8 ms·

Perhaps tangential, but is there a frequency of a gravity wave high enough to "cusp" due to one of the quantization limits?

FakeComments 7y ago
In even the classic theory, when the Swartzchild radius of the gravitational wavelet exceeds its width, wouldn’t it naturally form a black hole and end up a cusp? Like a gravity Kugelblitz.
- scottie_m 7y ago
  I think by definition it wouldn’t form a black hole, it would be a black hole. Gravitational waves are propagating disturbances through spacetime, so the kind of wave you’re describing would begin as a singularity. I don’t think (but am not sure) that the math allows for the emission of such a thing. It sounds non-physical, and I’d suspect that if you do the math you’d discover that you’d need to have giant black holes merging to generate such a wave, or FTL. In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form.
  - wallace_f 7y ago
    >In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form. Sorry if this is a bit naive and tangential, but I've always stumbled at the thought of how does gravity-information about the interior of a black hole propagate out of the event horizon? ...Gravitons/gravity waves travel at, c?
    - scottie_m 7y ago
      That’s a bit of a tricky question, because it’s math-heavy. The best way to describe it is to think of the event horizon as the black hole, and forget that there is even an interior. The black hole can be fully described by the conditions at the event horizon after all, and everything else is cut off from the surrounding universe completely. In that sense there is no propagation from the interior at all, which is good because if information could escape then theories describing black holes would be broken. Instead the black hole has mass, charge, and momentum (three kinds of momentum actually, but that’s not important). Whatever is going on beyond the event horizon, whatever that might be, has no effect that anyone can detect. Matter is accreted “onto” the event horizon which then expands in proportion to the mass of the volume of the hole. Maybe it’s destroyed beyond that point, or maybe it goes to another universe, but we can never know. The event horizon can also shrink if the surroundings are sufficiently cold (really really really cold) and the horizon is sufficiently hot. Still, all of this is surface phenomena, like dropping a bowling ball into a tub of water. The water only “knows” about the surface of the ball, which which gets properties from the whole ball without exposing the center. A bowling ball in water creates waves, but the interior isn’t interacting with the water anymore than the black hole interior interacts with space (assuming an idealized perfectly rigid bowling ball). In the same way gravity waves or fractions would be a function of how the space just beyond the event horizon is warped. Does that help?
      - wallace_f 7y ago
        Thanks for writing that out, very interesting. I find this concept of a black hole's surface having the contents inscribed on it to be really difficult for me to imagine; but I can take people's word for it, and it does explain away the paradox. This reminds me that from some vantage points the universe seems so arbitrary sometimes. This is probably just my human intuition, but personally it just appears to me that nature is not really always elegant, but rather has these work-arounds and different layers to it to keep it working. Not unlike my terrible code.
      - thaumasiotes 7y ago
        I have some naive questions too. This is basically just me rephrasing the question I understood wallace_f to be asking: - The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force. - The black hole within the horizon is a three-dimensional massy object and can and does exert a lot of gravitational force. - Assume at equilibrium our black hole is somehow exerting gravitational forces on its surroundings which are what you would predict if you accurately knew the black hole's actual mass. - Assume the black hole moves, e.g. because of inertia. - Now it should be exerting more force than previously on one half of the universe (the half it moved toward), and less force on the other half. - Say it moved toward you. After a speed-of-light delay, you should actually perceive more force on yourself towards the black hole. But this can't be because a messenger particle was transmitted from the black hole to you. How can it be? Assuming this shows that black holes cannot move seems unsatisfactory, given the recession of galaxies from one another, observations believed to show black holes colliding, etc. Where are my mistakes? Followup: one black hole collides with another black hole of roughly ten times its size. Is it necessarily the case that the center of mass of the new, combined black hole ends up at the point that was the center of mass of the small-hole/big-hole system just as the small hole crossed the big hole's event horizon?
        cperciva 7y ago
        > - The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force. Stop right there. A two dimensional surface can have mass if it has infinite density. And infinite density makes as much sense as any other sort of singularity...

FakeComments 7y ago

In even the classic theory, when the Swartzchild radius of the gravitational wavelet exceeds its width, wouldn’t it naturally form a black hole and end up a cusp? Like a gravity Kugelblitz.

scottie_m 7y ago
I think by definition it wouldn’t form a black hole, it would be a black hole. Gravitational waves are propagating disturbances through spacetime, so the kind of wave you’re describing would begin as a singularity. I don’t think (but am not sure) that the math allows for the emission of such a thing. It sounds non-physical, and I’d suspect that if you do the math you’d discover that you’d need to have giant black holes merging to generate such a wave, or FTL. In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form.
- wallace_f 7y ago
  >In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form. Sorry if this is a bit naive and tangential, but I've always stumbled at the thought of how does gravity-information about the interior of a black hole propagate out of the event horizon? ...Gravitons/gravity waves travel at, c?
  - scottie_m 7y ago
    That’s a bit of a tricky question, because it’s math-heavy. The best way to describe it is to think of the event horizon as the black hole, and forget that there is even an interior. The black hole can be fully described by the conditions at the event horizon after all, and everything else is cut off from the surrounding universe completely. In that sense there is no propagation from the interior at all, which is good because if information could escape then theories describing black holes would be broken. Instead the black hole has mass, charge, and momentum (three kinds of momentum actually, but that’s not important). Whatever is going on beyond the event horizon, whatever that might be, has no effect that anyone can detect. Matter is accreted “onto” the event horizon which then expands in proportion to the mass of the volume of the hole. Maybe it’s destroyed beyond that point, or maybe it goes to another universe, but we can never know. The event horizon can also shrink if the surroundings are sufficiently cold (really really really cold) and the horizon is sufficiently hot. Still, all of this is surface phenomena, like dropping a bowling ball into a tub of water. The water only “knows” about the surface of the ball, which which gets properties from the whole ball without exposing the center. A bowling ball in water creates waves, but the interior isn’t interacting with the water anymore than the black hole interior interacts with space (assuming an idealized perfectly rigid bowling ball). In the same way gravity waves or fractions would be a function of how the space just beyond the event horizon is warped. Does that help?
    - wallace_f 7y ago
      Thanks for writing that out, very interesting. I find this concept of a black hole's surface having the contents inscribed on it to be really difficult for me to imagine; but I can take people's word for it, and it does explain away the paradox. This reminds me that from some vantage points the universe seems so arbitrary sometimes. This is probably just my human intuition, but personally it just appears to me that nature is not really always elegant, but rather has these work-arounds and different layers to it to keep it working. Not unlike my terrible code.
    - thaumasiotes 7y ago
      I have some naive questions too. This is basically just me rephrasing the question I understood wallace_f to be asking: - The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force. - The black hole within the horizon is a three-dimensional massy object and can and does exert a lot of gravitational force. - Assume at equilibrium our black hole is somehow exerting gravitational forces on its surroundings which are what you would predict if you accurately knew the black hole's actual mass. - Assume the black hole moves, e.g. because of inertia. - Now it should be exerting more force than previously on one half of the universe (the half it moved toward), and less force on the other half. - Say it moved toward you. After a speed-of-light delay, you should actually perceive more force on yourself towards the black hole. But this can't be because a messenger particle was transmitted from the black hole to you. How can it be? Assuming this shows that black holes cannot move seems unsatisfactory, given the recession of galaxies from one another, observations believed to show black holes colliding, etc. Where are my mistakes? Followup: one black hole collides with another black hole of roughly ten times its size. Is it necessarily the case that the center of mass of the new, combined black hole ends up at the point that was the center of mass of the small-hole/big-hole system just as the small hole crossed the big hole's event horizon?
      - cperciva 7y ago
        > - The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force. Stop right there. A two dimensional surface can have mass if it has infinite density. And infinite density makes as much sense as any other sort of singularity...