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First and foremost: GR guarantees that an event horizon will form. Oddball exotic states of matter (quark stars, fuzz balls, whatever) might form in proper time

by qubex 9y ago

First and foremost: GR guarantees that an event horizon will form. Oddball exotic states of matter (quark stars, fuzz balls, whatever) might form in proper time within the event horizon, halting collapse before it reaches the singularity state of infinite compaction. (Since time freezes at the event horizon, these states would occur infinitely far in the future from the perspective of any external observer and might therefore very reasonably be considered ‘unphysical’.) Second, and this is surprisingly subtle: we often discuss black-hole formation in terms of mass, but it's really a matter of relative density: the 2.7 solar mass limit only pertains when one is considering a mass in a void, if for example one found oneself in an infinite universe with an undisturbed uniform density of (say) 2.7 solar masses per metre cubed the tug of gravity would be uniform in all directions and there would be no impetus to initiate gravitational collapse towards any specific point. Once perturbed, however, average density somewhere would rise, and the collapse would begin.

pdonis 9y ago
> Oddball exotic states of matter (quark stars, fuzz balls, whatever) might form in proper time within the event horizon, halting collapse before it reaches the singularity state of infinite compaction. No, this is not correct. Once an event horizon forms, everything inside the horizon will hit the singularity, because the singularity is not a place in space, it's a moment of time, which is in the future (and the not too distant future for holes of reasonable size--the time from horizon to singularity for a black hole of 10 solar masses is about 100 microseconds) for everything inside the horizon. > time freezes at the event horizon No, it doesn't. This is a common pop science misconception, but it's still a misconception. The correct statement is that the horizon is a null surface: a surface generated by outgoing light rays. To someone falling through the horizon, the horizon looks like any other surface generated by light rays, and nothing unusual happens there.
- IntronExon 9y ago
  While I’m totally in agreement with the second part of your comment, the first part may not be true. No, this is not correct. Once an event horizon forms, everything inside the horizon will hit the singularity, because the singularity is not a place in space, it's a moment of time, which is in the future (and the not too distant future for holes of reasonable size--the time from horizon to singularity for a black hole of 10 solar masses is about 100 microseconds) for everything inside the horizon. We don’t actually know if there’s a singularity within the event horizon. GTR predicts it, but that’s in the context of the breakdown of the predictive power of the theory. Until/Unless we have a viable theory of quantum gravity what is inside an EH is speculative. Quark stars or any form of conventional matter are right out obviously, but Fuzzballs or some other novel “structure” can’t be ruled out yet. Having said that, I’m not a string theory adherent, I’m just pointing out that within the event horizon we need a complementary theory to augment GTR and QM.
  - pdonis 9y ago
    > We don’t actually know if there’s a singularity within the event horizon. Not if we take quantum effects into account, no. I was describing what classical GR predicts. I agree that GR also predicts that it should break down in the regime close to the singularity. It's also worth noting that there is a school of thought among physicists that says that quantum effects are non-negligible even at the horizon of a black hole of stellar mass or larger. This is the "firewall" debate that is currently ongoing. I personally don't find the arguments in favor of a "firewall" convincing, but it is an area of ongoing debate.
    - IntronExon 9y ago
      Agreed on all points, and the firewall is interesting, but I can’t see how it doesn’t somehow create a privileged frame of reference. It’s still very cool/terrifying, even in the already notoriously cool/terrifying realm of black holes.
      - qubex 9y ago
        The whole controversy about the firewall hypothesis is that I’d does create a privileged frame of reference, directly contradicting general relativity (that indirectly forms the basis for predicting the firewall’s existence).
- qubex 9y ago
  That is what GR predicts. However we know that GR is incomplete because it is incompatible with quantum field theory. I was trying to illustrate what can be relied upon to be true even if ultimately GR gets superseded by deeper theories: namely, we can rely on an event horizon forming at the schwartzchild radius whatever the ultimate fate of the in-falling matter may be (in an infinite future). Furthermore, whereas you and I apparently know what a null surface is, I was trying to illustrate what infinite time dilation means without resorting to very specialist knowledge.
  - pdonis 9y ago
    > we can rely on an event horizon forming at the schwartzchild radius whatever the ultimate fate of the in-falling matter may be (in an infinite future) This is not necessarily true if the quantum "firewall" speculations end up panning out (I think they're unlikely, as I posted elsewhere in this thread, but it's an open area of debate).

by qubex 9y ago

pdonis 9y ago

> Oddball exotic states of matter (quark stars, fuzz balls, whatever) might form in proper time within the event horizon, halting collapse before it reaches the singularity state of infinite compaction. No, this is not correct. Once an event horizon forms, everything inside the horizon will hit the singularity, because the singularity is not a place in space, it's a moment of time, which is in the future (and the not too distant future for holes of reasonable size--the time from horizon to singularity for a black hole of 10 solar masses is about 100 microseconds) for everything inside the horizon. > time freezes at the event horizon No, it doesn't. This is a common pop science misconception, but it's still a misconception. The correct statement is that the horizon is a null surface: a surface generated by outgoing light rays. To someone falling through the horizon, the horizon looks like any other surface generated by light rays, and nothing unusual happens there.

IntronExon 9y ago
While I’m totally in agreement with the second part of your comment, the first part may not be true. No, this is not correct. Once an event horizon forms, everything inside the horizon will hit the singularity, because the singularity is not a place in space, it's a moment of time, which is in the future (and the not too distant future for holes of reasonable size--the time from horizon to singularity for a black hole of 10 solar masses is about 100 microseconds) for everything inside the horizon. We don’t actually know if there’s a singularity within the event horizon. GTR predicts it, but that’s in the context of the breakdown of the predictive power of the theory. Until/Unless we have a viable theory of quantum gravity what is inside an EH is speculative. Quark stars or any form of conventional matter are right out obviously, but Fuzzballs or some other novel “structure” can’t be ruled out yet. Having said that, I’m not a string theory adherent, I’m just pointing out that within the event horizon we need a complementary theory to augment GTR and QM.
- pdonis 9y ago
  > We don’t actually know if there’s a singularity within the event horizon. Not if we take quantum effects into account, no. I was describing what classical GR predicts. I agree that GR also predicts that it should break down in the regime close to the singularity. It's also worth noting that there is a school of thought among physicists that says that quantum effects are non-negligible even at the horizon of a black hole of stellar mass or larger. This is the "firewall" debate that is currently ongoing. I personally don't find the arguments in favor of a "firewall" convincing, but it is an area of ongoing debate.
  - IntronExon 9y ago
    Agreed on all points, and the firewall is interesting, but I can’t see how it doesn’t somehow create a privileged frame of reference. It’s still very cool/terrifying, even in the already notoriously cool/terrifying realm of black holes.
    - qubex 9y ago
      The whole controversy about the firewall hypothesis is that I’d does create a privileged frame of reference, directly contradicting general relativity (that indirectly forms the basis for predicting the firewall’s existence).
qubex 9y ago
That is what GR predicts. However we know that GR is incomplete because it is incompatible with quantum field theory. I was trying to illustrate what can be relied upon to be true even if ultimately GR gets superseded by deeper theories: namely, we can rely on an event horizon forming at the schwartzchild radius whatever the ultimate fate of the in-falling matter may be (in an infinite future). Furthermore, whereas you and I apparently know what a null surface is, I was trying to illustrate what infinite time dilation means without resorting to very specialist knowledge.
- pdonis 9y ago
  > we can rely on an event horizon forming at the schwartzchild radius whatever the ultimate fate of the in-falling matter may be (in an infinite future) This is not necessarily true if the quantum "firewall" speculations end up panning out (I think they're unlikely, as I posted elsewhere in this thread, but it's an open area of debate).