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Does black hole's gravity really travel out of it's own Event Horizon?

  1. Jul 6, 2007 #1
    To Physicists/Administrators/Moderators:

    The following two facts raises a doubt
    1) "No influence can travel from inside the Event horizon of a black
    hole to the outside of it AT or below the speed of LIGHT (V<=C)."
    2) "Gravitational influence travels AT (or below?) the speed of light only".

    The question is how the GRAVITY itself of a mass say, 'X' -which is located INSIDE the Event horizon of a Super Massive Black hole can influence a particle say, 'Y' which is located OUTSIDE the Event horizon IF at all gravity CAN'T cross the Event horizon at V=C?
    Eventually if 'X' so can't influence 'Y' then this Black holes can't GROW anyway by swallowing any such 'X' (it looks like a paradox as gravity is proportional to matter inside the black hole). Thanks.
    (I'm a new comer looking for some answers.Please help)
  2. jcsd
  3. Jul 6, 2007 #2


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  4. Jul 6, 2007 #3


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    Staff: Mentor

    In a static situation, the gravitational field (better, the curvature of spacetime) doesn't "travel". It simply "is".

    When we talk about the "speed of gravity" or the "speed of gravitational influences," we are really talking about the speed of changes in the gravitational field.

    To make an analogy with electrodynamics, a stationary charge has a static, unchanging electric field associated with it. If you start to shake that charge back and forth, the electric field (and magnetic field!) at each point starts to change, and those changes propagate outwards from the charge at speed c.
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