Time Crystals: A Thermodynamic Glitch?
When most people say the word ‘crystal’, they are likely reminded of a diamond, or some table salt, or essentially any other shiny stone. Physically speaking, a crystal is a homogeneous compound with a repeating geometric form with symmetrically aligned plane faces. The structure of these crystals has been well described for around a century. And while they possess many fascinating physical qualities, they are still simple solids.
And then there are time crystals.
A time crystal differs from normal matter in that its symmetry goes beyond the typical three dimensions and into time, allowing it to spontaneously break the symmetry of time translation. This repetition through the 4th dimension allows it to appear to be in perpetual motion — even in its ground states, it is intrinsically out of equilibrium, and therefore unlike a diamond or table salt, it is incapable of ever ceasing its motion. Though the reasons for this are quite technical, a relatively simple way of looking at it is to consider a hypothetical object, say, a coin. Suppose that you have a coin that, no matter what, will always flip over every few seconds, regardless of how cold it is, or what forces are acting on it. It simply exists this way. Its characteristic of flipping itself over is as fundamental to its being as its colour, shape, or value as a currency (okay, maybe that last one shifts a bit). On the surface, this appears to imply that the existence of this object completely obliterates the laws of thermodynamics as it is in perpetual motion.
While this revelation may have you foaming at the mouth as tears pour out of your eyes, or overturning a table whilst proclaiming the death of logic and reason, or some other proportional response to this astounding revelation, please temporarily holster your existential crisis.
In March 2017, a mere 5 years after the crystals were first theorized by nobel laureate Frank Wilczek, two lab teams at Harvard University and the University of Maryland were able to create time crystals in the lab environment. Amazingly enough, each team took a vastly different approach to the problem, yet had similarly successful results. In each case the crystals were open quantum systems, and therefore, in spite of their perpetual motion, they did not violate thermodynamics. Essentially because while in its ground state the crystal does not generate any work, it cannot convert thermal energy into mechanical work, and it cannot perpetually store work, thus preserving all three laws of thermodynamics (a more technically concise explanation may be found here). So, unfortunately and as fun as it may be to consider it, time crystals do not allow us to construct the long-fabled perpetual motion machine. With that being said, they are still a monumental discovery. For one, their consistency in time gives them great potential for use in quantum computing, and the discovery of the first non-equilibrium matter opens a new region of discovery on par with that of antimatter. But on top of that, there is simply the speed of this discovery. Time crystals were first theorized in 2012, the lab technique for creating them in 2016, and two teams finally independently created them in 2017. Within 5 years they went from not even being conceptualized to being physically created. That speed of progress is, in a very profound sense, astounding. The new avenues of research these open up and their practical applications promise a bright tomorrow for the world of physics.
