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

  1. Mar 31, 2013 #1

    I saw this:


    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?
  2. jcsd
  3. Mar 31, 2013 #2


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    Staff: Mentor

    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.
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