What are time crystals?
Ice crystals are more orderly than flowing water – right? Wrong. There is a so-called translational symmetry in the liquid. That is, no matter where we look in the stream, it always looks the same. However, when the water freezes, its molecules arrange themselves in a fixed lattice. Now, if we look at a particular spot, there is either a molecule there (if we are lucky) – or not (if we caught the interstices of the crystal). The translational symmetry of the system is broken, the physicist says, in terms of spatial coordinates. Moreover, crystals are normally in the energetic ground state. This will be important later.
Are there also systems where the same thing happens, only in the temporal dimension?
Back in 2012, U.S. physicist and Nobel laureate Frank Wilczek set up a model where this is possible. He called it a «time crystal» analogous to the well-known crystal. A time crystal is therefore a physical system in which certain parameters are periodically repeated. However, the time crystal should also be in the ground state. This means that you don’t have to add energy to it to make it keep its properties, just like you don’t have to add energy to ice to make it stay frozen.
With time crystals, this is a very special challenge. Because what do you call a machine which changes its properties periodically in time without adding energy (!). Correct: that would be a perpetual motion machine, and we already learned in school that thermodynamics simply forbids the existence of such systems. But that is not all. The translational symmetry of time seems to be built into nature. Researchers rely on it when they assume that the outcome of an experiment does not depend on its timing.
Nevertheless, physicists have managed to demonstrate several such systems in practice since 2017. Most recently, in September 2021, Google made headlines with it. This is despite the fact that scientists proved back in 2015 that a system at equilibrium (in the ground state) cannot become a time crystal. How were the researchers able to overcome this? They looked at systems that are not (yet) in equilibrium and keep them from ever coming to equilibrium either by adding energy from the outside. Since now the clock, the period, of the time crystal does not depend on the external excitation, the researchers call such systems also «time crystal».
What can time crystals be used for? Physicists still have no answers to this question. As is often the case in basic research, we do not yet know what it will be good for (certainly not for time travel). It was no different with quantum theory and relativity. Today, our everyday life would no longer function without them.