Hacker News Reader

Comment by Zaak | Hacker News Reader

The defining feature of a black hole is the event horizon, not the singularity.

Here is the current state of evidence for black holes, from http://chandra.harvard.edu/chronicle/0307/onbh/

"Yet although thousands of bursts have been observed from neutron stars, not one has been observed from a black hole candidate. The lack of X-ray bursts from black hole candidates is a strong argument against the existence of gravastars and other exotic alternatives to black holes."

What "well established theories" do black holes contradict?

Black holes are predicted by relativity. How could they not be reconciled with it? http://en.wikipedia.org/wiki/Schwarzschild_metric

View on HN

An event horizon sort of implies a singularity doesn't it? Otherwise what is it the horizon of?

The main theoretical problem with black holes is the information loss due to the no hair theorem. Look up "Black hole unitarity".

Another problem is that hawking radiation can send out anything, with no regard to the quantum numbers of what when in - so you put in a proton and return photons, which normally is impossible (conservation of the baryon/lepton/parity number).

No one really knows it that will really happen though, it's all theories and ideas.

But a black hole is not so much a prediction of relativity as it's the point where relativity fails. In that sense it was predicted by it, but relativity can not explain what happens in a black hole.

An event horizon sort of implies a singularity doesn't it? Otherwise what is it the horizon of?

Any clump of mass smaller than its own Schwarzchild radius will have an event horizon, but that doesn't necessarily mean that all that mass needs to wind up as a point. Under what we currently think we know about gravity a clump compressed to that radius will wind up as a singularity, but it's possible that we might have something wrong about gravity which would result in black holes with event horizons but no singularities -- instead you'd just have some high-but-not-infinite density mass in the middle being prevented from collapsing by... something we currently don't understand.

I'm pretty sure that the singularity is not the fact that it's a point, but that the gravity is so high that it wraps in on itself. i.e. that spacetime has a hole in it.

Because once gravity is that high going any higher has no meaning by any physics currently known. So by that definition once you crossed the event horizon anything inside has no meaning - and that is the singularity (i.e. the calculations of relativity have an infinity there, which is the singularity, not the mass in a point).

Also I did sort of assume that if matter can not collapse to a point a black hole is impossible. It does not appear that a quark star could be a black hole. And nothing is known about the other candidates listed on wikipedia (Preon star and Q star), but there sort of is the assumption that they could not make a black hole.

The article referenced for Q stars says 1.5 Schwarzschild radius, so it looks like to make a black hole you need a point mass.

I'm pretty sure that the singularity is not the fact that it's a point, but that the gravity is so high that it wraps in on itself. i.e. that spacetime has a hole in it.

I know Wikipedia isn't a good source for physics, but:

"While in a non-rotating black hole the singularity occurs at a single point in the model coordinates, called a "point singularity", in a rotating black hole, also known as a Kerr black hole, the singularity occurs on a ring (a circular line), defined as a "ring singularity". Such a singularity may also theoretically become a wormhole."

http://en.wikipedia.org/wiki/Gravitational_singularity

It seems a singularity is either a point or a line, but never a sphere for example.

Reply on news.ycombinator.com