Conservation of energy in general relativity & non-existence of PMMs

[sshai45]

Hi.

I saw this:

http://physics.stackexchange.com/questions/35431/is-the-law-of-conservation-of-energy-still-valid

The energy conservation becomes vacuous or invalid in the general theory of relativity and especially in cosmology.

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?

 

[mfb]

A law of energy conservation would need a proper way to define "the total energy at some point in time" - but in GR, there is no global "point in time" at all - and there is no clear way to define "the total energy" either. I doubt that you can use those mathematical issues to generate perpetual motion.