technically the evidence merely points to an object of black hole mass, which does not give off it's own light.
Regular black-hole candidates would be small compared to their mass - indicating "very dense" or, in the case of supermassive black holes, there is just too darn much mass. Have you had a go looking up the likely black hole candidates?
 

stevendaryl

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Technically the evidence merely points to an object of black hole mass, which does not give off it's own light.
Regular black-hole candidates would be small compared to their mass - indicating "very dense" or, in the case of supermassive black holes, there is just too darn much mass. Have you had a go looking up the likely black hole candidates?
The reason that I'm wondering is because of a recent paper (being discussed in the Beyond the Standard Model forum) suggesting that quantum mechanics may prevent black holes from forming (at least in some circumstances). It's not appropriate in the relativity forum to discuss QM, but I was wondering whether the claim that there is observational evidence for black holes counts as evidence against the claims of this paper. I was thinking that maybe the observations don't contradict the paper if they can't tell the difference between an actual black hole and a very massive dark star that is (for some reason) blocked from forming a black hole.
 

TumblingDice

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pervect

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This seems like a question that would be in the Relativity FAQ, but I didn't see it.

Briefly: I've seen the claim made that there is plenty of observational evidence for the existence of black holes. But I don't understand how, from the outside, one can tell the difference between a black hole and a very massive star that has not yet collapsed into a black hole. What are the key differences that are observable from far away?
There is fairly strong evidence for event horizons, see for instance http://iopscience.iop.org/0004-637X/701/2/1357. "The Event horizon of Sagittarius A*"

Basically, matter is falling into the BH candidates, but there isn't any observed radiation consistent with the infalling matter hitting any sort of surface coming back out. When stuff falls onto a neutron star, in contrast, we do see radiation characteristic of the matter hitting a surface.

Radiation from the accretion disk will have doppler shifts and other characteristics that make it different from radiation due to infalling matter hitting a surface.

These observations are consistent with the existence of event horizons, which are a feature of the black hole model, - and not consistent with many (if any) other models. This is a cursory review from memory, see the references for the full details of the argument.

 
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There is fairly strong evidence for event horizons
Technically, this isn't actually evidence for event horizons, but for trapped surfaces, i.e., apparent horizons. For it to be evidence for event horizons, i.e., absolute horizons, we would have to know for sure that apparent horizons are always associated with absolute horizons; but that may not be true, depending on how the black hole information loss problem finally gets resolved.
 

Chronos

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When you find orbital velocities around an unseen object no larger than our solar system suggests a mass of millions of suns, most scientists are inclined to believe it is a black hole.
 

pervect

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Technically, this isn't actually evidence for event horizons, but for trapped surfaces, i.e., apparent horizons. For it to be evidence for event horizons, i.e., absolute horizons, we would have to know for sure that apparent horizons are always associated with absolute horizons; but that may not be true, depending on how the black hole information loss problem finally gets resolved.
Yes - I thought the paper I quoted mentioned that, but I didn't find any such quote. Digging a bit, I found that what I probably remembered was in the paper "A note on the observational evidence for the existence of event horizons in astrophysical black hole candidates."
 

Simon Bridge

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The reason that I'm wondering is because of a recent paper (being discussed in the Beyond the Standard Model forum) suggesting that quantum mechanics may prevent black holes from forming (at least in some circumstances).
That's not a new idea - it's been around for slightly less time than the idea of black holes iirc.

It's not appropriate in the relativity forum to discuss QM, but I was wondering whether the claim that there is observational evidence for black holes counts as evidence against the claims of this paper.
Note the "in some circumstances" ... what about other circumstances?

I'll have to check the paper for what the authors are actually saying - but evidence of black holes certainly is evidence against any claim that black holes cannot form at all.

I was thinking that maybe the observations don't contradict the paper if they can't tell the difference between an actual black hole and a very massive dark star that is (for some reason) blocked from forming a black hole.
The observations place constraints on alternatives to the black-hole theory for what they are. Efforts are made to make sure that the possibility that the object is an hitherto unknown kind of object like a very massive star that gives off no light is slight. As more evidence rolls in, the picture gets clearer.

One of the trouble's with this is that most people only get the broad brush strokes. You've seen that there are ways to check obvious things like if it's a ball of non-radiating regular matter with a physical surface outside it's event horizon. It's amazing what gives off light. There are always going to be alternate theories to account for the light - that's just normal science. The trick is to choose between them. Eventually we'll end up with some statement like "the objects we have been calling black holes are actually..." for now, it's as good-a label as anything.
 

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