First proposed by Nobel-Prize winning theoretical physicist Frank Wilczek back in 2012, time crystals are hypothetical structures that appear to have movement even at their lowest energy state.
This ability violates a fundamental symmetry in physics called time-translation symmetry, but physicists have now demonstrated that it might actually be possible for time crystals to physically exist.
If none of this makes sense to you, don’t worry, we’ll run you through the backstory.
In 2012, Wilczek and a team of theoretical physicists at MIT suggested that it could be possible to add a fourth dimension - the movement of time - to a crystal, imbuing it with the ability to act as a kind of perpetual 'time-keeper', or clock.
In basic terms, Wilczek envisioned an object that could achieve everlasting movement by periodically moving and then returning to its original state over and over again in its lowest-energy state - known as its ground state.
As the energy of the ground state is known as the zero-point energy of a system, it means movement should theoretically be impossible - but perhaps not for time crystals.
As Bob Yirka explains for Phys.org, Wilczek proposed that it could be possible to construct a time crystal using a low temperature superconductor, because crystals naturally align themselves at low temperatures.
"[I]t seemed reasonable to assume that the atoms in such a crystal could conceivably move or rotate and then return to their natural state naturally, continually, as crystals are wont to do as they seek a lowest energy state," says Yirka.
The idea was that a ring of ions inside the crystal could be made to move independently inside the crystal - like a mouse exploring the inside of a snake's stomach - but he couldn't figure out how to build such a thing.
Within months, a separate team of physicists from Purdue University jumped in and said Wilczek’s plan could work - they just needed better ion traps, which was something that would conceivably be developed within the next few years.
Fast-forward to now, and those predictions are looking pretty spot-on.
Physicists from the University of California, Santa Barbara have proposed that it’s possible, in theory, to build a time crystal from a large system of trapped atoms, ions, or superconducting qubits - particles used in quantum computers to replace the bits of today’s computers - just as the Purdue team had predicted four years ago.
But they weren't looking for a method to produce a time crystal, specifically. They were more interested in trying to prove that time crystals could exist by tackling the biggest argument against them - that their existence fundamentally breaks time-translation symmetry (TTS).
Time-translation symmetry is a version of one of the three symmetries of space-time called translation symmetry, which states that the laws of physics are the same everywhere and at all times.
It's one of the most fundamental assumptions of our current understanding of physics, but the University of California team argues that you can actually break time-translation symmetry without having everything you know and love crumble around you.
"The crucial difference here is between explicit symmetry breaking and spontaneous symmetry breaking," one of the team, Dominic Else, told Lisa Zyga at Phys.org.
"If a symmetry is broken explicitly, then the laws of nature do not have the symmetry anymore; spontaneous symmetry breaking means that the laws of nature have a symmetry, but nature chooses a state that doesn't."
As Zyga explains, while spontaneously broken time-translation symmetry has never been observed before, almost every other type of spontaneous symmetry breaking has been - for example, how magnets get their north and south poles (what force decided which would be north and which would be south?), and how ordinary crystals look different when viewed from different angles in space.
Using a simulation, the team demonstrated how spontaneously broken time-translation symmetry could take place in a type of quantum system called 'Floquet-many-body-localised driven systems'.
They found that a simple crystal could be turned into such a system, and was able to achieve two things that allowed it and our current understanding of physics to co-exist.
First, it remained far from thermal equilibrium at all times, meaning the system never heated up, despite its periodic, oscillating motions.
And secondly, as the size of the system continued to grow, the time it took for a symmetry-breaking state to relapse into symmetry-respecting state increased, meaning that in an infinite system, the symmetry-respecting state can never be reached.
So time-translation symmetry can be broken indefinitely within the time crystal system, but this perpetually rotating object doesn't heat up, so the second law of thermodynamics remains intact - a crucial stipulation for a time crystal to exist within the laws of physics.
"The significance of our work is two-fold: on one hand, it demonstrates that time-translation symmetry is not immune to being spontaneously broken," one of the team, Bela Bauer, told Phys.org.
"On the other hand, it deepens our understanding that non-equilibrium systems can host many interesting states of matter that cannot exist in equilibrium systems."
The next step, of course, is for someone to go ahead and build such a thing for real. And with this evidence in place, there's never been a better time to carve out a time crystal.
The study has been published in Physical Review Letters.