As I understand it, the current theory is that black holes eventually evaporate. But to do this they need to lose mass/energy. So why do they not stop being black holes when they no longer have enough mass to overcome the force of gravity?
Because the only requirement for being a black hole is infinite density. You can have infinite density with any amount of mass. As long as the equation of a black hole’s density is x/0, the only way to get a result other than infinity is make it 0/0.*
Now… we think of mass as being important because one well-known method of generating black holes is the collapse of a large-enough star. This is a case where gravity overcomes the ability of the mass to resist compression and the only next step of compression is a black hole.
However, black holes can be created in high-energy particle collisions (like the LHC and cosmic rays) as long as there is sufficient mass/energy density.
- This may be sloppy thinking both in terms of physics and math. But it should illustrate the point.
Well yes, but to maintain that density, don’t you need a certain mass to overcome gravity?
No. In fact, I think you’re even wording it wrong, since gravity would like to have everything packed into tiny little point particles, if it could.
In a star, you’ve got various forces resisting the compression. First, there’s the heat and pressure of the gas. Then electron degeneracy and then neutron degeneracy. It takes a lot of gravity to overcome those forces and start the black hole.
But once you’ve turned matter into a black hole, there’s no pressure to return back to matter. We don’t actually know what’s in there under the even horizon, but the only thing that seems to change the black hole is the so-called evaporation
Well, maybe, but nobody’s ever seen it happen, and it’s probably just wishful thinking. Unless you want to hang an argument on the phrase “as long as there is sufficient energy”: A sufficiently-energetic particle collision presumably could create a black hole, but “sufficient energy” is probably far higher than the energy of any cosmic ray in the Universe.
I had thought that when a black hole evaporated to nothing, the singularity disappeared too. Is that correct? Otherwise, wouldn’t there be “naked singularities” hanging around where mini black holes used to be?
We don’t actually know what happens at the end of evaporation, since that’s squarely in the realm of quantum gravity. The prevailing notions are that they either reach some very small stable size and stop evaporating, or that the singularity goes away with the final evaporation, but there’s no real evidence for either.
As far as we know, black holes aren’t “made” of anything- General Relativity simply describes them as a singularity of zero size and infinite density, and the space surrounding them. Black holes do conserve the spin and the electric charge of the matter that fell into them but otherwise the composition of the matter that falls into them is irrelevent.
I though they just had to be dense enough to prevent light from escaping.
Nitpick: They also conserve magnetic charge. It’s not the hole’s fault that our Universe seems to have a severe shortage of those.
And treis, once something collapses far enough, there’s nothing that can stop it from finishing collapsing all the way.
Ah, I see. That answers it.