A newly theorised type of time crystal could revolutionise the potential of these fascinating structures. Unlike the time crystals that have been created to date, it would not require the application of an external stimulus to keep the atoms ticking.
The method hinges on inducing entangled particles to affect each other's 'spin' (a property like angular momentum) over a distance. But to understand the details of this latest approach, first we need to step back a little.
Time crystals may sound like some wacky sci-fi concept, but they're a real phenomenon, first theorised by physicist Frank Wilczek in 2012. From the outside, they look just like normal crystals. But inside, the atoms - arranged in an otherwise normal repeating lattice structure - are behaving quite peculiarly.
They oscillate, spinning first in one direction, and then the other. These oscillations - what is referred to as "ticking" - are locked to a very regular and particular frequency. So, where the structure of regular crystals repeats in space, in time crystals it repeats in space and time - hence, time crystals.
To date, time crystals produced experimentally have required an external stimulus (such as a pulse of electromagnetic radiation) at ground state, or lowest-energy state, to induce their ticking. This was achieved in 2016, but since then, there has been debate over whether this fits what we imagine a real time crystal to be like.
In fact, it has seemed very much that time crystals without an energy input to its ground state are simply physically impossible, according to a 2015 paper. In physics this is known as a no-go theorem.
But there is a notable exception to this theorem as it pertains to time crystals, and it's what Valerii Kozin of the University of Iceland in Reykjavík and Oleksandr Kyriienko of the University of Exeter in the UK used to approach the problem.
That 2015 paper assumes that interactions between particles grow weaker over distance. This is actually a pretty fair assumption to make - think of magnetic or gravitational forces weakening over distance, for example.
But there's a handy exception. Particles that are entangled have a relationship that doesn't grow weaker with distance. Measuring the spin of one particle will immediately determine the spin of its entangled partner, no matter how far away it is.
According to the physicists, in time crystals such interaction-at-a-distance could theoretically produce a time crystal ground state that needs no energy injection.
In their new paper, they propose a system of particles within the time crystal, each of which has a spin. They demonstrate that there is a way to describe the entangled particles' spins using a string theory model that meets the 2015 paper's definition of a time crystal.
Even if the particles were spinning out of sync, the interactions between the particles would produce the ticking of a time crystal, according to the researchers.
Now, this system would be incredibly complicated, with each particle able to spin in superposition - that is, in an undetermined state of both up and down at the same time.
In fact, the whole thing might not be feasible to create in a lab setting. Entangling particles in this manner is an idea that works well on paper, but is unlikely to be easily doable practically.
But time crystals themselves were a pretty wild idea when first proposed. The future could surprise us yet.
The research has been published in Physical Review Letters.