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Pulling 5% of mass from 2 black holes in a second

  1. Feb 17, 2016 #1
    OK, in solar mass terms, 29+36=62 with 3 left over. Or make that 3 left out.
    This is a little-advertised mechanism for pulling mass from a black hole. And, since the output is gravity waves, it doesn't seem to be directly related to Hawking Radiation.

    Somehow intense gravity can scoop out mass from a black hole - and the proportional amount of information that goes with it.
    But do those gravity waves really hold that much information? They seem to vary only by math functions based entirely on the mass and dynamics of the black hole. Can they encode more than that?
    It seems more likely to me that these black holes must have been shedding something more than GWs. What could that be?

    If I timed it right, could I cross the event horizon of one black hole just before the effects of another one pulled me back out and ejected me? I don't think so - but it certainly adds a new twist to that well-discussed journey.
  2. jcsd
  3. Feb 17, 2016 #2


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    Some of your mass energy might escape in the form of gravity waves, but it wouldn't really be you anymore.
    I don't think anyone really knows what happens to the information that falls into a black hole-- hence the information paradox.
    Last edited: Feb 17, 2016
  4. Feb 17, 2016 #3


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    None of the energy released as GW comes from energy crossing any event horizon. The energy comes from the orbital energy of the BH. Consider that shortly before merger, you have BH enormous KE (near light speed) in some COM like coordinates. The GW extract some of this energy. Note, the orbital decay and inspiral would not even occur without GW extracting orbital energy. Nutshell - GW carry energy that originates outside of the horizon(s).

    [edit: another way to look at this, to make contact with the notion of potential energy, is to consider how much energy would be needed to pull apart the inspiralling BH just before merger. You have gravitational potential energy -> orbital energy -> GW; in GR this is a complicated, non-linear process, but conceptually this is what is going on.]
  5. Feb 17, 2016 #4
    Besides of being less energetic, the result of the merger was also cooler than the original black holes.

    A cool thing can store same information with less energy.

    This is just simple thermodynamics - black hole thermodynamics is just simple thermodynamics, that's what I have learned, from Wikipedia.
  6. Feb 17, 2016 #5
    My mistake. The information is proportional to the area of the event horizon which is, in turn proportional to the square of the mass.
    So 23² + 36² < 62², and lots of additional information get's hidden, not revealed.
  7. Feb 19, 2016 #6
    Energy can be extracted from a black hole via spin and charge. The following is an extract from an old library entry for 'What is irreducible mass'-

    'As high as 29% of a black holes total mass can be extracted by the first process [rotational energy] and up to 50% for the second process [coulomb energy i.e. charge] (but realistically, charged black holes probably only exist in theory or are very short lived as they would probably neutralise quickly after forming).'

    Check out the following links-
    What is irreducible mass
    Black hole thermodynamics
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