Andrei G. Lebed, "Breakdown of the equivalence between gravitational mass and energy for a composite quantum body," http://arxiv.org/abs/1404.4044 [corrected a mistake in the following paragraph] He seems to argue that hydrogen atoms moving from one region of space to another, with a different gravitational potential, will make transitions from the ground state to an excited state. This violates the equivalence principle. He proposes a space-based experiment to detect the effect. I suppose this shouldn't be particularly surprising, since semiclassical gravity (a) doesn't seem to work reliably, and (b) has a tendency to produce results that violate the equivalence principle. I'm basing this on my non-specialist impression of the kind of stuff that people like Barcelo do, e.g., http://arxiv.org/abs/0902.0346 . They have to do all kinds of renormalizations, and when they're done they predict dramatic things happening at the event horizon of a black hole, which violates the e.p. My take on it has been that semiclassical gravity is simply bogus and shouldn't be trusted. Comments?