Berkeley Lab proposes 4D clock
‘Space-time crystal’ would outlast the universe
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It’s not quite a perpetual motion machine: scientists at the US Lawrence Berkeley National Laboratory have proposed a design for a timing crystal they say would theoretically outlast the universe.
In this paper, published on Arxiv, the researchers propose a design for "a 4D crystal that has periodic structures in both space and time”.
While it sounds blue-sky, the researchers say a space-time crystal would provide important inputs to understanding problems in many-body physics (complex interactions between large numbers of individual particles).
To create the “space-time crystal”, the paper suggests trapping particles of the same charge in space using an electrical field. Their Coulomb repulsion forces them into a ring configuration, at their lowest possible energy state.
In classical mechanics, that’s an end to the matter: the “lowest energy state” would mean the particles can’t move. This is, it seems, the characteristic of a 3D crystal in the macro world: the particles have organized into their lowest energy state. To get them moving – for example, in a computer’s timing crystal – external energy is needed.
However, there’s an escape clause at the quantum level: the ions can be given a push with a weak magnetic field, to get them rotating, and since they lose no energy to the outside world, that rotation should continue forever – even, according to research leader Xiang Zhang, after the “heat death” of the universe. And since there is no energy output from the crystal, it doesn’t break the rules to offer a perpetual motion machine.
As Zhang explains here, “a spatial ring of trapped ions in persistent rotation will periodically reproduce itself in time, forming a temporal analog of an ordinary spatial crystal. With a periodic structure in both space and time, the result is a space-time crystal.” ®
COMMENTS
Perpetual motion machine
I'd also like to point out that there's nothing in the laws of physics that forbid perpetual motion. They just forbid perpetual motion machines. A machine does work. A spinning mass in isolation from anything else cannot do any work, therefore it could keep spinning forever. However it wouldn't be a machine because it couldn't do work. As soon as the system starts interacting with something else it could be put to work, but its energy would be dissipated in the process.
Spinning forever? Quite possible. Perpetual motion machines? Not so much.
"how does one use them as a crystal?"
I think that part of the conundrum is explained in this paragraph:
"However, there’s an escape clause at the quantum level: the ions can be given a push with a weak magnetic field, to get them rotating, and since they lose no energy to the outside world, that rotation should continue forever – even, according to research leader Xiang Zhang, after the “heat death” of the universe. And since there is no energy output from the crystal, it doesn’t break the rules to offer a perpetual motion machine."
However, what is not clear to me is how one is meant to "read" this clock. The action in so doing would surely impart energy to the system, a sort of "observer effect"? If by observing one imparts energy to the system is it not then above the zero state referred in the article? It is of course highly likely that I am missing something here because this is not my field. Is there a physicist in the house?
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