3 ms·

What does it mean for a black hole to spin at the speed of light? Which point on it spins at that speed?

chupa-chups 7y ago
I wondered this myself, since the singularity is assumed to be a point, thus it should spin as fast as possible. But this appears to be more complicated: https://physics.stackexchange.com/questions/183832/typical-rotation-speeds-for-black-holes https://physics.stackexchange.com/questions/183832/typical-r... https://physics.stackexchange.com/questions/174247/is-there-a-horizontal-event-horizon-on-a-spinning-black-hole https://physics.stackexchange.com/questions/174247/is-there-... Taken from one of the stackexchange posts, this is quite interesting: http://www.nature.com/news/spin-rate-of-black-holes-pinned-down-1.13512 http://www.nature.com/news/spin-rate-of-black-holes-pinned-d...
fragsworth 7y ago
I am confused by this as well. The only thing I can imagine is "the matter falling into the hole at the event horizon moves at near the speed of light, relative to the center of the black hole". But I'm not 100% sure. The angular momentum inside the black hole should then also reflect this, but I don't know if physicists are too confident about what the matter inside a black hole looks like.
lalaithion 7y ago
Rotating black holes have a ring singularity, not a point singularity.
deleted 7y ago
[deleted]
zaarn 7y ago
Essentially, when spinning a black hole doesn't have a point at it's center but a ring (a point can't rotate). Theoretically a black hole can spin faster than the speed of light since the event horizon is just space, not matter, and space can certainly exceed the speed of light (see expansion of the universe). IIRC from relevant literature, the event horizon shrinks with increasing speed until it vanishes once the speed of light is exceeded and leaves behind a naked singularity; a black hole with no event horizon.
- emilfihlman 7y ago
  What is a black hole with no event horizon? Does that mean that once crossing, you are instantly subject to a great gravity? So there's a gravity gradient? How is that possible?
  - zaarn 7y ago
    It's a naked singularity. Light can escape from the center ring of the singularity and you could possibly observe it. What exactly it is we don't know, our physics just break down that deep into the gravity well. It could just be a barely visible ring that saws your spaceship in half if you fly through or a wormhole or possibly anything really.
  - harshreality 7y ago
    The event horizon is not a shield from gravity. It's the theoretical shell/radius inside which light cannot escape no matter what direction it's travelling. Photons sort of accumulate around the event horizon of a spinning black hole[0], because it takes them more orbits to escape the closer they get to the horizon, but without any matter/photos to observe, the event horizon is just a theoretical shell/radius limiting observation. A black hole without an event horizon is a naked singularity[1], a concept in theoretical physics resulting from looking at the mathematical equations that describe black holes and observing that the event horizon's radius becomes undefined at certain angular velocities[2]. It's not clear if it can exist in reality. AFAIK (not a physicist), gravitational force of a specific object (as a function of distance) cannot be discontinuous, can it? It's definitely not under the classical F=G(M1)(M2)/r^2 equation. Force can't be discontinuous unless mass changes discontinuously or space changes discontinuously. But I don't know if there are extreme conditions (and a naked singularity might qualify) where that equation breaks down, or is thought to break down? [0] to support the movie Interstellar, several people worked on an accurate rendering, based on known physics, of a rotating black hole: https://astronomynow.com/2015/02/14/interstellar-technology-throws-light-on-spinning-black-holes/ https://astronomynow.com/2015/02/14/interstellar-technology-... [1] https://en.wikipedia.org/wiki/Naked_singularity https://en.wikipedia.org/wiki/Naked_singularity [2] https://physics.stackexchange.com/questions/147034/rotating-black-holes-and-naked-singularity https://physics.stackexchange.com/questions/147034/rotating-...
wiml 7y ago
It means that the black hole is close to extremal. This means that the space near them is deformed in even more interesting ways than around a boring old non-spinning spherically-symmetrical hole. I think it's often assumed that most black holes in the universe would have relatively low spin, compared to their mass. The research here is saying that, perhaps, the way that stars collapse would produce near-extremal holes in most cases, so they would be common in the universe. I recommend reading up on black hole geometry ad Kerr(-Newman) holes; it's likely that any description that fits in a HN comment will leave you knowing less than when you started.

I wondered this myself, since the singularity is assumed to be a point, thus it should spin as fast as possible. But this appears to be more complicated: https://physics.stackexchange.com/questions/183832/typical-rotation-speeds-for-black-holes https://physics.stackexchange.com/questions/183832/typical-r... https://physics.stackexchange.com/questions/174247/is-there-a-horizontal-event-horizon-on-a-spinning-black-hole https://physics.stackexchange.com/questions/174247/is-there-... Taken from one of the stackexchange posts, this is quite interesting: http://www.nature.com/news/spin-rate-of-black-holes-pinned-down-1.13512 http://www.nature.com/news/spin-rate-of-black-holes-pinned-d...

fragsworth 7y ago

I am confused by this as well. The only thing I can imagine is "the matter falling into the hole at the event horizon moves at near the speed of light, relative to the center of the black hole". But I'm not 100% sure. The angular momentum inside the black hole should then also reflect this, but I don't know if physicists are too confident about what the matter inside a black hole looks like.

lalaithion 7y ago

Rotating black holes have a ring singularity, not a point singularity.

deleted 7y ago

[deleted]

zaarn 7y ago

Essentially, when spinning a black hole doesn't have a point at it's center but a ring (a point can't rotate). Theoretically a black hole can spin faster than the speed of light since the event horizon is just space, not matter, and space can certainly exceed the speed of light (see expansion of the universe). IIRC from relevant literature, the event horizon shrinks with increasing speed until it vanishes once the speed of light is exceeded and leaves behind a naked singularity; a black hole with no event horizon.

emilfihlman 7y ago
What is a black hole with no event horizon? Does that mean that once crossing, you are instantly subject to a great gravity? So there's a gravity gradient? How is that possible?
- zaarn 7y ago
  It's a naked singularity. Light can escape from the center ring of the singularity and you could possibly observe it. What exactly it is we don't know, our physics just break down that deep into the gravity well. It could just be a barely visible ring that saws your spaceship in half if you fly through or a wormhole or possibly anything really.
- harshreality 7y ago
  The event horizon is not a shield from gravity. It's the theoretical shell/radius inside which light cannot escape no matter what direction it's travelling. Photons sort of accumulate around the event horizon of a spinning black hole[0], because it takes them more orbits to escape the closer they get to the horizon, but without any matter/photos to observe, the event horizon is just a theoretical shell/radius limiting observation. A black hole without an event horizon is a naked singularity[1], a concept in theoretical physics resulting from looking at the mathematical equations that describe black holes and observing that the event horizon's radius becomes undefined at certain angular velocities[2]. It's not clear if it can exist in reality. AFAIK (not a physicist), gravitational force of a specific object (as a function of distance) cannot be discontinuous, can it? It's definitely not under the classical F=G(M1)(M2)/r^2 equation. Force can't be discontinuous unless mass changes discontinuously or space changes discontinuously. But I don't know if there are extreme conditions (and a naked singularity might qualify) where that equation breaks down, or is thought to break down? [0] to support the movie Interstellar, several people worked on an accurate rendering, based on known physics, of a rotating black hole: https://astronomynow.com/2015/02/14/interstellar-technology-throws-light-on-spinning-black-holes/ https://astronomynow.com/2015/02/14/interstellar-technology-... [1] https://en.wikipedia.org/wiki/Naked_singularity https://en.wikipedia.org/wiki/Naked_singularity [2] https://physics.stackexchange.com/questions/147034/rotating-black-holes-and-naked-singularity https://physics.stackexchange.com/questions/147034/rotating-...

wiml 7y ago

It means that the black hole is close to extremal. This means that the space near them is deformed in even more interesting ways than around a boring old non-spinning spherically-symmetrical hole. I think it's often assumed that most black holes in the universe would have relatively low spin, compared to their mass. The research here is saying that, perhaps, the way that stars collapse would produce near-extremal holes in most cases, so they would be common in the universe. I recommend reading up on black hole geometry ad Kerr(-Newman) holes; it's likely that any description that fits in a HN comment will leave you knowing less than when you started.