The distant past. A long time ago. Right now. Right here. With the way we speak about time and space highlighting their deep parallels, is it any wonder science keeps uncovering new similarities between the two?
This time it’s crystals. Defined as a repeating pattern, familiar “space” crystals such as salt, diamond, and emerald feature atoms arranged in regular shapes. But what would it mean for this phenomenon to exist in time, wondered Nobel Prize-winning physicist Frank Wilczek in 2012. Now, two teams have an answer: A time crystal is a system featuring a behavior that repeats in time.
Lots of things form patterns that repeat, including trains and bouncing basketballs, but what sets those cases apart is the idea of symmetry. If a snowstorm interferes, or a basketball player starts dribbling a little harder, those systems could easily slow down or speed up. They have no physical preference for any particular speed.
The idea that you can shift a train schedule forward or back a minute or an hour with no physical consequence (other than frustrated travelers) is called temporal symmetry, and time crystals break it.
“The surprising thing about the time crystal is that it’s stable,” Norman Yao, a physicist and author of an influential paper published in January, told Gizmodo. The time crystal would need to prefer a certain vibrational frequency, different from the frequency of the periodic nudge. Under a few different nudges, the preferred vibrational frequency doesn’t change.
Graduate student Soonwon Choi’s group at Harvard based their time crystal on a familiar space crystal: diamond. They swapped out about a million of the usual carbon atoms for the similarly shaped but slightly smaller nitrogen atoms, creating impurities in the diamond lattice known as “nitrogen vacancies.” These spots could be described by the quantum mechanical property of spin as pointing up or down. The team then hit the diamond with repeated pulses of microwaves and lasers, and the vacancies responded by establishing a pattern of flipping from up to down that took place at half the speed of the driving pulses – the hallmark of a time crystal.
Initially, Dr. Wilczek imagined time crystals could exist independently, an idea that seemed to fly in the face of normal intuition that all moving systems lose energy and settle down eventually. A 2014 paper confirmed that this original conception was impossible, paving the way for the development of the modified time crystals realized in these experiments, ones that depend on an external source of energy, a periodic pulse or drive.
But even if they aren’t perpetual motion machines, they do handle the energy provided by the drive in odd ways.
As Vedika Khemani, a junior fellow at the Harvard Society of Fellows who was involved with the Harvard experiment previously explained to The Christian Science Monitor, “The idea is that while the system doesn't absorb any net energy from the drive, the drive is needed to keep the system ‘tickled’ and prevent it from relaxing.”
A team at the University of Maryland chose an element called ytterbium as their canvas. Fixing 14 charged atoms in place, they started a pattern of flipping the atoms’ spins from up to down and down to up using lasers. Once the system settled into a routine, the spins again flipped at a speed that was actually slower than the flashes of the laser driving the system. Tweaking the speed of the laser flashes had no effect on the speed of the flipping, characterizing the system as a time crystal.
That would be like dribbling a basketball once per second, but somehow the ball responds by bouncing once every two seconds, a speed that refuses to change even if you start dribbling a little harder.
What’s even more interesting about the two experiments is how different they are. “Both systems are really cool. They’re kind of very different,” Dr. Yao told Gizmodo. “They look at two different regimes of the physics. The fact that you’re seeing this similar phenomenology in very different systems is really amazing.”
Applications at this point are few, but physicists are excited because the experiments open up a novel field of study. Physics strives to reveal reality’s hidden rulebook by discovering exactly what is and isn’t possible, and time crystals represent a whole new chapter, which physicists have tentatively titled "non-equilibrium matter."
The creation of time crystals “has allowed us to add an entry into the catalog of possible orders in space-time, previously thought impossible,” said Dr. Khemani.