B What really is a black hole?

1. Mar 6, 2016

Arceus74

I have a question that when the fuel of star runs out there is a imbalance is temeperature-gravity equilibrium.
When it collapses to a single point where mass gets compressed so much,but shouldnt the temperature at instant increase so much that the mass could not be compressed to infinite density at all.This should result in a stellar plasma.But people claim this is false.Could you explain why?

2. Mar 6, 2016

rootone

It's only collapsing in the first place because gravity has overcome all other forces which previously had held the star in equilibrium.
There are no longer significant nuclear or EM interactions to oppose gravity.

We don't actually know what happens to the collapsing mass once inside the event horizon, but arriving at a single point having infinity density sounds improbable.
Some sort of phase change resulting in a new stable state, but within the event horizon seems more likely to me.

3. Mar 6, 2016

phinds

This "single point" is a mathematical representation that is not believed to represent physical reality. Contratry to the incorrect statements in pop-science, "singularity" does not mean "point" in means "the place where our model breaks down and the math gives nonphysical results and we don't really know WHAT is going on.". It is believed that a solid, empirically demonstrable theory of quantum gravity will give a better picture of what is really happening at the heart of a black hole.

4. Mar 6, 2016

Arceus74

Well gravity did overcome all the phases during that instant but when it is about to reach a high dense state surely temperature should have a impact isnt it.I mean where does all the kinetic energy go?

5. Mar 6, 2016

Arceus74

So if it doesnt become a point then what force opposes it from not becoming a point.It must be temperature.

6. Mar 6, 2016

phinds

All I can do is repeat what I said

7. Mar 6, 2016

rootone

We don't know since we can't investigate inside a black hole and we can't reproduce the conditions.
My bet is still on some kind of change of state to a condition which is at present unknown to us and isn't predicted by current theories.
A 'Quark star' is one such suggestion which seems reasonable, but there is no evidence to support that idea.

8. Mar 6, 2016

Jonathan Scott

A black hole does not require infinite density. According to GR, gravitational collapse is inevitable regardless of the strength of the material if an object collapses to within its Schwarzschild radius, which is given by $2Gm/c^2$ for mass $m$.

This is somewhat denser than a typical neutron star of one to two solar masses. It is not clear exactly where the limit lies for neutron stars lies; it is thought that neutron stars would give way under the pressure somewhere around that point, but it seems theoretically possible that there might be a more dense form such as a "quark star" which is more massive than a neutron star but can still resist total collapse. Either way, the Schwarzschild radius goes up linearly with the mass but the volume at a given density only goes up with the square root, so there is a threshold not much higher where nothing can prevent collapse.

One way of looking at collapse is that the effective escape velocity exceeds the speed of light in that case, so not even light can escape. The question of what happens behind the event horizon isn't covered by current physics, but if GR holds there is only a finite proper time before a falling observer hits the singularity at the centre, and there is no way to escape that.

9. Mar 6, 2016

Jonathan Scott

The hypothetical 'quark star' is something which is a bit heavier than a neutron star but hasn't collapsed into a black hole. According to GR, there is only a limited range of masses over which such an object could exist. I'm not aware of any current evidence for the existence of such stars.

10. Mar 6, 2016

QuantumQuest

In order to describe the collapse of a star, it's better to talk about a gravity - pressure equilibrium and what happens when gravity is the winner. Talking in simple terms and without all the details involved, for a small star, when the fuel of star is exhausted, meaning that there are no more nuclear reactions to fight gravity, then due to degeneracy pressure among electrons, there is no further collapse, so we usually have cooling of star and creation of a white dwarf. If the star is massive enough (about twenty times the mass of our sun or more) and a supernova explosion leaves a massive core (> about 2.5 times mass of the sun), there is no known repulsive force inside the star that can prevent gravity to collapse the core and the result is a black hole.

As phinds pointed out, singularity is a mathematical point not a physical one. So, because the model breaks down there, there is no point to state that this becomes a point or not.

11. Mar 6, 2016

rootone

No there isn't evidence of quark stars, I agree it's only hypothetical and requires physics we don't yet know about and exotic forms of matter which have never been observed.
That being said, it does seem more reasonable than proposing 'the singularity' is a real object having zero dimensions and infinite density.
What properties the exotic matter of a (hypothetical) quark star might have is unknown.
Can such matter degenerate further? - maybe. maybe not.

12. Mar 6, 2016

Jonathan Scott

Within an event horizon as predicted by GR there are simply no possible paths through space-time which do not inevitably hit the singularity within a limited time, regardless of what form the matter takes. At present, the nature of the singularity itself is not addressed by physics.

13. Mar 6, 2016

rootone

Which is why a quantum theory of gravity is somewhat the 'holy grail' for theoretical physics at present.
If we nailed that then the awkward singularity might go away.

14. Mar 6, 2016

bcrowell

Staff Emeritus
Are you claiming that nothing inside the event horizon is covered by current physics, or only that what happens at the singularity (or at the Planck scale) isn't covered? If the former, then I don't know why you would say that.

Same question.

15. Mar 6, 2016

Jonathan Scott

No, I'm not saying nothing is covered. Perhaps I should have said what happens "eventually". I was answering someone who seemed to think it might be possible to have something stable behind the event horizon, and saying that as far as GR goes that's not possible.

16. Mar 6, 2016

Jonathan Scott

Also at present, what happens behind the event horizon is by definition unobservable, so it's not very easy to apply the scientific method to it!

17. Mar 6, 2016

bcrowell

Staff Emeritus
I wouldn't overstate this. For example, we have pretty good models of the interior of Jupiter. I don't think there's any realistic prospect of ever testing those models directly and in detail, since we can't use seismic waves as we do on earth, but that doesn't mean we can't believe the models. Another good example would be predictions of the future evolution of our sun, which we will never live to verify directly.

And it's actually not true that the interior of an event horizon is unobservable. You can go in there and observe it. You just can't publish the results in Physical Review.

Even from the outside, there are things you can check. You can check that, for example, the mass, charge, and angular momentum of the collapsing body remained constant during the collapse to a black hole and are in some sense "still in there." You can also check negative predictions, such as the prediction that nothing comes flying out through the event horizon.

18. Mar 7, 2016

Arceus74

[QUOTE=" I was answering someone who seemed to think it might be possible to have something stable behind the event horizon, and saying that as far as GR goes that's not possible.[/QUOTE]

Why isnt anything stable,Just because we cant observe it doent mean there is nothing stable?

19. Mar 7, 2016

Jonathan Scott

The mathematics of GR extends beyond the event horizon, and says that the shape of space-time is such that there are no possible paths forward in time which stay in one place. Every possible path ends up in the singularity after a limited time.

20. Mar 7, 2016

Arceus74

You mean that black hole itself becomes a singularity and matter which once became a black hole behaves differently?