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Comment by kibwen | Hacker News Reader

Very interesting, I didn't realize if was possible for anything to survive the heat death of the universe. The answer to my immediate question:

"'While a space-time crystal looks like a perpetual motion machine and may seem implausible at first glance,' Li says, 'keep in mind that a superconductor or even a normal metal ring can support persistent electron currents in its quantum ground state under the right conditions.' [ ... ] Li is quick to point out that their proposed space-time crystal is not a perpetual motion machine because being at the lowest quantum energy state, there is no energy output."

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Lots of things will survive heat-death, but they will all be in maximum entropy, thermodynamic equilibrium so you can't perform any useful work after that.

I was also thinking of this as a perpetual motion machine but perhaps the slightest attempt to use force of the moving ions would cause the rotation to stop. My non-physicist brain says if you have something that will move forever, it sounds like free energy!

I believe that because we are talking about quantum effect energy, then it is a case of any form of measuring would effect it. That is to say any attempt to extract (which could be deemed a form of measuring/observing) any energy from it would be at the expense of the source. Can think of it as quantum friction if that helps. Perhaps a betetr way to explain it would be a atom, which has the classic solarsystem effect with orbiting electrons. Now we have managed to tap into that energy and that is the atomic bomb and later nucleur power stations. Now you know in those cases the original source of material/energy is changed and that is it in a nutshell, by measuring/observing/tapping into that level of energy will effect it and in a way introduces friction into a frictionless state of energy.

So while it might survive the heat death of the universe, there could be no way to measure it.

You would measure it by giving it energy, but not enough to perturb it. You give it exactly as much as it gives back, so you don't cause any changes.

This comment is speculation and is wrong. You cannot take energy out of a ground-state configuration but it has nothing to do, at least directly, with measurement.

It doesn't move forever, it moves until you stop it. It is a lot like a spinning satellite - the only reason it keeps going is because there is no friction.

By free energy one usually means the excess energy going out of a system after putting energy in. Since this type of transformation is not possible we never see truly free energy. There are lots of systems which have things in motion, and could in principle stay in motion forever, that do not provide free energy because the amount of energy going out = the amount of energy going in = zero. A pendulum in a vacuum is an example.

If you want an analogy to the quantum situation of a time clock spinning around you can think of the penny rolling in a funnel. https://www.youtube.com/watch?v=C2awnAeuFbc If there were no friction then the penny would never fall through the hole. It would reach a 'ground state', where it's as far down the funnel as its initial angular momentum will allow, and go no further but continue to whiz around. This is still not free energy and is not a quantum effect.

"My non-physicist brain says if you have something that will move forever, it sounds like free energy!"

Counterexample, hydrogen atom with electron circling in lowest (boring ole unexcited ground) energy state. Classical theory says that electron will radiate and auger in leaving the world with nothing but a sea of neutrons, but quantum mechanically its in its lowest energy state.

Electron orbits are not terribly interesting. Yes the s orbital is really boring and the p d and f orbitals are much more interesting looking (talk to a chemist) but fundamentally a bunch of blah.

In comparison this time crystal is an exceedingly weird looking complicated configuration, which means it'll probably be much more interesting.

"An object in motion will remain in motion unless acted upon by an outside, unbalanced force."

Keeping the ions moving once the material superconducts (reducing its "electromagnetic friction" to effectively nothing) requires very little, if any, energy. It can run entirely off its own inertia.

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