What happens to the energy in a photon incident upon a vampire?

I see three possibilities:

1. Both the R-photon and S-photon carry all the energy of the plain ol' photon.
   
2. The R-photon and S-photon each carry some proportion of the energy.
   
3. The energy goes Quantum (hand-wave) and is found by the first person to go looking for it.

(1) has the obvious problem that it violates conservation of energy; (2) has the problem that objects viewed in a mirror through space occupied by a vampire will look dimmer and/or vampires will themselves look dimmer when viewed directly; (3) avoids that objection, but only until both the reflection through the vampire and the vampire are simultaneously observed.

Thinking about it further, the only option that makes sense is (1), with the vampire acting as an energy source — maybe that's why vampires can't cope with sunlight? But even then, where does the energy go when an R-photon hits a sensor or an S-photon hits a mirror?

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.

Y'see, that's where I was going to start with a description of diffraction phenomena too.

Why doesn't (2) make sense? They might indeed be dimmer.

But then vampires are visible in mirrors, which violates the premise of the whole thing! Being visible as a patch of dimness is just as bad as being visible as an evil-looking fanged person, from the point of view of a vampire trying to sneak up behind a maiden admiring herself in her bedside mirror. Either way, she's going to notice something fishy is up, turn round, scream, and thereby summon stake-wielding security guards.

Hmm, good point. Perhaps there would have to be a split in energy such that the S photons (if those are the ones that are reflected by the vampire but not then by the mirror) get most of the energy and the R photons come out with hardly any, like a gentle infrared signal. Ooh, in fact, you could play with the energy split in terms of wavelength and come up with some nice stuff. Because I needed something else to do this weekend like a hole in the head...

Well (he says, noticing he has a vampire-duck userpic) the amount by which the vampire was dimmer viewed directly would have to be the complement of the amount by which the background was dimmer viewed in a mirror. One or the other would have to be at least 50% dimmer than usual, which would surely stick out like a sore thumb?