Yes, this came up in the pub. My best answer was that the vampire must heat up or cool down as required: if both the R- and S-photons end up interacting with something then the vampire must lose energy, and if neither does then the vampire must gain energy. Unfortunately that gives rise to an obvious thermodynamics problem (shine bright light on a vampire such that most of the generated R- and S-light actually ends up being used, and the vampire must eventually chill to absolute zero in defiance of 2LoT – and then what happens if you continue to shine light on it?), and also a worrying action-at-a-distance effect. Though the latter at least might have interesting applications in long-distance communication; hmm.

Shine a laser into a distant galaxy; interpose a vampire. When does the vampire lose energy? If at once, you have a predictor for whether or not the R-photons are going to hit an object in a few million years' time; if the energy loss only occurs when the R-photon strikes something, or maybe even subsequently when the information about the strike has been returned to the vampire by some kind of "R-antiphoton"… what happens if someone stabs the vampire with a stake in the intervening aeons?

Well, then you have an R-antiphoton containing negative energy which is going to end up hitting something at random and causing an unexpected phenomenon.

If you don't like that, I think the ball is in your court to propose an alternative optical model of vampirism which has fewer problems with conservation of energy :-)

I think it's high time we stopped faffing about with the theoretical physicists and started taking some measurements off an actual vampire in the optics lab.

Assuming 2LoT holds and the vampire is unable to reach thermodynamic equilibrium with its surroundings (rate of energy loss too high), then the energy of subsequent R and S photons will decrease as they carry away energy from the vampire.

A fail-proof vampire detector could then be constructed. In any brightly lit room, the vampire will be the inky black shape.

Hmm, perhaps an illuminated, observed vampire would be an almost perfect cold load for heat engines.

Obviously, if you shine a bright enough light (eg sunlight) on a vampire, it will rapidly cool down past the point where life (or unlife) can be sustained, and it crumbles to ex-vampiric dust. Which, being no longer a vampire, interacts with photons in the normal manner, hence we don't need to worry about it hitting 0K.

I don't think the well-lit observed vampire cooling effect violates the 2LoT, does it? The entropy is being carried away from the vampire, but that doesn't mean entropy is decreasing universally-- just that we're pumping the entropy elsewhere via R- and S- photons. Local decreases in entropy are the only reason we're all here, anyway-- we pump entropy elsewhere in order to organize our cells/bodies/planet/solar system/etc.

As for the absolute zero issue, I suspect that once the vampire hits absolute zero, the energy would have to come from mass, which explains why a vampire might crumble to dust in direct sunlight-- they begin losing mass to extra photons, and you end up with structures that can't sustain themselves anymore and collapse under their own weight. At the extreme case, I suppose either you'd end up with a pile of bits that is no longer a vampire (and thus the process would halt) or the vampire would dissolve entirely into r- and s- photons.