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."

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!

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.

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.

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.

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.

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.

"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.

"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.

How would you read the state? If, at the lowest energy state, there is no energy output, would even photons change the state? If so how would you read the "time" from it?

Does that make any sense?

Via quantum entanglement, possibly? I honestly have no clue either, but they hint at entanglement, and they don't seem to be worried about observation:

"Peng Zhang, another co-author and member of Zhang's research group, notes that a space-time crystal might also be used to store and transfer quantum information across different rotational states in both space and time."

You could send photons arranged such that they don't change the state of it. You would have to supply the external energy necessary to read it.

But you arrange things such that any energy you give it is immediately given back to you, so you don't change the state.

Can someone explain how this thing is different than an ordinary ring of like atoms (e.g., benzene) rotating in a magnetic field?

What experiment can I perform to distinguish a "space-time crystal" from a non-STC in ordinary peroidic motion?

Can you manipulate future and past states by manipulating the current state? I'm not too knowledgeable about the matter but it seems like if the crystal's states at certain points in time are fixed from its inception then it can be manipulated to transfer information to future and past states of the crystal.

Its only state is its very exotic looking ground state, at least compared to all the other ground states I can think of at this moment. So no manipulating the state because the definition of the thing is its a constant ground state.

There's a fun thermodynamic argument that does not fit in the margins of this hacker news explaining why you can not transmit information by lowering entropy on something already in its ground state.

As for transferring information in general, see "light cones". Going forwards you can't send info further away than the speed of light would reach, and going backwards there isn't much of a light cone...

This prevents information from being stored inside the system, but does not prevent the system from being used "in bulk" as information storage. Crude analogy is you could make a Turing machine where "mark" equals drop one of these crystals on the tape "space" equals wipe it off or flip it upside down or whatever. Or you could make essentially a punchcard by building a 10x10 array of them and smashing some of them in a pattern... theoretically the unsmashed ones would never ever decay, and the smashed ones are too complicated to spontaneously reform, so a large collection of them aligned in certain patterns would actually make a pretty decent ultra long term storage media.

I can imagine some future analogue of Danny Hillis constructing one of these things, entombed inside the a planet sized crystal extracted from the heart of a gas giant, parked in intergalactic space. The purpose would be to save the universe from heat death by ensuring that time will always have meaning.

Is that entire article just trying to say that the ion-ring, which is in it's lowest energy state, rotates at a perfect rate (will never speed up or slow down) and hence you have the central peace of a very accurate time keeper?...

Is the speed of the electrons quantized, or can you "tune" the period continuously? Either would be useful.