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serp777
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Suppose a black hole travels at something like v = 0.999999999c relative to some observer. Does the black hole's event horizon becomes length contracted, thus appearing to turn into a black disk?
Simon Bridge said:That would be the same as asking what the geometry of the event horizon would be from the POV of an observer traveling relativistically wrt it.
Note: an observer stationary wrt a large sphere would see the sphere as a disk.
Observe: Sun, Moon, Planets, near starts etc: all look like disks.
Measure the diameter, in different directions re travel direction, as you pass one at speed, and they look like ellipses.
Very fast and you get a thin ellipse.
Simon Bridge said:artistic interpretations aside, BHs are unlikely to look black.
serp777 said:Suppose a black hole travels at something like v = 0.999999999c relative to some observer. Does the black hole's event horizon becomes length contracted, thus appearing to turn into a black disk?
Louis Philippe said:I was more interested in the ramifications for the gravity of the black hole combined with the relativistic mass increase of the black hole as it approaches c
Louis Philippe said:What effect would this have on the imaginary time which would be experienced by an object falling into a black hole itself moving at relativistic speed?
But how does the Schwarzschild geometry outside the horizon look like in a frame where the BH is moving fast? Or, alternatively the Schwarzschild geometry of an massive star, that isn't a BH yet. Is it similar to a contracted version of the usual Schwarzschild geometry?PeterDonis said:But this reasoning doesn't apply to a black hole's horizon, because there's no way to measure the diameter; it doesn't have one.
Read the OP more carefully:rootone said:The black hole is approaching a speed of c in relation to what?
I'm confused here. Its true that an EH is a null surface but that applies to non-moving BHs as well. So it seems to me its not related to the state of the motion of the BH.PeterDonis said:But the "world tube" of the event horizon is composed of null curves, not timelike curves
serp777 said:Suppose a black hole travels at something like v = 0.999999999c relative to some observer. Does the black hole's event horizon becomes length contracted, thus appearing to turn into a black disk?
Shyan said:So it seems to me the worldtube of the EH can't be composed of null curves.
martinbn said:This would imply that the cross section area will be the same (for a stationary black hole) for all observers. So it seems natural to ask about geometric properties other than area as well.
Louis Philippe said:Would we find black holes moving near the speed of light in nature? Orbiting very close together or blasted out of a violent galactic nucleous? Or is it just a thought experiment?
Depends what you mean. From the perspective of a black hole, it's a cinch that there are particles passing it at very high fractions of c. From the point of view of the particles, it's the black hole that is moving at high speed, and the point of relativity is that either perspective is valid. So in that sense, yes, there are billions upon billions of particles encountering black holes moving at substantial fractions of c all the time. However, I tend to agree with Drakkith that we're unlikely to be able to see this directly ourselves.Louis Philippe said:Would we find black holes moving near the speed of light in nature? Orbiting very close together or blasted out of a violent galactic nucleous? Or is it just a thought experiment?
Louis Philippe said:Thank you for clarifying about the the two perscpectives. BTW I just found dozens of serious articles about stars moving at near the speed of light. They have not yet been observed because they a far away and faint but astronomers believe that when two supermassive black holes collide, as will happen with the Milkyway and Andromeda, as they get locked in the proverbial death dance some stars will be hurled out at relativistic speeds. And if stars get accelerated that way then why not black holes?
Agreed. However, an object being ejected at relativistic speeds must be rare, since it's effectively a case of it winning big in a once-in-a-stellar-lifetime energy lottery, and black holes are rarer than stars. I'd tend to think that black holes moving at relativistic speed with respect to other stellar-mass stuff near them are extremely rare, at best.Drakkith said:I see. I don't see any reason black holes wouldn't have the same thing happen under similar circumstances. I guess I've just never read about relativistic stars.
Ibix said:Agreed. However, an object being ejected at relativistic speeds must be rare, since it's effectively a case of it winning big in a once-in-a-stellar-lifetime energy lottery, and black holes are rarer than stars. I'd tend to think that black holes moving at relativistic speed with respect to other stellar-mass stuff near them are extremely rare, at best.
Ibix said:Agreed. However, an object being ejected at relativistic speeds must be rare, since it's effectively a case of it winning big in a once-in-a-stellar-lifetime energy lottery, and black holes are rarer than stars. I'd tend to think that black holes moving at relativistic speed with respect to other stellar-mass stuff near them are extremely rare, at best.
For rarity of black holes, or at least detectable black holes this 2008 paper notes the existence of 58 stellar mass black hole candidates and around one intermediate or supermassive black hole per globular cluster and galaxy. Other sources, such as the Royal Astronomical Society of Canada, cite on the order of 150 black hole candidates in 2012, in part based on the paper I linked. This still seems small compared to the number of stars.WannabeNewton said:Do you have references for any of these claims?
Good link. Very good explanation for popular science.Simon Bridge said:Aside: artistic interpretations aside, BHs are unlikely to look black.
http://blogs.discovermagazine.com/b...you-dont-know-about-black-holes/#.U03yjFT_RXo
To the observerrootone said:The black hole is approaching a speed of c in relation to what?
... technically you can't just say "the observer" in GR like you can with SR. In SR the observer is usually inertial. Where a black hole is involved, the observer is unlikely to be inertial so you'll see discussion involving distant and close observers, observers free-falling, and so on. Do you mean to ask about a relativistic black hole from the POV of a very distant observer? Perhaps headed directly away or directly towards the observer? Or are you thinking of a black hole passing by so it starts out distant, comes close, but not s close the observer gets gravitationally bound to it, and then retreats?To the observer.
A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape from it. This occurs when a massive star dies and collapses in on itself, creating a singularity with infinite density and zero volume.
Technically, a black hole itself cannot travel at the speed of light since it has a fixed position in space. However, objects near a black hole can reach speeds close to the speed of light due to the extreme gravitational pull.
The speed of light plays a crucial role in the formation and behavior of a black hole. It is the maximum speed at which anything can travel, including light. As an object gets closer to a black hole, the gravitational pull increases, and the speed required to escape the black hole's pull becomes closer to the speed of light.
It is not possible for a black hole to travel at the speed of light since it is a fixed point in space. However, if an object near a black hole were to reach the speed of light, it would essentially be stuck in the black hole's gravitational pull and unable to escape.
It is unlikely that anything could survive traveling at the speed of light near a black hole due to the intense gravitational forces. Any object or matter would likely be torn apart and absorbed by the black hole.