Is the universe a perpetual motion machine?

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    $\begingroup$ What does that even mean? $\endgroup$ Mar 14 '14 at 22:45
  • $\begingroup$ Some comments were deleted/censored. Please keep the language civil. $\endgroup$
    – Qmechanic
    Mar 15 '14 at 0:12
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    $\begingroup$ Hey cool it guys. I ask myself questions as ill-posed as the OP's all the time: maybe he/she could have thought more about it, maybe not and found the concept too bewildering. As onlookers we are in no position to judge. Kleingordon has given a wonderful answer that illustrates how working out how to ask the ill-posed questions is a highly nontrivial part of the scientific method. How we bring meaning to "perpetual motion" when in general there is no cosmological global definition of time and thus no conservation of energy is an ... $\endgroup$ Mar 15 '14 at 3:07
  • $\begingroup$ ... excellent question and one that Kleingordon shows we can still answer precisely. There has been some wonderful physics illustrated here as a result of the OP's question that I have thoroughly enjoyed seeing. $\endgroup$ Mar 15 '14 at 3:09
  • $\begingroup$ @WetSavannaAnimalakaRodVance, philosophically, the question is meaningless; the Universe is simply all there is, was and will be - including the laws of physics. The laws of physics are not apart from or independent of the Universe. The Universe - Existence - exists. There are no laws that govern Existence for that would imply something outside, independent of, and apart from Existence. The notion of perpetual motion, in any form, has no meaning independent of the context of Existence. $\endgroup$ Mar 15 '14 at 3:29

Nope, not if you use the standard definition for "perpetual motion machines of the first kind", which can indefinitely produce work. Entropy increases monotonically over the entire universe, and eventually all free energy will be gone. This is known as heat death.

Will particles in the universe continue moving forever? Probably. But that's not what perpetual motion thought experiments are typically about.

  • $\begingroup$ A fantastic answer bringing precise meaning to a slightly wobbly question: I ask myself questions as ill-posed as the OP's all the time, and the way that you have worked out how to ask the question and answer it so clearly is a wonderful illustration of the theoretical scientific method. $\endgroup$ Mar 15 '14 at 3:08
  • $\begingroup$ You're very welcome. This is one of the many reasons I've come to adore Physics SE. $\endgroup$ Mar 15 '14 at 8:14
  • $\begingroup$ Couldn't the universe be infinitely large with infinite matter? So are you assuming the Universe is curved? $\endgroup$
    – William
    May 28 at 17:31

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