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Berkeley Lab proposes 4D clock

‘Space-time crystal’ would outlast the universe

Four dimensional tic tac toe

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.” ®

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