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Comment by lscharen | Hacker News Reader

The core idea is not too strange, actually.

A "normal" crystal has a repeating structure in space; a time crystal is a construct with a repeating temporal structure.

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Does that mean it is continuously changing in a repetitive fashion?

apparently yes, but because it's in a low energy state, it's impossible to extract any energy from it and do work (otherwise it would be a perpetual motion machine.)

It's interesting it is a perpetual motion machine, but it works only as long as you don't use it.

Any atomic nucleus with orbiting electrons fulfills that requirement, you could loosely call those time crystals too.

Their half life for most stable atoms is so long that perpetual applies for most practical uses of the word.

Which makes me wonder if these time crystals have a half-life too.

Electrons do not orbit atomic nuclei in the quantum description of atoms. Instead, in the lowest energy state the electron wave function is spread out in a sphere around the nucleus, and this wave function stays constant in time. Almost all quantum systems do something like this in the ground state, which shows why the time crystal is something new and interesting.

It must be able to tunnel into some other structure. The half-life is defined by how much energy it needs to "melt" as the article says.

The universe has a perverse sense of humor.

right, now i'll have to read the article to understand why there is no energy loss :)

it does seem to be breaking some rules.. but I guess that's the achievement here. Intuitively there should be some concept of friction/heat loss but maybe on this scale things are different. Maybe it could be exploited as a battery? A way of capturing energy into a stable kinetic "state". (Like a flywheel..)

Moving doesn't implement energy loss automatically. When things are set in motion, they will continue to move infinitely unless interfered by outside forces.

Right, but physical movement within a material without loss due to friction implies some kind of mechanical superconductor.. I mean, that's a pretty cool concept.

Can any old oscillator be called a time crystal?

No - oscillators need constant energy input in order to operate. Time crystals require no energy for temporal repetition.

That's not really right. You can oscillate without constant energy input. Two particles can orbit forever.

But normally that oscillation represents stored energy.

They key to a time crystal is that it's oscillating while it's in a minimum energy state. It would take energy to make it stop moving.

A pendulum in a vacuum will oscillate forever, requiring energy to stop. Is it like that?

It requires force to stop. But you can output energy as you do so. Not so with these. They have 'no' energy despite movement.

Don't quantum harmonic oscillators oscillate even in their ground state? How is a time crystal different from a garden-variety QHO?

Nope. Like in the ground state of the hydrogen atom, the ground state of the quantum harmonic oscillator has the wave function spread out in a sphere, and this wave function stays constant in time. Almost all quantum systems do something like this in the ground state, which shows why the time crystal is something new and interesting.

I'm curious what does the temporal structure look like?

Edit: have just looked it up. Looks like it's basically 3-D structure with repeated motion over time.

So ... like a regular wave?

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