# What happens as a neutron star collapses into a black hole

1. Nov 16, 2004

### brian0918

I've heard this explained numerous times, most recently in my General Relativity course today, where he talked about how smaller stars will collapse into white dwarfs, while more massive ones will overcome the electron fermi gas pressure, effectively forcing the electrons into the protons, so they make neutrons to go to a lower energy state. Then, as a neutron star, there is a fermi gas pressure (or pauli exclusion) that keeps the star from collapsing further.

This is where it gets vague. If the star is more massive, the gravitational pull will larger than the neutron pressure... but what exactly happens to the neutrons? How is it that Pauli exclusion apparently just doesn't apply?

Can anyone make this a little clearer?

Thanks.

2. Nov 16, 2004

### Garth

As the gravitational mass, the neutron star, becomes even more condensed the gravitational force, otherwise the weakest of the four fundamental forces, continues to increase in magnitude. If the overall mass is beyond to 2 -3 solar mass limit for a neutron star (the actual value is a little vague) there are no other (known) forces that can withstand it. In this situation the mass continues to shrink and the gravitational force continues to spiral upwards in value. There is nothing to stop the collapse and so, as the theory goes, the central mass collapses into zero volume, the singularity of the Black Hole. Around this singularity there will form an event horizon at which the 'escape velocity' is the speed of light, inside this nothing, not even light, can escape.

What happens as the singularity is approached is a matter for further research. A Quantum Gravity theory is required that does not yet exist. Many researchers think that here some kind of quantum effect will cause a 'bounce', perhaps into another universe? Your guess is as good as mine, or theirs!
Garth

Last edited: Nov 16, 2004
3. Nov 16, 2004

### Mike2

If two particles were entangled before the collapse, and one ends up inside the black hole while the other is left outside the black hole, would they still be entangled afterwards the collapse?

4. Nov 16, 2004

### turbo

There is a special case of this situation that is responsible for Hawking Radiation. When only one member of a particle/antiparticle pair of virtual particles is captured by a black hole, the member still outside the event horizon is immediately promoted from "virtual" to "real" status. This has been cited as a means by which the black hole can evaporate, but in truth the jury may be out on this one. One feature that may appeal to you in particular (in light of some past exchanges) is that in its promotion from quantum probability to "real" status, the particle outside the event horizon has now contributed to the information level of our universe, depending on your definition of information.

By definition, we cannot know what happens to the virtual particle that got captured beneath the event horizon, but intuitively I would suspect that it too is promoted to "real" status by virtue of its separation from its counterpart and as such may represent an increase in the information in the universe beyond that event horizon. I'm taking liberties with Smolin's black hole universe concept in this case, but you get my drift.

5. Nov 16, 2004

### Chronos

Just to clarify, there are some known stopping off points before the black hole stage in gravitational collapse. The first is the electron degeneracy pressure. We know from particle physics that atoms can normally only be packed so close. However, if you squeeze them hard enough, the electron shells will collapse. The amount of force required to do this is called the electron degeneracy pressure. If the mass/gravity of a collapsing star is not great enough to reach the electron degeneracy pressure, it will stabilize as a white dwarf. If it is great enough [~1.4 solar masses], it will collapse into exotic matter: crystaline iron, superfluid neutrons, superconducting protons, and stranger subatomic particle matter. This mess is called a neutron star. It too, however, can also be crushed, albeit a lot more force is required. This is called the neutron degeneracy pressure. The mass required to do this is not precisely known, just that anything more than about 3 solar masses will surely do the trick. We think, but are not sure it becomes a black hole at that point. It is, however thought it may even make another stop before a black hole is formed. Quarks themselves may further resist getting squished, in which case you would have a quark, or strange star. One suspected [ http://antwrp.gsfc.nasa.gov/apod/ap020414.html ], but no confirmed candidates are known. They are/would be even harder to detect than neutron stars.

Last edited: Nov 16, 2004
6. Nov 16, 2004

### brian0918

It just seemed like the pauli exclusion principle (no two fermions, in this case neutrons, can exist in the same state) was a steadfast unbreakable rule. In the case of gravitational collapse, however, it seems like it is suddenly no longer a rule, and everyone explaining it seems to ignore this.

Can anyone make this clearer? How does pauli exclusion suddenly no longer apply, and what actually happens to the neutrons?

7. Nov 16, 2004

### Mike2

Perhaps if you crush electrons enough, they turn into photons or other bosons which do adhere to the exclusion principle.

8. Nov 16, 2004

### turbo

It is possible that the Pauli exclusion principle still applies to the neutron star, but that the density of the mass involved isolates the body from our universe by enfolding it in an event horizon beyond which we cannot see.

9. Nov 16, 2004

### brian0918

Events still occur inside the event horizon. We just can't know about them (apparently).

10. Nov 17, 2004

### Chronos

The limit for electron degeneracy is very well known. The neutron degeneracy limit is not very well understood. The evidence for neutron stars is compelling. The evidence for condensed matter states beyond that is not very compelling.