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Gravity wave light wave.

  1. Jul 7, 2010 #1
    near black hole we are able to feel the gravity..as in the gravity force. when light enters we can no longer see it come back. since the gravity wave is able to come out does it mean that it travels faster than light wave?... sure I'm wrong.. could someone explain.
  2. jcsd
  3. Jul 7, 2010 #2
    It seems you perhaps conceptually misunderstand a few things here.

    You cannot see light coming out a black hole when near it
    That depends a bit on the observer. For instance a free falling observer who has his foot with an attached flashlight past the event horizon can still see it even when his head has not yet past the event horizon.

    Being able to feel the gravity near a black hole
    I assume you are talking about tidal effects here as (the first order) gravitational acceleration is not felt because the frame of reference accelerates as well.

    The intensity of the felt effect depends on the mass of a black hole. For a very heavy black hole the effect is virtually non-existent. In other words a free falling observer passing the event horizon of a very black hole will practically speaking not notice anything.

    Black holes sending out gravity waves
    Black holes as modeled by the Schwarzschild and Kerr metric do not send out (or receive) gravitational waves.
  4. Jul 8, 2010 #3


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    No. Nothing can escape from the EH, that has nothing to do with the state of motion of the observer.
  5. Jul 8, 2010 #4
    Perhaps you should accurately read what was stated.

    When the light of the flashlight moves towards the event horizon (to be followed by being sucked in again) the head of the observer approaches and eventually passes the event horizon also. The observer will clearly see his feet.

    If you disagree, please refer to the literature where it states that a free falling observer cannot see his own feet when his feet pass the event horizon but not yet his head.
  6. Jul 8, 2010 #5


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    I did:
    He will not see the flashlight past the event horizon until his head is also past the horizon. There is no way a flash from inside the horizon can be seen by the head when it has not yet passed the horizon.
    If you disagree, you're simply wrong. If you agree, perhaps you should accurately write what you want to state.
  7. Jul 8, 2010 #6
    I have to agree with Passionflower. I was asked this exact question in an exam with Prof. Bartelmann, and the answer is (sadly it's not the one I gave) that the event horizon is only an event horizon in the frame of an observer at infinite distance. In the case discussed here, the observer will see his feet at all times. Remember, it is only a coordinate singularity, no special physics is going on there.
  8. Jul 8, 2010 #7


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    This might not have been made clear by the previous posts, so I'll clarify here. Gravity waves and gravitational force are not the same thing. Gravity waves travel at the speed of light and cannot escape the black hole, however, this does not mean you cannot feel the gravitational force of the hole. Gravity waves carry information about changes in the gravitational field, but they are not carriers of the gravitational force.
  9. Jul 9, 2010 #8


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    Either you don't remember the details of the question exactly, or Prof. Bartelmann used a nonstandard definition of "event horizon" (i.e. apparent horizon as opposed to absolute horizon).
    The standard definition is that of an absolute horizon, meaning essentially that the horizon is what observers at infinity see. IIRC, "the boundary of the region that is not in the causal past of future null infinity".
    Yes, because the apparent horizon recedes.
    I urge you to read carefully, too: I did not say that the observer can't see his feet. I said that he can't see a specific event at his feet (e.g. the flashlight, triggered inside the horizon) until his head is also inside the horizon.
    So while he sees his feet until near the singularity, there is not a single photon escaping the event horizon. There's nothing observer-dependent about this fact.
  10. Jul 13, 2010 #9
    OK i got it. gravitational wave and force are different phenomena.
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