Black hole traveling near the speed of light

  • #26
WannabeNewton
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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.
Do you have references for any of these claims?
 
  • #27
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Well it does seem highly improbable that it would be a commonplace occurrence to have just the right gravitational circumstances necessary to accelerate an enormously massive object by that amount.
The minimum requirement would be to have several other super massive objects present and those having a very precise and unusual orbital arrangement.
 
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  • #28
Ibix
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Do you have references for any of these claims?
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.

For the rarity of stars ejected from galaxies at relativistic velocities, Brown et al detect 26 stars exiting our galaxy at up to around 720km/s as a result (they say) of interaction with our own galaxy's supermasive black hole. Hui and Becker report a neutron star travelling at around 1100km/s. Again, this is small numbers and none of them comes close to the OP's 0.999...c speeds.

Major galaxy collisions seem to have happened about once per galaxy in the last 9 billion years according to this, which is the easy reader version of Lotz et al. That is about "once in a stellar lifetime". I gather that other estimates for the frequency of collisions vary over an order of magnitude.

I think that covers everything I said. Do I gather from your question that you think I'm reading more into this than I should?
 
  • #31
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The black hole is approaching a speed of c in relation to what?
To the observer
 
  • #32
Simon Bridge
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To the observer.
... 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?

Did you have a look at the 10-things article about black holes? (See link post 29 above.) - the object that gets observed is probably not going to be a black ball for instance. What are you referring to when you talk about a black hole - the event horizon or the collapsed object that causes the event horizon ... or maybe you mean the light from the accretion disk or the optical effect on the background stars?

As you refine your question - you'll be able to get a better idea of the answers.
 

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