I What problems would 'black holes' not being formed solve?

[DarkStar42]

So when a star collapses, if the event horizon doesn't form, but the object remains in ordinary space, in increasing gravitational time-dilated collapse, what problems with these objects would be solved or more easily addressed?
I would think at least the information loss problem, and then there is the conservation of rotational momentum, and then there is the production of the magnetic field.

 

[mathman]

Your question is about a physically impossible situation. It seems very hard to answer.

 

[DrStupid]

So when a star collapses, if the event horizon doesn't form, but the object remains in ordinary space, in increasing gravitational time-dilated collapse

As mathman already mentioned, the event horizon will always form in such a collaps. But I wonder if the singularity could be avoided. Is it possible that the black hole evaporates before the singularity forms?

 

[Chronos]

In a classical sense a singularity is the inevitable result of gravitational collapsei of any sufficiently massive body. Ir is not entirely clear if a gravitational singulariy is the truly the physical consequence of such an event. There is no real disseny over the formation of an event horizon. There is good reason to doubt this happens even if quantum corrections conspire to preclude the formation of a singularity. Whether or not an event horizon is accompanied by a singularity does not appear to be relevant to event horizon properties given there is little to suggest a singularity is even necessary

 

[stefan r]

... Is it possible that the black hole evaporates before the singularity forms?

My impression was time dilates to infinity as you approach an event horizon. Assuming I understood that correctly then a falling particle would not make progress until the black hole evaporates enough to allow progress.

The singularity is there anyway. I thought this was the reason for the no hair theorem.

 

[phinds]

My impression was time dilates to infinity as you approach an event horizon. Assuming I understood that correctly then a falling particle would not make progress until the black hole evaporates enough to allow progress.

Your impression is wrong. Objects fall through the event horizon as thought nothing is there because in physical terms, that is exactly correct. Nothing is there. You can't get back out again after you cross it, but other than that it really doesn't exist.

 

[phinds]

Your impression is wrong. Objects fall through the event horizon as thought nothing is there because in physical terms, that is exactly correct. Nothing is there. You can't get back out again after you cross it, but other than that it really doesn't exist.

The time dilation of which you speak is an observational phenomenon of an observer far removed from the BH, not something that happens to the infalling object.

 

[DarkStar42]

but say that 'something' does prevent the event horizon forming, what problems with these objects would be addressed in part, or in full?

 

[phinds]

but say that 'something' does prevent the event horizon forming, what problems with these objects would be addressed in part, or in full?

This question amounts to "if the laws of physics did not apply, what would the laws of physics say about <insert nonsense of your choice>?"

 

[Chronos]

They could comprise part of the macho population contributing to the DM budget without violating gamma ray background constraints imposed by observational astronomy. They could also cause gravitational lensing and possibly waves without triggering EM emissions, or basically a variety of phenomena.that are not necessarily accompanied by significant EM events. Keep in mind, however, that gravitational collapses are not entirely free from collateral consequences

 

[stefan r]

but say that 'something' does prevent the event horizon forming, what problems with these objects would be addressed in part, or in full?

We have found objects that collapsed and did not form an event horizon.

 

[DarkStar42]

We have found objects that collapsed and did not form an event horizon.

Yes I have heard of neutron stars. I was rather meaning objects in a continual state of collapse, that would usually be considered to form event horizons.

Or alternatively, if you like, you could considered an unknown force/process, which prevents the object from collapsing enough to form an event horizon.

I find it more intuitive to imagine an object conserving its angular momentum if it remains an actual object. Plus, as I mentioned, the generation of the magnetic field.

 

[guywithdoubts]

The problem is not with event horizons. Any body with an escape velocity of at least c will necessarily form an event horizon, there is no doubt about this, as it's inevitable. The question is whether there is an actual physical singularity "behind" the horizon or not.

 

[Nugatory]

We have found objects that collapsed and did not form an event horizon.

Yes, but these do not collapse past their Schwarzschild radius, so General Relativity predicts that an event horizon will not form. It's only if the collapse reduces the size of the object to less than that radius that we expect the horizon to form.

 

[Ilythiiri]

https://en.wikipedia.org/wiki/Magnetospheric_eternally_collapsing_object

"...He argued that all proposed black holes are instead quasi-black holes rather than exact black holes and that during the gravitational collapse to a black hole, the entire mass energy and angular momentum of the collapsing objects is radiated away before formation of exact mathematical black holes ... Mitra argues that he has proven that the world-line of an in-falling test particle would tend to be lightlike at the event horizon, independent of the definition of "velocity"."

 

[mathman]

There is plenty of evidence for the existence of black holes, very large ones at the centers of galaxies and companions in some binaries. Speculations involving physics which contradict observations are pointless.

 

[DarkStar42]

There is plenty of evidence for the existence of black holes, very large ones at the centers of galaxies and companions in some binaries. Speculations involving physics which contradict observations are pointless.

But how would the observations differ between an event-horizon black hole and the above given magnetosphere eternally collapsing object?

 

[mathman]

But how would the observations differ between an event-horizon black hole and the above given magnetosphere eternally collapsing object?

I'm missing something. What is "magnetosphere eternally collapsing object"?

 

[DarkStar42]

I'm missing something. What is "magnetosphere eternally collapsing object"?

the one in post #15.

 

[Chronos]

Without venturing too deeply into the swamp, it is safe to say Einstein was opposed to the notion that an event horizon could be physically realized without a singularity forming. And the fact most physicists are opposed to the notion that a singularity can be physically realized leaves very much a conundrum. Suffice it to say the easiest trap to fall into [aside from a black hole] is an unfortunate choice of reference frame and inconsistent usage throughout a solution that results in realization of an event horizon. I am not aware of any widely accepted solution for this problem, but, am resist the notion that such a solution is not possible. We know that a neutron star can get close to forming an event horizon [depending, of course, on how strictly you elect to define 'close'] so I am willing to concede it should be possible to span the remaining gap without defying the currently known laws of physics.

 

[mathman]

the one in post #15.

How does that explain the evidence for black hole existence? If they aren't there what is causing the observations?

 

[DarkStar42]

How does that explain the evidence for black hole existence? If they aren't there what is causing the observations?

some other collapsed object.

 

[mathman]

some other collapsed object.

What makes this different from a black hole? If its volume is small enough and mass is high enough, wouldn't the escape velocity be greater than c? In which case it is a black hole. The main problem as I understand it is that current theory cannot really describe what is happening within the event horizon.

 

[sophiecentaur]

This question amounts to "if the laws of physics did not apply, what would the laws of physics say about <insert nonsense of your choice>?"

It’s just another example of lack of respect for the body of science. The worst offenders in this respect are, of course, Politicians with Journalists a close second. [emoji846]

 

[DarkStar42]

It’s just another example of lack of respect for the body of science. The worst offenders in this respect are, of course, Politicians with Journalists a close second. [emoji846]

depends whether the 'body of science' is an Egyptian pyramid or a space elevator..

 

[Chronos]

You need to look hard at GR to understand why Einstein concluded a singularity is needed in order for an EH to form. He wa pretty bright for a crusty old turn of the century kind of guy.

 

[DarkStar42]

You need to look hard at GR to understand why Einstein concluded a singularity is needed in order for an EH to form. He wa pretty bright for a crusty old turn of the century kind of guy.

and if you take a collapsing star, the singularity would form at the centre, right? And the event horizon would expand from there as the star fell into it..

But why would a singularity form at all? As time dilation rose, then the matter compressing in the area would rise, slowing compression. I don't see how time dilation could ever reach infinity at any point, like I said, as matter at the centre compressed, the process would slow down, and prevent infinite time dilation occurring and hence a singularity..

 

[russ_watters]

But why would a singularity form at all? As time dilation rose, then the matter compressing in the area would rise, slowing compression. I don't see how time dilation could ever reach infinity at any point, like I said, as matter at the centre compressed, the process would slow down, and prevent infinite time dilation occurring and hence a singularity..

Setting aside the singularity itself, which is essentially thought to be a math error (but it isn't possible to know for sure), there is nothing to stop the formation of the event horizon. Time dilation is not a local phenomena, so locally a particle falling toward the event horizon just passes through it without even noticing. There is nothing to *actually* stop the collapse, it only *appears* to stop to remote observers.

...I'm also seeing (though I'm not a physicist), that the infinite time dilation at the event horizon vs a remote observer only applies to a stationary point (under extreme acceleration) at the event horizon, not someone/thing falling in.

This is a subject that has been discussed before on PF and seemed to make the rounds a few years ago because I think Stephen Hawking said something provocative about it. But it isn't considered to be a real issue for the theory.

 

[Nugatory]

I don't see how time dilation could ever reach infinity at any point, like I said, as matter at the centre compressed, the process would slow down, and prevent infinite time dilation occurring and hence a singularity..

Thinking in terms of time dilation pretty much guarantees confusion because time dilation is a consequence of your choice of coordinates; you can make it come or go or be finite or infinite just by choosing different coordinates (that is, your convention for labeling points in spacetime) to describe what's happening to the infalling matter. The fallacy in using time dilation to show that the singularity cannot form is similar to the fallacy of trying to use time dilation to show that the event horizon cannot form.

No matter what choices you make here and no matter what the time dilation is, when you solve the equations of general relativity (the Oppenheimer-Snyder solution would be a good start - Google for it) for a sufficiently large mass of collapsing matter you'll end up with a singularity.

It is possible that something happens to prevent the singularity from forming, but if so we don't yet have any theory abut what that something might be. For an analogy, we could consider Coulomb's law for the force between two charged point particles: $F=Q_1Q_2/r^2$. Everyone is comfortable with his law and we know that it's good physics - yet it has a singularity at $r=0$. The conclusion we draw from this is that Coulomb's law is a good description of the physics when $r$ is not equal to zero, and it's a mistake to try using it when $r$ is zero; and likewise the singularity that general relativity finds at $r=0$ in a black hole is just telling us that general relativity does not provide a complete description of conditions there.

 

[alantheastronomer]

I find it more intuitive to imagine an object conserving it's angular momentum if it remains an actual object.

Black holes DO conserve their angular momentum - they're called "Kerr black holes" and their event horizons are oblate, bulging out along their equator due to their extreme rotation. There's also a region outside the event horizon called the "ergosphere" where space itself is induced to take part in the rotation in a process called "frame dragging"! If you're interested, there's a section on Kerr black holes on Wikipedia.