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Lets say we have a black hole that has a diameter > 1 light year and we can somehow push and pull this black hole {though the push and pull itself can never exceed lightspeed of course}.Will the other side of the black hole move in less than a year? Why or why not?

Qmechanic
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Mephistophilus
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  • How will you push/pull a black hole without another black hole? – Sam Jan 04 '20 at 13:12
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    Why does it have to be a black hole? No object can instantaneously transmit a force across it, including black holes. – bemjanim Jan 04 '20 at 13:21
  • BTW, a Schwarzschild BH with an event horizon of 1 lightyear diameter has a mass of over 1.6 trillion solar masses. Good luck moving that around... But as bemjanim said, it's pointless anyway. As far as we know, gravitational waves travel at the speed of light. – PM 2Ring Jan 04 '20 at 13:45
  • Related: https://physics.stackexchange.com/q/2175/2451 and links therein. – Qmechanic Jan 04 '20 at 14:00
  • @safesphere Sure, the speed of light at the horizon is zero, as determined by the observer at infinity, using Schwarzschild coordinates. However, a nearby freefalling observer doesn't see that. In general relativity, the local speed of light is always c. That's a consequence of the fact that GR reduces to SR in an infinitesimal region of specetime. But we've had this discussion in the past, several times, so I don't expect you to agree. ;) – PM 2Ring Jan 07 '20 at 19:20

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A black hole is not a rigid object and you can't push it. The event horizon is not a physical shell surrounding the black hole, it's just a place in space.

What actually happens if you try to push a black hole, e.g. with a long pole, is surprisingly complicated due to the time dilation and length contraction that happen near the event horizon. See Thought Experiment - Poking a stick across a Black Hole's Event Horizon for more on this. However the bottom line is that you can't push the black hole by pushing it with a pole (or anything else).

Hurling heavy objects into the black hole would cause oscillations in the event horizon, but those oscillations propagate at the speed of light so you couldn't use this for FTL communication either.

John Rennie
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