Could black holes form without a singularity?

[Cosmo Novice]

This is not a question - more a topic of discussion; as I am not expecting definitive answers.

At the center of black holes we stipulate singularities, now I understand that mathematically a singularity does not make sense; infinite density and zero spatial volume being irreconcilable and an indication of a breakdown in current theory.

So my discussion is this:
However would/could black holes form without a singularity?
and
Is it possible for singularities to form from our reference frame, ie: the reference frame of the external observer? Would it not be impossible for a singularity to form on the basis of properties of time dilation in extreme gravity - at least from our reference frame as we would be mapping a finite observer time to an infinite coordinate time?
or
Am I looking at this too simply, and its is curvature that reaches infinite proportions?

Just looking for some general discussion and possible laymans easy reading.

Thanks in advance, Cosmo.

 

[mathman]

Simple answer: we really don't know what goes on inside a black hole. Singularity results from assuming General Relativity applies exactly and quantum theory can be ignored. When attempting to apply both at the same time, the results are nonsensical. We need a theory which takes both (quantum and gen. rel.) into account (string theory or loop quantum gravity or something else).

 

[bcrowell]

At the center of black holes we stipulate singularities, now I understand that mathematically a singularity does not make sense; infinite density and zero spatial volume being irreconcilable and an indication of a breakdown in current theory.

I disagree with this reasoning. If we didn't know about quantum mechanics and the Planck scale, then I'd be perfectly happy to believe in black-hole singularities. There's nothing inherently wrong or illogical about the concept.

Simple answer: we really don't know what goes on inside a black hole.

But there is a singularity (or almost-singularity) that isn't hidden inside an event horizon: the big bang singularity.

In any case, the distinction between a GR-style singularity and a compression of matter to the Planck density is not necessarily a meaningful distinction. I'm not aware of any method that would allow us to distinguish one from the other (unless, say, Penrose's CCC is right -- but I wouldn't bet a six-pack on that). If description A and description B can never be distinguished by any observation, then the distinction between A and B is not a scientific distinction.

 

[PeterDonis]

I disagree with this reasoning. If we didn't know about quantum mechanics and the Planck scale, then I'd be perfectly happy to believe in black-hole singularities. There's nothing inherently wrong or illogical about the concept.

I've seen people say things like this before, and it makes me wonder. Do you really not see anything physically unrealistic about infinite spacetime curvature? This question doesn't depend on whether the infinite curvature is hidden inside an event horizon or not, so it would also apply to:

But there is a singularity (or almost-singularity) that isn't hidden inside an event horizon: the big bang singularity.

In both cases (black hole and big bang), I agree with the OP that infinite spacetime curvature doesn't make sense; the theory we are using (GR) has simply reached the limit of its operating envelope, because it makes a prediction, infinite spacetime curvature, that is clearly physically unrealistic.

In any case, the distinction between a GR-style singularity and a compression of matter to the Planck density is not necessarily a meaningful distinction. I'm not aware of any method that would allow us to distinguish one from the other

I would agree with this only if you add the strong qualification, "using our present or currently foreseeable science and technology". Things may look very different when we have an actual working theory of quantum gravity.

 

[WannabeNewton]

In both cases (black hole and big bang), I agree with the OP that infinite spacetime curvature doesn't make sense; the theory we are using (GR) has simply reached the limit of its operating envelope, because it makes a prediction, infinite spacetime curvature, that is clearly physically unrealistic.

Why do you find the concept of infinite tidal forces unrealistic?

 

[bcrowell]

I've seen people say things like this before, and it makes me wonder. Do you really not see anything physically unrealistic about infinite spacetime curvature?

Yep, that's why I said so. I actually find singularities very philosophically attractive. They provide a sweet way of dealing with otherwise intractable issues, such as the ultimate cause.

I would agree with this only if you add the strong qualification, "using our present or currently foreseeable science and technology". Things may look very different when we have an actual working theory of quantum gravity.

I would consider that an extremely weak qualification. If the Big Bang really got to the Planck density at some point, then penetrating past that point with observations is the most absurdly grandiose ambition I can imagine. More grandiose than reorganizing all the stars in the observable universe into the shape of a valentine for my wife. (Of course, there is heavy theoretical uncertainty here. If Penrose is right, then we already have seen past the BB. Theoretical uncertainty is qualitatively different from uncertainty about the future progress of technology.)

 

[PeterDonis]

Yep, that's why I said so. I actually find singularities very philosophically attractive. They provide a sweet way of dealing with otherwise intractable issues, such as the ultimate cause.

Hmm...not sure how a "standard" infinite curvature singularity helps with that, though I could see how something like Hawking's "no boundary" proposal, which basically gets rid of the "initial singularity" by making spacetime as a whole compact, like a sphere, so there is no "initial" point, might do it. (At least, that's my highly heuristic understanding of Hawking's "no boundary" proposal.)

If the Big Bang really got to the Planck density at some point, then penetrating past that point with observations is the most absurdly grandiose ambition I can imagine.

It's probably as grandiose as I can imagine too. But all of our imaginations are highly constrained by how early we are in the history of knowledge. In the 19th century, scientists thought that figuring out what the Sun was made of was absurdly grandiose, and then spectroscopy came along. But of course this is just a personal opinion of mine, I freely admit that it's not any kind of scientific judgment, or even a gesture in the direction of a scientific research program. It's just my best guess.

If Penrose is right, then we already have seen past the BB.

Can you elaborate on this? I'm not sure what you're referring to.

 

[bcrowell]

Hmm...not sure how a "standard" infinite curvature singularity helps with that, though I could see how something like Hawking's "no boundary" proposal, which basically gets rid of the "initial singularity" by making spacetime as a whole compact, like a sphere, so there is no "initial" point, might do it. (At least, that's my highly heuristic understanding of Hawking's "no boundary" proposal.)

Causality breaks down at singularities. There's the famous Earman quip about how anything can come out of a singularity, including green slime and your lost socks. To me, it's very attractive to have exactly one naked singularity (the Big Bang) and give it all the responsibility for explaining where everything came from.

Can you elaborate on this? I'm not sure what you're referring to.

Penrose has a theory called CCC, Conformal Cyclic Cosmology, where the universe loses its sense of scale at some point very late in expansion, and then it becomes a big bang and everything starts over again.

http://www.nature.com/news/2010/101210/full/news.2010.665.html
http://blogs.discovermagazine.com/cosmicvariance/2010/12/07/penroses-cyclic-cosmology/
http://arxiv.org/abs/1011.3706
http://arxiv.org/abs/1012.1305
http://arxiv.org/abs/1012.1486

 

[PeterDonis]

Causality breaks down at singularities. There's the famous Earman quip about how anything can come out of a singularity, including green slime and your lost socks. To me, it's very attractive to have exactly one naked singularity (the Big Bang) and give it all the responsibility for explaining where everything came from.

Ah, I see; to you "anything can come out of a singularity" is a feature, not a bug.

Penrose has a theory called CCC, Conformal Cyclic Cosmology, where the universe loses its sense of scale at some point very late in expansion, and then it becomes a big bang and everything starts over again.

Thanks for the links, I wasn't aware of this theory. More light bedtime reading.