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Black Holes and Light

  1. Jul 17, 2007 #1
    Black holes exert such a powerful gravitational force that even light cannot escape its grip. But doesn't this imply that the rule about how nothing can exceed the speed of light has been violated? If a black hole can suck in light, then doesn't that mean that the gravitational force of a black hole can exceed the speed of light?
  2. jcsd
  3. Jul 17, 2007 #2


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    No - force and speed are different quantities. The force of gravity doesn't have any particular speed.
  4. Jul 18, 2007 #3


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    Mathman gave you a correct answer. However I am interested in what I think is your mental picture. It seems as if you imagine the Force having a speed, as if it consisted of little force particles running after the light particles, trying to catch them! And these force particles have certain speed, limited by the usual speed limit.

    If the Force police actually WAITED until a light particle tried to make a run for it, then I think you would be right! The force would never catch up and haul them back!

    But actually the force is embodied in the GRAVIATIONAL FIELD that is already out there around the black hole.
    You can think of it either as a force field or as a curvature of the GEOMETRY of spacetime, which GUIDES almost every outgoing lightray back into the hole.
    With almost no exception, all the geodesics---the straightest lines light can follow in the curved geometry---lead back to the hole.

    Or if you dont like geometry, think "force", but it is the same in gravity case. Changes in the field do propagate like waves, it is thought, probably at the speed of light. But for a steady black hole the field is not changing and it doesnt have any speed. THE FIELD IS ALREADY OUT THERE ready to send the light back.
    Last edited: Jul 18, 2007
  5. Jul 18, 2007 #4
    Okay, thanks, that makes sense......
  6. Jul 18, 2007 #5
    1 universe.
    Billions of galaxies.
    Each galaxy contains billions of stars.
    Each star has at least one planet orbiting it.
    Surely there’s more life out then us, and more intelligent
  7. Jul 20, 2007 #6
    If gravity does propagate at c, this would mean that there could be areas of space time that were never "disturbed", wouldn't it? What would be the properties of such localities, zero inertia?
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