Sorry! I think I figured the mistake out right after I posted! This thread can be closed. I imagine a train powered by an engine of this sort: A region of hotter temperature evaporates some water and turn the wheels, and the evaporated water is at a lower temperature. So I am in the train and I observe that the engine works in the way described above. Relative to myself, I claim that I am not the one in motion but the station platform is in motion. But to claim that the station platform is in motion I must also claim that the wheel turns in the other way (edit: sorry, mistake here. give me a moment to sort this out). This implies that the evaporated water must fall back into its liquid form from a colder temperature to a hotter temperature. But by the second law of thermodynamics, this is impossible. So I must conclude that I am the one in motion and not the platform despite measuring in my own frame of reference. So, can I really not tell if I am in motion?