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Comment by ars | Hacker News Reader

A black hole is a singularity.

But we don't have empirical evidence of black holes - we have circumstantial evidence of very dense blobs - they could be neutron stars or other things.

The reason they assume they are black holes is that if a neutron star was that big it would be a black hole - but if loop quantum gravity is right, then it would become a degenerate star instead and not a black hole.

Getting rid of black holes would help a lot BTW because they contradict a lot of well established theories, plus they can not be reconciled with relativity.

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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

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.

plus they can not be reconciled with relativity. What do you mean? black holes indeed were a prediction of general relativity.

They are only sort of a prediction, more accurately black holes are the point where the theory breaks down resulting in infinities (division by zero).

For example relativity states that time stops totally past the event horizon, on top of that you can't even reach the event horizon because time dialates so much that you can never reach it - yet stuff falls in.

Or for example because length contracts based on speed it's possible for something to look like a black hole if you are moving fast, but not be a black hole if you are moving slow. That causes all sorts of contradictions.

Basically relativity can not calculate anything about the inside of a black hole.

Or for example because length contracts based on speed it's possible for something to look like a black hole if you are moving fast, but not be a black hole if you are moving slow.

How does length contraction make something "look" like a black hole? Gravitational force generated by an object cannot possibly change based on what an observer measures its length to be; No matter how fast or slow the observer is moving.

I don't know an awful lot of physics. For me, it appears that you are either intentionally throwing up random stuff to confuse others, or you are yourself confused. [No offense meant]

Gravitational force is also a function of distance - the closer you are to the object the stronger the force.

So if the object contracted and got really small, you could come very close to it and thus feel a gravitational force strong enough to act like a black hole.

It doesn't work like that, dude. Take a GR class.

Time continues as you continue past the event horizon. The myth that you can't reach the event horizon is an artifact of poor coordinate choices. The length contraction argument is also invalid.

Relativity cannot calculate anything "inside" a black hole because that's the regime where that particular theory breaks down. This is a feature of physics.

The time continues for the item falling, but for the rest of the universe it appears to take an infinite amount of time. It means that the math works, but the actual universe would never have anything fall in a black hole since it can never actually get there.

Why is the length contraction argument invalid? Just saying it's "invalid" doesn't help me much.

"Relativity cannot calculate anything "inside" a black hole because that's the regime where that particular theory breaks down. This is a feature of physics."

I said that didn't I? That was my point - that black holes break a theory, and there are no other theories to replace it. So not having black holes in the first place would help.

When you say feature do you mean "benefit" or "fact"? Because if you mean benefit I don't see how that is.

Please don't forget my original point: that if loop quantum gravity makes black holes impossible that is a plus for that theory. I'm not arguing about whether or not black holes exist, or if their math works.

> But we don't have empirical evidence of black holes - we have circumstantial evidence of very dense blobs - they could be neutron stars or other things.

We have images of stars rotating around some very dense invisible stuff. Too dense to be supported by neutron degeneracy. What more do you want? We can't exactly get pictures of the interior.

We do not have pictures of stars rotating around dense stuff. All the pictures of galaxies could be stars rotating around each other. Add in dark matter and you really have no idea what they are rotating around.

What we do have is pictures of gravitational lensing, and jets, and some other evidence, but certainly not stars rotating around dense stuff. And you are quite overestimating our ability to measure the size of the object - it could be very dense, but large, so not a black hole, but something else.

You quite missed the point - the whole point was that loop quantum gravity says that you can not get infinity dense, so even if neutron degeneracy can not hold it up, the loop quantum degeneracy will.

Perhaps quark degeneracy exists?

There is no evidence that black holes (as infinitely dense singularities) exists. There is only evidence of very big things, and our current theories don't know what else it could be. But that is NOT the same as evidence of a black hole.

I thought a black hole was an object sufficiently dense as to have an event horizon.

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