Could an object convert heat in the air around it to mechanical energy in order to accelerate itself?
In other words, the craft would take in surrounding air composed of random-velocity molecules and then eject the gas back out in a more unified direction so that it accelerates in the opposite direction.
This would use only that as a power source. So, for example, jet engines wouldn’t work because they use internally stored fuel in addition to outside air. Also I’m really just interested with fluid dynamics in mind. So like no collecting air molecules then fusing nuclei together or anything like that.
Would such a craft be possible? Or would some principle get in the way. For example, maybe the drag caused by collecting the gas would always cancel out the force caused by ejecting it, or something along those lines, I don’t know.
Please let me know if I should phrase the question more clearly.