Comment by ww520 | Hacker News Reader

Comment by ww520 | Hacker News Reader

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Every time the discussion of time crystal comes up, a simple analogy can explain what it is.

Regular crystal has repeatable structure in space, i.e. its lattice is the same shape over and over again across in space.

Time crystal is the kind of material that has repeatable structure across time, at the lowest energy state. It just means it changes its shape periodically over time.

An example of that is quartz crystal vibration where its structure bends and unbends over time. Quartz bending and unbending (vibration) is due to the piezoelectric effect when an external electric force is applied. The external force requirement disqualifies it as a time crystal.

The "at the lowest energy state" requirement means time crystal can "vibrate" by itself without external force applied. "Vibrate" is used loosely here. It could be the nuclear-spin flipped periodically, or some other structural changes that need no outside influence.

What you have described is an ideal time crystal. Unfortunately the time crystals we can create in the lab do need an external force, but this force can have a higher frequency than the oscillation of the crystal.

A powerful and mysterious device, whos mystery is only exceeded by its power.

Would it be accurate to say that the "Vibration" or nuclear-spin flipping doesn't emit energy or is neutral in the thermodynamic sense?

I guess if it’s emitting energy then it’s not at the lowest energy state. The emitted energy draws the energy state down further.

If you could coax work out of it in a ground state, that would be a perpetual motion machine, so no, it had better not emit energy. But you could detect the spin flips to make a clock

Spin waves are similar to vibrations (phonons) in a lot of respects, you still need energy to change a spin.

That said, some aspects of thermodynamics are counter-intuitive (well, actually most of it is). Vibrations are present even at 0 K because of the uncertainty principle, and that’s true for spin waves as well as actual vibrations. It does not mean that any energy is created. The difference with time crystals is that usually these vibrations don’t involve any symmetry breaking.

Time crystals are consistent with standard thermodynamics.

> Vibrations are present even at 0 K because of the uncertainty principle

Are you sure that's true? I can't remember the exact details but recall that being a common misinterpretation of the uncertainty principle

Yes it’s true. Kinetic energy is never exactly zero, that’s called zero-point energy. Perfect stillness does not exist at the atomic scale. It would involve knowing perfectly both positions and velocities (in case of atoms, the equivalent is true for spins).

https://en.m.wikipedia.org/wiki/Zero-point_energy

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