There’s no point denying that in science is partly an issue of marketing: branding matters. Would we be so interested in gravitationally collapsed stars if they had not been dubbed “black holes”? (The term is generally attributed to the aphoristic American physicist John Wheeler.) Would we care about the non-commutation of certain quantum variables, as the experts would phrase it, if Werner Heisenberg had not called it (once translated from the German) the Uncertainty Principle? Scientists themselves regularly complain about how a trendy new term—nanotechnology, quantum dots, buckyballs, proteomics—creates a bandwagon that bedazzles funding bodies.
There are certainly dangers of that, but there seems nothing intrinsically wrong in coming up with a catchy name that distils an image and fires the imagination. One of the latest, though, has provoked yet more grumbling and accusations of hype from some quarters. And you have to admit that “time crystals” is a term seemingly designed to hook media interest with a whiff of Dr Who zaniness: a phrase that looks almost tailor-made to grace the cover of New Scientist.
When he coined the term in 2012, physicist Frank Wilczek appended another buzzword to the title of his paper: these weren’t just time crystals but “quantum time crystals,” for they derived from the notoriously strange quirks of quantum theory. But time crystals don’t, as you might anticipate, do anything weird to time itself. Rather, they are crystals that exist in time rather than in space, and Wilczek’s paper showed how they might exist at all.
People take notice of Wilczek, a 2004 physics Nobel laureate who displays the kind of magisterial overview of his discipline that is rare now that physics has fragmented into a thousand different sub-branches. To hear Wilczek speak, or to read his recent book A Beautiful Question, is to see ideas connected in a way that persuades you, if only temporarily, that at last you see how it all fits together. And in his 2012 paper on time crystals Wilczek demonstrates where this broad vision can lead you: to inventive new ideas based on premises so apparently simple that you wonder how no one thought of them before.
What defines a crystal is that it has an atomic-scale structure that repeats again and again in space. In short, it is rather like a grid, or a stack of oranges packed together in a regular array on the greengrocer’s stall, each orange an atom.
It’s this regular packing of atoms that gives crystals their flat, geometric facets. We tend to think of structures like this as symmetrical, but actually scientists say that they form through a process of symmetry-breaking. When the atoms are all jumbled and disorderly, as they are in a liquid, no direction in space looks any different, on average, from any other. The randomness sets up a symmetry between all directions of space. But if the liquid freezes to a crystal—water turning to ice, say—then the orderly packing of the atoms and molecules picks out some directions in space as “special,” so the symmetry is actually lowered. It is broken.
Symmetry-breaking is one of the most fundamental principles in physics. It’s a process by which some things that were previously indistinguishable become distinct, so that we must describe the system mathematically as having less symmetry than before. In the process, a kind of order appears from randomness. The separation of fundamental forces from some more primal “ur-force” is a kind of symmetry breaking: that’s how a force called the “electroweak,” in the very early universe, instants after the Big Bang, separated into the electromagnetic and weak nuclear forces we see today. Most physicists believe that all four of the known fundamental forces were once a single force, unraveled by progressive stages of symmetry breaking.
These questions in fundamental physics are the deep waters in which Wilczek normally swims. But in his paper on time crystals he proposed a deceptively simple notion. If regular crystals break symmetry in space, having structures that repeat regularly throughout space, might there be crystals with structures that break symmetry in time?
After all, one of the messages of Einstein’s theories of relativity—it was also inherent in the ideas of mathematical physicists Hendrik Lorentz and Hermann Minkowski that presaged relativity—is that time and space are in some ways equivalent, just directions in a four-dimensional spacetime. So if crystals can form from symmetry breaking in space, shouldn’t the same apply to time?
What would such a time crystal look like? One way to imagine it is to think of an arrangement of atoms that moves in some cyclical manner, returning periodically to the same configuration. A rotating ring of atoms, say, repeating the same arrangement like the hands on a clock returning to the same state every 12 hours.
Thinking of such a periodic cycling of a system’s state as a “time crystal” is in fact nothing new. Any kind of regular wave is like that. And you can find such waves arising of their own accord in a bunch of atoms, for example in the oscillations of chemical state long known to occur in some special chemical mixtures, called clock reactions. Thinking of chemical waves, and the related phenomenon of waves of electrical excitation that cause heartbeats, as “patterns” or even crystals in time led American theoretical biologist Arthur Winfree to give his 1987 book on the topic the equally Whoesque title When Time Breaks Down.
However, chemical waves and clock reactions eventually run out of steam unless they are constantly fed with energy. What was strange about the quantum time crystals Wilczek was proposing is that they never run down. They keep cycling forever, even without an external energy supply. In one manifestation, that would seem to suggest that the atoms would keep moving even in their lowest-energy state—which sounded uncomfortably like a perpetual-motion machine. A time crystal would not quite be that, because there would be no way of extracting energy from that movement without disrupting it. Still, it seemed highly counterintuitive, despite Wilczek’s theoretical demonstration that such things might be possible.
But are they possible? In 2015, two Japanese physicists, Haruki Watanabe and Masaki Oshikawa, looked carefully at Wilczek’s argument and found that it didn’t quite stack up. They proved that it is impossible to create a time crystal from any system in its lowest-energy state. The order promised by this kind of spontaneous, wavelike cycling of configurations is a mirage.
That isn’t the end of the story, though. There was a loophole in the argument of Watanabe and Oshikawa, as they acknowledged themselves: you can get oscillations of the kind Wilczek proposed if a system is not in its lowest-energy state. In that case, there needs to be some source of energy to drive the system into a higher-energy state—which is to say, to drive it away from equilibrium.
But this possibility is well known, the Japanese researchers said. Non-equilibrium oscillations are precisely what we see in the chemical clock reaction and in other chemical waves. In fact, we see such spontaneous oscillations throughout nature: in heartbeats, say, and in the cyclic rise and fall of populations of predators and their prey. Such things “should not be called time crystals without further justification,” Watanabe and Oshikawa wrote.
Yet Wilczek's term seems just too catchy to ditch, and two teams of physicists have retained it for demonstrations that jump through the loophole left by the Japanese physicists' work. Their two papers, both just published in Nature, report the experimental creation of such “non-equilibrium time crystals” (links below*). Both groups have made them from arrays of so-called “atomic spins”: basically atoms that have a quantum property called spin, making them crudely analogous to compass needles. In these time crystals, the “compass needles” rotate over time, coming back into alignment at regular intervals. But to make that happen, the researchers must deliver kicks to the spins, provided by a laser or pulses of microwaves, to keep them out of equilibrium. The time crystals are sustained only by constant kicking, even though—crucially—their oscillation doesn’t match the rhythm of the kicking.
The experiments are ingenious and the results show that this modified version of Wilczek’s vision is feasible. But are we right to award the new findings this eye-catching new label, or are they really just a new example of a phenomenon that has been going on since the first primeval heart started beating? If these fancy arrangements of quantum spins deserve to be called time crystals, can we then say that we each already have a time crystal pulsing inside of us, keeping us alive?
*http://www.nature.com/nature/journal/v543/n7644/full/nature21413.html
http://www.nature.com/nature/journal/v543/n7644/full/nature21426.html
http://www.nature.com/news/the-quest-to-crystallize-time-1.21595
