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How can a black hole lose its mass? Is Hawking radiation the only way?

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Hawking radiation is not the only way. Mass can also be lost in the form of gravitational waves since it takes energy to create these waves and energy is directly proportional to mass (times $c^2$).

Also note these waves are created in the event of two black holes spiraling and colliding with each other.

Guppy
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  • Note that gravity waves and gravitational waves are different. – Qmechanic Nov 22 '17 at 20:41
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    How does a black hole produce gravitational waves? – ProfRob Nov 22 '17 at 21:10
  • maybe if it is spinning? – innisfree Nov 23 '17 at 04:15
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    @innisfree Absolutely not, which is why I made the comment. Only merging black holes emit GWs. But even there, there is no way a black hole can accrete more mass (including another black hole) and end up less massive. It is only that the final black hole has a lower mass than the sum of the contributing black holes. That is not the same as saying an individual black hole loses mass (which they never do). – ProfRob Nov 23 '17 at 05:39
  • @RobJeffries What about hyperbolic orbit? I suppose there are times when you have gravitational wave emission without a merging event. In this case, is it alright to say the black hole loses mass? – OTH Nov 24 '17 at 03:55
  • @Otto Ah, no, but that can come off from kinetic energy. Haven't seen proof that it is impossible to lose any energy from mass, though. – OTH Nov 24 '17 at 04:08
  • See the 'shell with springs' analogy in the accepted answer here: https://physics.stackexchange.com/questions/237249/did-merging-black-holes-in-gw150914-give-up-entropy-and-information-to-the-gravi @RobJeffries is correct in saying that individual black holes do not lose mass. For me, its more correct to think of gravitational wave energy being lost as potential energy is converted into kinetic energy. The entire system has less kinetic energy than it would otherwise. – Keith Knauber Nov 28 '17 at 19:43
  • @RobJeffries In the interval while they are spiraling in and send the gravitiational waves we heard in the LIGO, before total fusion, each black hole will be losing mass by radiating the gravitational waves detected. – anna v Jan 11 '18 at 05:12
  • @annav Not so. The energy comes from the orbit. – ProfRob Jan 11 '18 at 07:49
  • @RobJeffries Are you saying that the fusion happens as a delta function in time? Now the two masses with M1+M2 and then less than M1+m2? The signal seems continuous. It would be nice if you have a link for this. – anna v Jan 11 '18 at 11:31
  • @AnnaV Individual black holes cannot lose mass. Obviously once the event horizons have made contact then there are no individual masses, only a system mass. The system mass can and does get smaller, but any calculation must include the energy associated with the angular momentum, both of the initial black holes and the orbit. The figures you see quoted for LIGO are the masses before merger and the masses after merger. I doubt that the mass during the merger is a well-defined thing. – ProfRob Jan 11 '18 at 12:18