Maybe there's a zone between neutron stars and black holes where the object consists at least partly of quark-gluon plasma because the neutrons have been forced together until they break, but it's a hair too small to form a black hole.
There is a maximum mass for white dwarfs and neutron stars; for white dwarfs, it's about 1.4 solar masses, and for neutron stars, it's somewhere between about 1.5 and 3 solar masses. We can't pin it down for neutron stars any closer because we have a poor understanding of the equation of state of matter in the relevant density and pressure regime.
I don't know if quark-gluon plasma is even stable in the relevant density and pressure regime, but if it is, I suppose there could be a maximum mass for a quark-gluon plasma "star" that was somewhat larger than the maximum mass for a neutron star, so there would be a range of masses where gravitational collapse could stop at a quark-gluon plasma star instead of a neutron star, and would not go on to form a black hole.
The first sentence could be true, but it's not implied by the second sentence being true.
So a collapsing object that is under the maximum mass limit for some stable state other than a black hole (white dwarf, neutron star, etc.) could be said to be "too small to form a black hole" in one sense (since it won't end up collapsing to a black hole), but I think it's more useful to view things in terms of it not being too large to form a stable state short of a black hole.
I'm not aware of any such state that is known, and ajross commented upthread (a sibling to my original comment) that he thinks a quark-gluon plasma state has been ruled out by QCD work. I have seen some papers about the "dark energy stars" he mentions in the same comment, but I don't know of any proposed observational test for such objects and they seem highly speculative to me.
Note, btw, that there isn't an awful lot of room for objects more dense than a neutron star but which can remain stable against collapse to a black hole. There is a theorem called Buchdahl's Theorem that says that any stable object that doesn't collapse to a black hole must have a radius at least 9/8 of the Schwarzschild radius corresponding to its mass. The denser neutron stars are already fairly close to this limit.
As the article notes, this is a speculation in string theory. Since it has no testable consequences, it remains a speculation.
> falling past an event horizon, let alone reaching a singularity, seems like it can't be physical
"Seems like" is not a good indicator in relativity; lots of things in relativity are counterintuitive. This is one of them. The classical GR model of black holes is perfectly consistent. It's possible that there are quantum corrections that prevent that exact model from being realized in our actual universe, but until we have a good theory of quantum gravity that has actually passed some experimental tests, the classical GR model of black holes is the best we have.
But why should it not be the default? I wonder if you're reacting to "string theory" which I am not promoting specifically, just the idea there is something that doesn't collapse any further, call it quarks or strings or whatever.
The page says there are testable consequences, I think, but assuming there are none, what are the testable consequences of the contrary - i.e. the existence of a singularity or the possibility of crossing the event horizon?
Reading popularized accounts of renormalization gave me the idea that when a physical theory spits out infinities, it's got to be inadequate. Whatever ideas don't give infinities are probably better. Regardless of whether you can test just yet.
If you disagree, why? Why say "this is the way relativity is" when relativity is guaranteed to be wrong?
Why should what not be the default? The speculations of string theory? Um, because they're speculations?
>* just the idea there is something that doesn't collapse any further*
Because our best current theories, the ones we have actually tested experimentally, say that once you are over the maximum mass limit of stable states of matter, there is no alternative to collapse to a black hole.
If someone comes up with a better theory that makes different predictions, and has that better theory confirmed by evidence, then our default will change. But until that happens, our default is what our best current theories that have been confirmed by evidence say.
> what are the testable consequences of the contrary - i.e. the existence of a singularity or the possibility of crossing the event horizon?
We cannot directly observe events at or below the event horizon from the outside, but we can test for the possibility indirectly by looking for the evidence that we would expect to see if something else happened, i.e., evidence that collapse stops somewhere short of the horizon. So far, in cases of objects which are above the known maximum mass limits, we have seen no evidence that anything stops the collapse short of the horizon. If we ever do see such evidence, it will be evidence that our best current theories need to be modified. But so far, as I have said, we have not seen any such evidence.
Speculating about other possible theories, in the absence of any evidence such as I have described, is fine as long as they are understood to be speculations. But in the absence of such evidence, the "default" remains as I have stated it above.
> Reading popularized accounts of renormalization gave me the idea that when a physical theory spits out infinities, it's got to be inadequate.
Not inadequate; incomplete. Yes, physicists pretty much agree that GR must be incomplete because it predicts infinities in certain particular regimes. But that only means the theory is incomplete in those regimes: at the singularities predicted at the centers of black holes. The black hole event horizon is not such a regime; GR predicts no infinities there whatsoever. So there is no reason to consider GR incomplete at the black hole event horizon, or even well below it, all the way down to the singularity, on these grounds.
> Whatever ideas don't give infinities are probably better.
Not at all. The fact that a current theory is believed to be incomplete because it has infinities in a particular regime, does not at all imply that any theory that does not have infinities in that particular regime must be better. That is simply faulty logic. The fact that many string theorists proclaim it does not make the logic any less faulty.
> relativity is guaranteed to be wrong
No, relativity is believed to be incomplete in one particular regime. That is very, very different from "wrong".
So, if there is evidence that the fuzzball theory is not accurate, then why did you say it's not testable?
Please be clear - I don't know if it's testable, nor if it's been (explicitly or implicitly) tested.
My point is that I think the correct application of Occam's razor in the absence of evidence would be to prefer theories that don't involve unlimited collapse providing they are consistent with known physics outside the extreme conditions in a black hole.
I have no expertise in physics, but this is a philosophical issue/opinion.
Even without any theory that prevents total collapse, due to relativistic time dilation, we can't see that total collapse in any amount of time, so it's always going to be premature to declare the matter settled.
That's not what I said. I said we have no evidence that gravitational collapse of an object over the maximum mass limit for any stable state other than a black hole, stops short of the event horizon of a black hole. In other words, we have no evidence that would require us to modify our best current theory of gravity, GR.
That is not at all the same as saying that we have evidence that the fuzzball theory is not accurate. To have such evidence, we would first have to have some actual predictions made by the fuzzball theory as to what we should expect to see in gravitational collapse of objects over the maximum mass limit for all other known stable states (neutron stars, etc.). I am not aware of any such predictions from the fuzzball theory. All I am aware of are hand-waving speculations that have not led to any testable predictions. That's why I said the fuzzball theory is not testable.
> I think the correct application of Occam's razor in the absence of evidence would be to prefer theories that don't involve unlimited collapse providing they are consistent with known physics outside the extreme conditions in a black hole.
Even if I were to accept this claim for the sake of argument, it would still require that we know fuzzball theory is consistent with known physics outside of a black hole event horizon. We don't even know that much; string theorists have yet to prove that string theory actually matches the predictions of our best current theories in all regimes outside of the extreme cases under discussion. They believe that string theory does match those predictions, but believing is not the same as proving.
This is a speculative hypothesis at this point, since we do not have a good theory of quantum gravity. A lot of physicists think it will turn out to be true once we do have a good theory of quantum gravity, but it's still a speculative hypothesis at this point.
> This is also the maximum mass of an elementary point particle.
This is part of the same speculative hypothesis.
This is also why Plank units aren’t used much outside of theoretical physics and cosmology which are fields where using absolute value based on current measurements won’t mean that much.
If you can figure out a way to cram the mass together.
But that was the point of the first comment anyway, that if you didn't quite get enough mass+density that you might get something weird.
This discovery is significant, because we never found a collapsed object in the 2-3 solar masses range, but it definitely doesn't need new physics to be explained.
A simple thought is that a 2.6 solar mass object could form by two ~1.3 solar mass objects merging.
The wikipedia article mentions a number of hypothetical exotic star types.