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How do black holes move?

  1. Nov 17, 2011 #1
    It is my understanding that time within black holes is slowed down incredibly, and all of the mass is found within this slowed down area. How then could a black hole move if time is slowed down so much (possibly infinitely)? Shouldn't this slowing of time also reduce the speeds of the mass within? Why would a slowed down particle retain its speed?
  2. jcsd
  3. Nov 17, 2011 #2
    All inertial movement is relative and does not depend on local (proper) time.

    Does the Earth move around the Sun or does the Sun move around the Earth or both? The answer is that it is all relative.
    Last edited: Nov 17, 2011
  4. Nov 17, 2011 #3
    I see. So does an observer at earth counting the total seconds contained within one revolution of mars around the sun with an atomic clock arrive at a different seconds count than a observer in earths orbit with a second atomic clock? What if the observer in orbit also moves really fast, does that change anything? What if they count the amounts of seconds within one earth year?

    I've always been under the impression that an observer would see everything speed up if that observer goes slower, including any of such movements.
  5. Nov 17, 2011 #4


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    According to classical GR, as you drop through the event horizon of a big black hole, you will notice nothing terribly different. Time will not slow down for you.

    There are many notions of time in GR, and the notion of time slowing down near a black hole is more related to the fact that if you send light to a distant observer, it will be very red shifted.

    Try http://casa.colorado.edu/~ajsh/schwp.html" [Broken] of gravitational time dilation and gravitational red shift, which are essentially the same thing: "That the redshift factor is the same as the time dilation factor (well, so one's the reciprocal of the other, but that's just because the redshift factor is, conventionally, a ratio of wavelengths rather than a ratio of frequencies) is no coincidence. Photons are a good clocks. When a photon is redshifted, its frequency, the rate at which it ticks, slows down".
    Last edited by a moderator: May 5, 2017
  6. Nov 17, 2011 #5
    Yes, although it is very small.

    There are three factors, one is the fact that the mass of the planets if different, and this will influence the relative clock rates and second the (pseudo-)elliptic orbits precess and one would have to deal with what constitutes an exact orbit from different perspectives (think Thomas rotation) and thirdly all orbits (except for light) in GR very slowly decay.
  7. Nov 17, 2011 #6
    Hmm well I didn't think an observer going in would notice his own inside clock going slower, but compared to the outside everything would be slower right? I'm having a hard time visualising why everything would go slower, except these inertial movements.
    How would gravity act upon a slowed down object? Would it pull as strongly, or would the accelerations be reduced? If it pulls as strongly, wouldn't the acceleration due to gravity be comparatively large in a heavily time dillated area compared to other kinds of slown down movements?

    Well I'm mainly curious if motions seen in space from a position would be time dilated in the same way as the rest of time is dilated. Or if it would be seen at some static speed regardless of time dilation. From your answer I couldn't really derive the answer of that.
    Last edited by a moderator: May 5, 2017
  8. Nov 17, 2011 #7


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    Basically light can be strongly bent in general relativity. If you use light from a distant object to say how time is passing for that object, there will be situations in which the light is so strongly bent that it will never reach you. So you will say that time has slowed down infinitely for that object according to this method of defining time. That's all, nothing deep.

    You can get the same thing in special relativity at the http://gregegan.customer.netspace.net.au/SCIENCE/Rindler/RindlerHorizon.html" [Broken]. The only difference is that in GR the light doesn't reach you because of spacetime curvature, whereas in SR it is because you are accelerating away from those objects.
    Last edited by a moderator: May 5, 2017
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