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I understand that a quantum description of gravity would necessitate the transmission of gravity through a mediating particle, the graviton. This makes me wonder about black holes. A black hole would have to be emitting gravitons all the time. It seems to me that an isolated black hole -- even ignoring Hawking radiation -- would have to be losing energy just through the emission of gravitons and thus lose mass. Is this a correct extrapolation? Thank you.
Answer 1:

Unfortunately, the picture of forces being mediated by particles isn't all that accurate. The reason we use this picture is because in the best quantum theory we have, called quantum field theory (QFT), there's an approximate technique for doing calculations called Feynman diagrams (we have to use approximations because unfortunately, doing exact calculations in QFT is really hard!).

In these Feynman diagrams, we draw lines representing different types of particles, and where the lines meet is where we say the "particles" are "interacting." I put those words in quotes because in this context, "particles" is sort of a hand-wavy term, and shouldn't really be thought of as fundamental objects (there is a more rigorous way to define particles in flat spacetime, but that's not relevant here).

Now, the particles that transmit forces (called "bosons") are what are called virtual particles - that means that they only exist in these Feynman diagrams and are not detectable. Since they can't be detected, there's not much sense in calling them "real," and so the picture of massive objects losing energy through radiation of virtual particles is not an accurate one (of course, black holes do lose energy through Hawking radiation of real particles, but that's another story).



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