Hi. I saw this: http://physics.stackexchange.com/questions/35431/is-the-law-of-conservation-of-energy-still-valid Now, if that's the case, then I wonder about a related question: can general relativity be used to create perpetual motion machines? If not, then why not (i.e. how does energy conservation break down in such a way that the impossibility of PMMs is maintained)? If it can, then what would they look like?