Henry wrote: << I attended a talk last night by an MIT professor who is working with "photonic crystals", which have the property that they can selectively allow photons of certain wavelength/energy to propagate, while disallowing other photons. . . . This is not a perpetual motion machine of the first type, because energy is conserved, but isn't it a perpetual motion machine of the second type?

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I don't know from "PMM's of the nth type" -- but wouldn't a photon moving through empty space without hitting anything be a kind of PMM ? Hmm, I suppose that since force fields, though attenuated, extend (AFAIK) indefinitely through space, any photon would realistically experience "friction" with various force fields, so would gradually slow down . . . --Dan