We discuss a challenge to the second law of thermodynamics in an optical setting, in which two black bodies at strategically chosen points inside a perfectly reflecting cavity of appropriate shape apparently transfer energy asymmetrically so that one body experiences a net gain of energy at the other's expense. We show how the finite sizes of the black bodies lead to a resolution of the apparent paradox. We describe a simulation that allows us to follow the paths of individual rays and show numerically that the second law requirement of energy balance is satisfied. We also demonstrate that the energy balance condition is satisfied in the more general situation where the cavity and black bodies are of arbitrary shape.
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