What is a time crystal?
In: Planetary Science
Toss a bunch of marbles into a bowl of water and they’ll bump and jostle their way to the bottom. Eventually they’ll all settle down into an unmoving state. This is their *resting state*, and it represents the point at which they have the least amount of energy. If you want them to move around any more you have to add energy back into the system (by, say, shaking the bowl around).
But imagine if I’m that lowest energy state they were still moving. Let’s say they continue to swirl around the bottom of the bowl. They’re at the lowest possible energy they could have but they’re still moving. Obviously, marbles in a bowl of water *don’t* do this so it can be difficult to picture, but that’s basically what a time crystal is: a system where its lowest energy state is still moving.
It’s a little more specific than that though, which is where the “crystal” part comes in. We define a (spatial) crystal as a structure that repeats in space: take any one part of the structure and you’ll be able to find identical parts at regular intervals in any direction within the structure. A time crystal, by analogy, exhibits the same repeating behavior. Instead of repeating in space though, it repeats in time. So that still-moving lowest energy state? The same arrangement of the system will occur time after time. If you take a snapshot of it any given moment you’ll be able to find it in that same arrangement at a regular interval in time. Usually this interval is measured in something like microseconds.
Now it should be noted that we’ve never created a time crystal that doesn’t involve *some* input of energy (i.e., shaking up the bowl) but we have created ones that continue oscillating for much longer than we’d expect for the energy put in. Most recently something like 40 seconds off a pretty weak laser pulse.
So what use are they? No idea! They’re a new thing to play with and physicists are still trying to figure out what to do with them. It’s pretty common in physics—and science in general—for things that eventually become important to not have any immediately obvious uses.
Latest Answers