Let's say we just had a normal atom in energy state E, and one of the electrons jumps down to a lower orbital E'. Of course, E - E' is positive because E' is a lower orbital than E, so the energy of the photon emitted must be positive. But what if E' were a higher orbital than E? Could the electron in the atom spontaneously jump up to a higher orbital and release a photon of negative energy? Virtually? Theoretically? I don't expect that we've observed such a thing, or I'd probably have heard of it, but from what I've read on the matter, Einstein's field equations don't forbid negative energy.