Scientists have developed the first-ever time crystals that are visible to the human eye, and they could one day make their way onto $100 bills.
The time crystals emerge from the kinks that appear when light is shone onto liquid crystals — the same as those found inside LCD screens .
Unlike previous time crystals, these "psychedelic tiger stripes" are visible, researchers said in a statement. This gives scientists a new window to probe this bizarre phase of matter and also has a number of practical applications, from telecommunications to anti-counterfeiting designs and data storage. The researchers published their findings Sept. 4 in the journal Nature Materials.
"They can be observed directly under a microscope and even, under special conditions, by the naked eye," study lead author Hanqing Zhao, a graduate student in the Department of Physics at the University of Colorado Boulder, said in the statement.
First proposed in 2012 by the Nobel prize-winning physicist Frank Wilczek, time crystals are groups of particles that repeat in time, much like other crystals repeat in space. This behavior is intriguing to physicists — physical laws are entirely symmetrical in space and mostly in time, leading to outcomes that are the same regardless of their direction in either space or time.
But crystals break this symmetry, arranging themselves in a preferred spatial direction. This means that, despite physical laws retaining their underlying symmetry, these laws create different outcomes for crystals depending on the direction they act upon them.
And in the same way that crystals break symmetry in space, time crystals break them in time. They exist in the lowest possible energy permitted by quantum mechanics, and oscillate between two states without slowing down.
These remarkable properties have led to claims that time crystals are perpetual motion machines that violate the second law of thermodynamics, but this isn't the case. The crystals, which are driven by photons, or light particles, cannot lose or gain energy; all the light hitting them does is cause them to repeat their two-state shuffle.
Since Wilczek's proposal, physicists have created and studied time crystals inside diamonds, quantum computers and rubidium atoms excited to hundreds of times their typical size. Yet these crystals can't be directly seen, and are instead studied through the fluctuations of laser light.
To design their visible time crystals, the researchers behind the new study sandwiched liquid crystals — rod-shaped molecules that behave both like solids and liquids — between two dye-coated pieces of glass. If they're squeezed together in the right way, these molecules form kinks that can move around and even behave like atoms.
"You have these twists, and you can't easily remove them," study co-author Ivan Smalyukh, a professor of physics at the University of Colorado Boulder, said in the statement. "They behave like particles and start interacting with each other."
By shining light on the glass pieces, the scientists caused the dye molecules coating them to shift in response and squeeze the liquid crystals in turn, forming thousands of new kinks that danced across the solution in interactions that repeated regularly over time. Even when the researchers raised or lowered the temperature, the kinks' movements remained unchanged.
"Everything is born out of nothing," Smalyukh said. "All you do is shine a light, and this whole world of time crystals emerges."
Beyond advances in fundamental physics, the researchers say their new system could one day be developed into "time watermarks" whose distinct patterns will make higher denomination bills harder to counterfeit. Stacking the crystals could also create even more complicated patterns that could store large quantities of data.
"We don't want to put a limit on the applications right now," Smalyukh said. "I think there are opportunities to push this technology in all sorts of directions."
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