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Information out of BH

  1. Sep 4, 2012 #1

    Metric and electromagnetic field outside BH are determined by mass, angular momentum and charge of BH. Does this imply that information of the values of the three parameters come from BH through event horizon? Is it possible?

  2. jcsd
  3. Sep 4, 2012 #2


    Staff: Mentor

    No. That information comes from the object that originally collapsed to form the BH. More specifically, at any event outside the BH's horizon, the observed fields at that event are determined entirely by the sources of those fields that are in the past light cone of that event, and those sources will therefore have to be in the collapsing object, before it falls below the event horizon.

    This is actually not limited to BHs; it's true in general in relativity, as a consequence of the general principle that information cannot travel faster than light.
  4. Sep 4, 2012 #3
    Thanks. According to your explanation, I can assume that all the blackholes should have their history of collapse. There cannot be natural born BH. Is this assumption right?

  5. Sep 4, 2012 #4


    User Avatar
    Gold Member

    What is a "natural born BH" ?
  6. Sep 4, 2012 #5


    Staff: Mentor

    If by "natural born" you mean a BH that has always existed and was never formed by the collapse of a massive object, then there is a mathematical solution of the Einstein Field Equations that describes such a BH (it's called the "maximally extended" Schwarzschild solution), but as far as I know, nobody considers that solution to be physically reasonable. So any actual BH will have been formed by the collapse of a massive object.

    Btw, one of the reasons the maximally extended solution isn't considered physically reasonable is that the answer I gave to your question doesn't work, because in that solution there are no "sources" anywhere--the entire spacetime is vacuum. What allows that solution to still have a "mass", even though it is vacuum everywhere, is the presence of a "white hole" singularity in the past, as well as the black hole singularity in the future. But as far as I know, nobody thinks a "white hole" is physically reasonable either, so that's another reason why the maximally extended solution is not considered physically reasonable.
  7. Sep 5, 2012 #6
    Thanks phinds and PeterDonis for taking me up to the higher level of understanding. Regards.
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