Consider a black hole in vacuum at Temperature T. The setup that I am interested is a one that collects thermal photons from a black hole by enclosing in a spherical photographic plate and then lowering it back into the black hole.
If one assumes that photographic plates are massless.(which they are not). Then the very fact that I can lower the photographic plates(after the photons are collected) back into the black hole allows for a possibility of for the creation of a perpetual motion machine, from the potential energy generated by lowering the collected photons back into the black hole.
The mass of the photons emitted is equal to the mass of the black hole that has evaporated away. When I drop the photons back in the black hole, The mass of the black hole is equal to the original mass that we started off with. Which is why i thought it could be perpetual. This can continue for ever.
Of course I know something is wrong with such a setup. I think It can be traced to my assumption about photographic being massless.
My understanding of black hole thermodynamics is pretty elementary. Perhaps some one could comment on what is going on?