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>There is no such thing as "too small to form a black hole". >There is no minimum mass for a black hole. The first sentence could be true, but it's not implied

>There is no such thing as "too small to form a black hole". >There is no minimum mass for a black hole. The first sentence could be true, but it's not implied by the second sentence being true.

pdonis 6y ago
Hm, yes, I should probably clarify what I meant: what determines whether an object undergoing gravitational collapse forms a black hole is not whether or not it is too small to form a black hole, but whether or not it is too large to form anything else. There is no minimum mass for a black hole, but every stable state of matter we know of short of a black hole has a maximum mass. (And there are good theoretical reasons to suspect that every stable state of matter short of a black hole must have a maximum mass, even if we haven't yet discovered all of the possible ones.) 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.
- deepspace 6y ago
  I am pretty sure that is what the parent commenter to your first reply meant, at least that is how I understood their question. Semantics aside, I would like to know if one of the possible states you mention might be "more dense than a neutron star but not dense enough to collapse into a black hole"? I.e. some kind of quark-gluon plasma or other exotic state?
  - pdonis 6y ago
    > I would like to know if one of the possible states you mention might be "more dense than a neutron star but not dense enough to collapse into a black hole"? I.e. some kind of quark-gluon plasma or other exotic state? 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.
    - perl4ever 6y ago
      There's something on Wikipedia about how maybe all black holes are basically a ball of subatomic stuff, termed a "fuzzball", with a diameter equal to where escape velocity reaches c. This appeals to me, because falling past an event horizon, let alone reaching a singularity, seems like it can't be physical. https://en.wikipedia.org/wiki/Fuzzball_(string_theory) https://en.wikipedia.org/wiki/Fuzzball_(string_theory)
      - pdonis 6y ago
        > "fuzzball" 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.
        perl4ever 6y ago
        >Since it has no testable consequences, it remains a speculation. 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?
        6y ago

>There is no such thing as "too small to form a black hole". >There is no minimum mass for a black hole. The first sentence could be true, but it's not implied by the second sentence being true.

pdonis 6y ago

Hm, yes, I should probably clarify what I meant: what determines whether an object undergoing gravitational collapse forms a black hole is not whether or not it is too small to form a black hole, but whether or not it is too large to form anything else. There is no minimum mass for a black hole, but every stable state of matter we know of short of a black hole has a maximum mass. (And there are good theoretical reasons to suspect that every stable state of matter short of a black hole must have a maximum mass, even if we haven't yet discovered all of the possible ones.) 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.

deepspace 6y ago
I am pretty sure that is what the parent commenter to your first reply meant, at least that is how I understood their question. Semantics aside, I would like to know if one of the possible states you mention might be "more dense than a neutron star but not dense enough to collapse into a black hole"? I.e. some kind of quark-gluon plasma or other exotic state?
- pdonis 6y ago
  > I would like to know if one of the possible states you mention might be "more dense than a neutron star but not dense enough to collapse into a black hole"? I.e. some kind of quark-gluon plasma or other exotic state? 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.
  - perl4ever 6y ago
    There's something on Wikipedia about how maybe all black holes are basically a ball of subatomic stuff, termed a "fuzzball", with a diameter equal to where escape velocity reaches c. This appeals to me, because falling past an event horizon, let alone reaching a singularity, seems like it can't be physical. https://en.wikipedia.org/wiki/Fuzzball_(string_theory) https://en.wikipedia.org/wiki/Fuzzball_(string_theory)
    - pdonis 6y ago
      > "fuzzball" 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.
      - perl4ever 6y ago
        >Since it has no testable consequences, it remains a speculation. 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?
        6y ago