[jiggawatts]

> In fact mass-energy equivalence is a pretty core part of quantum mechanics.

It may be stated as such, and added in to equations as an external piece of knowledge from relativity, but this is cheating a bit.

Essentially, when we state that H2O has less mass than H+H+O, what we actually mean is that H2O bends spacetime a little bit less than the three atoms individually that made it up. There's no accepted variant of QM or the Standard Model that explains this. The dynamics of spacetime curvature rearranging as the photon is emitted as the hydrogen atoms burn is not explained by modern science. This is fundamentally the "QM is incompatible with GR" issue.

My point was that it isn't just near black holes that a GR-compatible microscopic theory is relevant.

It's relevant even in the flame of a candle. It's a small effect, but it's there. The inconsistency in the theories occurs at all scales.

[tsimionescu]

While you're right about the inconsistency between GR and QM applying at any level, you're wrong about needing GR to talk about the mass of the water molecule.

Even in pure QM, the water molecule will have less inertia than unbonded hydrogen and oxygen atoms. This should in principle be measurable by applying a known force to the water molecule and to the three atoms, and measuring their acceleration. The difference should perfectly match the inertial difference predicted by SR and GR.

GR adds the observation that, if the water molecule has less inertia, it should also bend space-time less, and it is this bending of space time that can't be explained by QM.

Though I should add that I've had a reply to a different comment once that explained that QM is actually compatible with the flat-ish but not perfectly flat space times that GR predicts anywhere not very close to a black hole. They were claiming that in fact modern QFTs can even predict things like the gravitational lensing produced by our sun, and that they only break down when near the event horizon of a black hole.

[naasking]

> Essentially, when we state that H2O has less mass than H+H+O, what we actually mean is that H2O bends spacetime a little bit less than the three atoms individually that made it up. There's no accepted variant of QM or the Standard Model that explains this.

I'm not sure this is correct. It bends spacetime less simply because it's in a lower energy state. It's correct to say that the Standard Model doesn't explain spacetime curvature, but the curvature in GR is implied by the energy which is explained.