Feynman got his dollar

so hattifattener and I can each have fifty cents: a vampire casts no shadow. It must transmit 100% of incident light. Still, a vampire is visible; it does reflect light. Hence, a vampire has total optical gain exceeding unity.

Placing any optical element of this sort within a sufficiently well-mirrored chamber will create a runaway optical amplifier, which if it is to be applied to peacetime uses must be equipped with an exceedingly fast-reacting system modulating absorbent damping. If this hurdle can be overcome -- perhaps using a nonlinear element? -- vampires provide unlimited free and non-polluting energy.

uh oh, we forgot something

Vampires don't appear in mirrors. Light is lost at some point in the path from source to vampire to mirror to eye. We don't know where in that path it is lost. If vampire-reflected light reflects from mirrors, and merely is imperceptible to the human eye, then the cell succeeds, though it cannot be used directly in a zero-fuel lighthouse built for isolated locales, but we should consider the more troublesome case. If vampire-reflected light is absorbed by mirrors, the mirrored vampire power cell is in trouble. (Experiment: see if a heated mirror radiates strongly in the pallid band reflected by vampires.)

At first thought, one is tempted to insert a vampire into a loop of fiber-optic cable. However, total internal reflection presumably constitutes a mirror. A graded-index fiber, though?

(My mail informs me that thatmathchick has already called us on this whole mirrors problem.)

If all else fails, vampiric light amplification may be left to more-advanced civilizations, ones capable of assembling a mass concentration approaching a black hole. For simplicity, take a black hole, and consider dipping a vampire into the photon sphere, at R_* = 3/2 R_s, where light travels in a circle. (No vampire report I am aware of suggests that this could act as a mirror.) The useful photons for public-utility purposes are those just outside R_*, which will receive repeated vampiric gain and then escape.

Huh. Dropping a vampire into a black hole would be very, very bad.

No, well, in some sense it may eat the universe, but I think not within finite time in an external observer's timeline. I think. What do you think?