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Keep in mind in GR singularities don't really make sense either: i.e. when you pass the event horizon of a black hole, under GR the rules say that you must alwa

by XorNot 3y ago

Keep in mind in GR singularities don't really make sense either: i.e. when you pass the event horizon of a black hole, under GR the rules say that you must always been traveling towards the singularity. But the corollary of this is that it's not actually possible - mathematically - to arrive at the singularity (because then you'd be moving parallel with it rather then towards it). So while we can define what happens very well under GR for the "most" of the inside of a black hole, we can't actually explain what happens to objects which reach the center. There has to be some type of discontinuity whatever happens - i.e. if the object disappears, that's still a discontinuity even if it would resolve the paradox. EDIT: Which is of course where GR and QM need to reconcile as well - as you get infinitely close to the singularity, the size scale is getting small enough that QM should be well in play.

richardw 3y ago
Can they reach the center? And how long would that take? I ask because time will slow down for them as they accelerate towards the center.
- XorNot 3y ago
  Your first question is the problem: they must always be moving towards the center - which implies they must be getting closer to it. Because if they can never actually reach the center, then in 4D spacetime they're no longer moving towards it - they'd be traveling parallel with it. Which would imply that singularity isn't a singularity - i.e. the "hole" it makes would in fact be highly curved spacetime in one dimension, but completely flat in another (i.e. it would be a cylinder). Which creates a hole lot of weird infinities in the system: i.e. a black hole becomes a finite bounded volume on the outside, but contains an infinite amount of space on the inside (since no matter how much you distort spacetime, the molecules of whatever falls in can just pull themselves together more tightly provided the distortion doesn't happen too quickly).
- SpaceManNabs 3y ago
  Time doesnt slow down for them. They reach the center in their time as normal. We never see them reach the center. You can use kruskal coordinates and other tools to understand this.
  - wholinator2 3y ago
    Right, time doesn't slow down for them, but it would appear so for an outside observer. So would the "experience" of a particle sent in to be that it circles closer and closer for some time on the order of years and at some point when it is arbitrarily close to the center it just pops out some billion or trillion years later once the black hole has evaporated? Assuming it isn't destroyed and somehow can experience it's own time.
    - SpaceManNabs 3y ago
      It would be more than a few trillion years I think, but that is the thinking unless you are a certain contentious Berkeley astrophysicist. The particle does see itself “reach” the black hole though. And so would an outside observer if they had infinite time. The black hole dissipating sure would make some of the experience odd. I would need to revisit my notes to make a claim about that, but discontinuities like that arent uncommon.

by XorNot 3y ago

richardw 3y ago

Can they reach the center? And how long would that take? I ask because time will slow down for them as they accelerate towards the center.

XorNot 3y ago
Your first question is the problem: they must always be moving towards the center - which implies they must be getting closer to it. Because if they can never actually reach the center, then in 4D spacetime they're no longer moving towards it - they'd be traveling parallel with it. Which would imply that singularity isn't a singularity - i.e. the "hole" it makes would in fact be highly curved spacetime in one dimension, but completely flat in another (i.e. it would be a cylinder). Which creates a hole lot of weird infinities in the system: i.e. a black hole becomes a finite bounded volume on the outside, but contains an infinite amount of space on the inside (since no matter how much you distort spacetime, the molecules of whatever falls in can just pull themselves together more tightly provided the distortion doesn't happen too quickly).
SpaceManNabs 3y ago
Time doesnt slow down for them. They reach the center in their time as normal. We never see them reach the center. You can use kruskal coordinates and other tools to understand this.
- wholinator2 3y ago
  Right, time doesn't slow down for them, but it would appear so for an outside observer. So would the "experience" of a particle sent in to be that it circles closer and closer for some time on the order of years and at some point when it is arbitrarily close to the center it just pops out some billion or trillion years later once the black hole has evaporated? Assuming it isn't destroyed and somehow can experience it's own time.
  - SpaceManNabs 3y ago
    It would be more than a few trillion years I think, but that is the thinking unless you are a certain contentious Berkeley astrophysicist. The particle does see itself “reach” the black hole though. And so would an outside observer if they had infinite time. The black hole dissipating sure would make some of the experience odd. I would need to revisit my notes to make a claim about that, but discontinuities like that arent uncommon.