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B How come black holes have a mass?

  1. Mar 21, 2016 #1
    If the sun disappear right now, we won't notice before 8 and something minutes due to the speed of light. The earth will keep going it's way on it's orbit because, as everything, gravity cannot exceed light speed.

    The same would apply if we were orbiting à black hole.

    I often heard of the graviton, an hypothesis for a particle carrying gravity. Nothing can escape à black hole (only degenerated particles over very long period of time) so why is gravity escaping?

    In other words, why do black holes have a mass?
    Does this question makes any sense?
  2. jcsd
  3. Mar 21, 2016 #2


    Staff: Mentor

  4. Mar 21, 2016 #3
    Mabe I should have asked my question in another way. I know black holes are made of matter, but I wonder why gravity is escaping out of them.

    Gravity travels at the speed of light, not fast enough to escape à black hole...
  5. Mar 21, 2016 #4

    Jonathan Scott

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    Gravity is effectively an interaction with the gravitational field, not with the source of it. The distant gravitational field is created when the black hole is formed (and does not change at all for example if an object of a given mass collapses in a spherically symmetrical way).
  6. Mar 21, 2016 #5


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    Einstein's great accomplishment with GR was showing gravity is the geometry of spacetime. Nothing need escape a black hole to warp spacetime.
  7. Mar 21, 2016 #6


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    So the merging of the two black holes that caused the LIGO chirp happened outside of the Schwarzschild radius, and that's why we could detect the event?
  8. Mar 22, 2016 #7

    Jonathan Scott

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    The gravitational waves were produced while the black holes were orbiting one another, initially well outside the Schwarzschild radius. Even a black hole has to follow the shape of space-time, so no communication behind the event horizon is necessary for that to happen.

    Gravitational waves are driven by the sources of the fields being moved rapidly, causing a ripple of a slight change in direction to move through space-time at the speed of light. By conservation of momentum, it's not possible for one source to move on its own, so on average the field changes caused by a pair of masses orbiting one another cancel out (which means that there is no "dipole" radiation). However, as the system moves from maximum elongation to minimum elongation, there is effectively a slight squashing and relaxing of space perpendicular to the elongation, and that propagates as a "quadrupole" ripple through space.

    When the black holes get close enough, they start to interact and combine directly, with the event horizon deforming until it joins and encloses both, creating a single spinning black hole. That happens during the "ring down" phase. I think the idea is that the initial merged shape is like a dumbbell but rapidly becomes more circular, after which there is no longer any gravitational signal. Again, there is still no need for anything to communicate behind the event horizon, in that each part of the system is still simply following the shape of space-time.
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