Ask HN: Can anyone explain space-time crystal for dummies?

A space-time crystal is a "microscopic" object that breaks the time symmetry. Time symmetry means "if you inverted time, the process would look the same". Imagine this is as if you were going back in time. Literally.

First, an analogy in macroscopic physics. Assume an Engine, like in your car. As each piston moves through time, it goes through 3 phases. Injection, Compression, Ignition.

If you inverted this process, going from ignition, to compression, to injection, your engine would not work. It would probably destroy itself, but it would at least not work. Therefore, your engine breaks time symmetry. Obviously. Most macroscopic processes are not time invariant.

On the microscopic scale, most quantum systems are believed to be time invariant. If you invert time on a hydrogen atom, nothing would change about it.

But a spacetime crystal is a system that exerts state transitions, like your engine does. If you were to "record" its behaviour and then play the recording backwards, you would observe a physical process that is impossible. Therefore, it is not time invariant.

Disclaimer: This explanation is, as requested, for "dummies". It is wildly inaccurate at best, but seeks to explain the problem without introducing any of the necessary physics.