Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Doesnt information travel faster than light in case of black holes?

  1. Jan 22, 2012 #1
    i was wondering,
    for a blackhole, as everyone knows,the escape velocity is greater than light.
    this means that the information about the existence of the blackhole in terms of light never reaches the observer.
    so if the blackhole were the cease to exist at an instant of time, the observer will never see the event happening. He will only feel the instantaneous change in the gravitational field. so in a way both gravity and information has traveled faster than light.

    an alternative way of considering the same problem would be by expressing gravitational force to be transferred by gravitons. in this case the gravitons reach the observer but the photons don't. so gravitons can escape the gravitational field or 'climb over' the curvature of space time while photons cannot.
    so gravitons can be considered to travel faster than the escape velocity giving the same consequences as the first case.

    where is my thought process incorrect?
  2. jcsd
  3. Jan 22, 2012 #2


    User Avatar
    Gold Member

    None of this is possible or correct. Gravity does not travel instantaneous, that was the whole point of General Relativity.
  4. Jan 22, 2012 #3
    so what is the explanation or correct thought process for this?
  5. Jan 22, 2012 #4


    User Avatar
    Gold Member

    As far as what happens if a black hole were to all of a sudden cease existing, there is no explanation because that's not possible. There is something to be said about information that passes into a black hole though ( http://en.wikipedia.org/wiki/Information_loss_paradox ).
  6. Jan 23, 2012 #5


    User Avatar
    Science Advisor

    During black hole formation there is no instantaneous change in the gravitational field. Gravitational waves are generated during the collaps outside / before the formation of an event horizon. In perfectly symmetric collaps w/o gravitational radiation for an outside observer the gravitational field does not change at all, the Schwarzschild metric is stationary.

    No. The Schwarzschild metric is stationary, and gravitational waves generated during the collaps outside / before the formation of an event horizon travel with v=c.

    It's a common misconception to think that gravity is mediated solely by gravitational waves (gravitons - which we don't need b/c we do not quantize gravity here).

    Compare that to electrodynamics and QED: there is a well-known and exact formulation of QED where by integrating out unphysical i.e. gauge degrees of freedom one finds a Hamiltonian consisting of
    1) a stationary Coulomb potential for charge densities plus
    2) fluctuations i.e. transversal photons.
    So there is a stationary Coulomb potential which does not rely on any photon to be exchanged. In addition one can prove explicitly that this formulation is fully Lorentz covariant (even so the stationary Coulomb term looks strange; the algebra of all Lorentz generators closes w/o anomalies, showing manifest Lorentz covariance in the QED Hilbert space).

    Something similar is known in gravity. If you look at the effective gravitational potential used for the calculation of perihelion precession it looks like an instantaneous Newtonian potential plus (instantaneous) correction terms; but of course there is no instantaneous interaction; the formulation does neither break symmetries of GR nor does it violate c a s universal upper bound for physical interactions.

    As explained gravitons neither can not need 'climb over' the curvature of space time; the external gravitational field is not generated by gravitational waves leaving the black hole but is nothing else but the gravitationsl field that already existed before the collapse.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook