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A meditation teacher of mine once said, “You know, the mind can be very weird,” and as it turns out, so can matter. Big in science news last week was the story, as reported for Nature and Scientific American by Elizabeth Gibney, that a new form of matter has been created. So-called “time crystals” aren’t a video game power up or a sci-fi plot device – they’re a bizarre state of matter which shows a crystal structure not in space, but over time. The newly created time crystals are a development of an idea from 2012, and are slightly less improbable than the original concept. However, as researcher Norman Yao, who is an author on both of the papers reported on, said, “It’s less weird than the first idea, but it’s still fricking weird.”
So what’s a time crystal? Well, substances with a crystal structure have their atoms arranged in repeating patterns. A time crystal is a group of atoms which display a pattern not in space, but in time. The idea originally came from Nobel Prize–winning physicist Frank Wilczek, who came up with the notion in 2012. Crystals represent what physicists call symmetry breaking – their atoms are arranged differently depending on where you look.
It occurred to Wilczek to wonder whether states of matter could exist where symmetry breaking would occur in time, rather than space, and he hypothesized a state of matter that, in its lowest energy state, would flip back and forth between two states without any additional external energy input. It would be a kind of perpetual motion machine, except you couldn’t harvest any energy from it, or use it to do any work.
“This is a new kind of order, one that was previously thought impossible. That’s extremely exciting.”
Vedika Khemani, Harvard University
As it turns out, this particular version of a time crystal turned out to be impossible, but two groups of researchers working independently have created a very intriguing variation on the original ideal. Scientists at the University of Maryland, and another group at Harvard, have created time crystals in which groups of atoms flip between between different states at specific intervals. The states in question are the direction, in groups of atoms, of a quantum property called spin, which is analogous to classical spin but different in key ways. For one thing, quantum spin is quantized, or restricted to specific values; it only comes in 1/2 integer increments (0, 1/2, 1, and so on). For another, quantum spin produces a minute magnetic field. Normally spins of various particles cancel each other out, but if the spin of atoms in an object are aligned, this produces a magnetic field for the whole object.
In both experiments, the time crystals are getting energy from an outside source (a laser) but – and this is the tricky bit – the period of flipping between states is independent of the period of the energy input. It’s as if you were pushing a pendulum, but for every push, the pendulum swung three times – or, as Yao says, “It’s like playing with a jump rope, and somehow our arm goes around twice but the rope only goes around once.” Harvard’s Vedika Khemani emphasizes the very unusual nature of these experiments, saying “This is a new kind of order, one that was previously thought impossible. That’s extremely exciting.”
So, if you’re into horology and you hear about an oscillator whose period is independent from the frequency of the driving energy, you immediately wonder, could this be used as a frequency standard for a clock? I asked Gibney.
“As I understand it,” she replied by email, “the oscillation is certainly well fixed (up to a point, too much perturbation and it breaks down) and so time crystals do have some potential future for use as a form of atomic clock, though that prospect is very speculative: it was only mentioned to me in a rather hand-wavy way during my reporting. Others also pointed out that current atomic clocks are currently way more stable as timekeepers than these time crystals.”
Still, the possibility seems to exist; the frequency of the transitions between states stays the same even if you slightly vary the frequency of the driving force. Crucially, this stability actually arises out of random interactions between the atoms making up the time crystal. Says Gibney, “What is interesting (and potentially exploitable) though is that the phenomenon is ’emergent,’ i.e. the ticking fundamentally emerges from interplay between the atoms. This is very different from an atomic clock, where you have a whole bunch of individual atoms, but only because you want to get a good signal to noise ratio, fundamentally you don’t need to use lots.”
Whether or not time crystals ever become the basis for a timekeeping device better than the current generation of atomic clocks is uncertain – it seems unlikely though. Ytterbium lattice atomic clocks like the one you see above are already accurate to better than ± 1 second over the current age of the universe (13.82 billion years). But the reality of a state of matter that exhibits a crystal structure inherent to its internal quantum interactions, that persists over time rather than space, is fascinating on its own. For more, read Elizabeth Gibney’s original story, “The Quest To Crystallize Time,” in Scientific American. Also highly recommended: the same story in Nature, which has an excellent diagram illustrating the relationship between ordinary crystals and time crystals.
Images: Wikimedia Commons
Article from: Wristwatch News, Reviews, & Original Stories — HODINKEE, by Jack Forster
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