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[BankItDrew]

Could the universe be considered a maching of perpetual motion?

[MadMojoMonkey]

Could the universe be considered a maching of perpetual motion?

This is complicated.

What do you mean by universe?

If it is a wholly self-contained system, inside which energy is neither created nor destroyed, and from outside which no energy is added or removed... then maybe. Otherwise, no.


What do you mean by motion?
The universe is expanding. If it keeps expanding above a certain threshold, then it overwhelms the gravitational attraction, and expands forever. This seems like a strong case for perpetual motion.

If it expands below that threshold, then the gravitational attraction will eventually overwhelm the expansion, halting it and turning it into contraction. This would, presumably, result in another Big Bang, and another universe. If the next universe starts with the same total energy as our universe, then this also seems like a strong case for perpetual motion. It must have the same (or more) energy to be perpetual. The more bit would likely explode it completely at some stage, but I think we can still call this a perpetual motion machine.

However, if the rate of expansion is just so, then it will approach an asymptote of some finite size, and never quite reach that size, ever slowing as it gets ever closer. This seems like a very weak case for perpetual motion. As far as we can tell, this final option is the case in our universe. That is, our universe is essentially "flat" from an GR point of view.