The articles on the subject often say : a part of the mass of the 2 celestial bodies became energy for gravitational waves after merging : E(gravitational waves) = (M1 + M2 - M_merged).c^2

Intuitively I want to say that because there's a gravitational wave, there's a gravitational particle.

The rationale is that if gravitational waves are not quantised, then that would mean every body of mass is losing a little bit of mass all the time by generating arbitrarily small gravitational waves.

If I'm correct GR and QM can be unified. Please teach me where I'm wrong.

Related unanswered question: Energy Conversion from Mass to Gravitational Wave

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    $\begingroup$ I don't think that you want to say that intuitively. You want to say that because you have heard many other people say that before you and your brain is evolved to repeat things it has heard before. In this case that's a bad strategy as the literature on failed attempts to quantize gravity in the weak field (linearized gravitational wave) limit will tell you. As a counterexample... where are the quantum theory versions of ocean waves or acoustic waves in gases? Shouldn't they exist? Why do they not exist? $\endgroup$ – CuriousOne Feb 25 '16 at 23:46
  • $\begingroup$ Thank you. Is the conclusion that there's indeed a wave, that's carrying energy, but not in a quantified way and rather like a wave in the ocean ? I think you just taught me that I must learn more about the physics of fields. $\endgroup$ – TheNaturalTanuki Feb 25 '16 at 23:54
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    $\begingroup$ We do know that there is a classical wave-like behavior. We also do know that this, in itself, is not enough to formulate a quantum theory. That, of course, doesn't mean that one doesn't have to try and many really smart people tried really hard. The naive attempts have failed, now we have some less naive ones with string theory and loop quantum gravity. Either may still be missing the actual situation, or, better, they may be misjudging it. There may be another layer between a quantum theory of gravity and the large scale gravity that we are observing. I would keep an open mind about that. $\endgroup$ – CuriousOne Feb 26 '16 at 0:01
  • $\begingroup$ Thanks for the insight. Now I would like to ask specifically if by moving around I'm generating gravity waves and if so am I losing mass. Should I ask another question ? $\endgroup$ – TheNaturalTanuki Feb 26 '16 at 0:06
  • $\begingroup$ The energy that is radiated away does not come from the mass of the objects, but rather from the energy of the bound system, including their motion. There are many issues standing in the way of a quantum theory; there are no easy answers. $\endgroup$ – Peter Diehr Feb 26 '16 at 3:07

Your argument is that gravitational waves must be quantised because otherwise arbitrarily small amounts of energy could be transferred to a gravitational wave and therefore all objects would be steadily losing energy to gravitational waves.

Your argument doesn't work because (in principle at least) all objects with an oscillating quadrupole in their mass distribution are indeed steadily losing energy to gravitational waves. However the rate of energy loss is too low for us to measure.

When we say gravitational waves are quantised we mean that energy can only be transferred to or from them in discrete chunks that we call gravitons. However we expect that the energy of an individual graviton is so small that it is likely to be forever beyond experimental measurement. So any system we can observe is operating in the classical limit and the quantisation of the gravitational wave will have no observable effect.

Objects cannot just lose energy to a gravitational wave because gravitational waves are generated by an oscillating quadrupole in the mass distribution. The simplest example of this is a rotating dumbbell, which is basically what a binary star is, and indeed we have measured gravitational wave emission in the Hulse-Taylor binary pulsar system.

But the coupling of the gravitational field to mass is extremely weak so unless you have extreme sytems like a binary pulsar the amount of energy lost to gravitational waves is undetectable. For more on this see Is it possible to produce gravitational waves artificially?.

  • $\begingroup$ I'm not sure the OP was only referring to astrophysical objects. My take on the OP's Question: Take an atom or a protein - every motion should radiate GR waves (however little). Since these structures are stable - we can thus say that gravity is quantized. $\endgroup$ – Tom Andersen Mar 3 '16 at 23:06
  • $\begingroup$ @TomAndersen: the problem with that is that if you take the Schrödinger equation for the hydrogen, and replace the E&M constants with gravitational ones, you get "atoms" larger than the solar system. $\endgroup$ – Jerry Schirmer Dec 5 '16 at 18:39

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