As I understand it, the second law means that energy always tends to get delocalized. A blunt way to put it is that you can't make energy flow from a cold source to a hot one without doing work ( ie localizing a point by delocalizing another ) But here is an idea that struck me Suppose you have a hot body and a cold body. both sepereated by a long tube. Trough this tube , radiation goes back and forth between the two. Suppose now that we place a refractive medium between them ( glass or anything I don't know ) Since the hot body and cold body radiate different frequencies of light, the get refracted through this medium at different angles, and now we adjust it so that the radiation from the cold body reaches the hot one but the radiation from the hot body is refracted in such a way that it never meets the cold body. So haven't we successfully made heat travel from a cold body to a hot one without doing any external work?