When the existence of time crystals was first proposed in 2012 by the physicist Frank Wilczek, the scientific community thought that it was a simple theoretical artifice, an exotic consequence of quantum mechanics that could occur in systems formed by many particles. Wilczek argued that these strange “crystals” had the ability to break temporal symmetry, changing to periodically return to the same state. Something very similar to what ordinary crystals do in space.
However, several experiments later proved that time crystals they were not just theorists, but they could, to some extent, actually be created in a laboratory, keeping them in imbalance through some external force.
And now, a team of physicists led by Norman Yao, from the University of California at Berkeley, suggests in an article just published in Nature Physics that time crystals can arise without the need for quantum physics. In their article, the researchers argue that purely classical oscillator systems, such as pendulums, could have the same behavior as their quantum “alter ego.” And what’s more, the Time crystals could exist, also, in Nature.
Classic vs. quantum systems
A discrete time crystal responds to a periodic driving force. And it does so by showing some kind of temporal oscillation with a different period, usually an integer multiple of the impeller. Already in the 1830s, Michael Farday He showed that, in theory, a type of periodically controlled oscillators that are now known as “parametric resonators” can suffer a “doubling of periods”, which means that they oscillate at half the driving frequency. And that is precisely the type of “subharmonic” response that characterizes time crystals.
There has been much discussion about whether classical systems (that is, not quantum) can show the same type of behavior as a time crystal. Last year, for example, a team from the Swiss Federal Institute of Technology in Zurich (ETH) demonstrated that two docked oscillating chains showed a doubling of the period. The researchers also noted that there is a close analogy between that behavior and that observed in quantum time crystals.
But a real time crystal needs something else, Yao and his team point out in their study. Discrete time crystals (DTC) are open systems that remain out of balance because of an energy supply from their environment. But in the classical world, that constant contribution would cause the entire system to slowly warm up. With enough time, the temperature could rise without limit, and eventually “Melt” the time crystal, so that the periodic order would disappear forever. Which does not happen in quantum time crystals, where the “location of many bodies” inhibits the exchange of energy and prevents the spread of heat.
However, in a classical system there is no analogue of the “location of many bodies”, so it is not clear whether the classic time crystals could remain stable against heating. That is precisely the problem with Faraday’s parametric resonators and also with the oscillating chains of the Zurich scientists. In their study, Yao and his colleagues point out that in this way noise is added to the system, and it is not clear whether the oscillations of the time crystals could withstand it.
A first approach
However, researchers have managed to identify a simple classic system that could show the behavior of a time crystal even in the presence of noise. It is a series of pendulums or oscillators, arranged in a row and connected to each other by springs. «And that – affirms Michael Zaletel, co-author of the study – is something that had not been seen until now ».
In Yao’s experiment, the behavior of the time crystal does not last indefinitely, but gradually declines to end up “melting.” In Yao’s words: «We don’t have a real classic time crystal. Although ours dies in very long times ». It isn’t, but it looks a lot like one. It’s what researchers call «activated time crystal».
Yao believes that a time crystal that persists indefinitely could be achieved if the oscillators or pendulums were joined in more complex ways. He is even convinced that systems like his, in which the oscillations of time crystals are maintained for a long time, could be found in nature, specifically in living systems such as colonies of cells that interact with each other. Such periodicity «could be very useful in Biology – adds Zaletel-. And usually it would be enough to have it for finite, but long periods.