Time crystals are an unusual form of matter made up of particles that “keep time,” meaning they move back and forth in a steady, repeating cycle. Scientists first predicted its existence and then confirmed its existence about 10 years ago. Although practical applications have not yet been developed, these systems are considered promising for future technologies such as quantum computing and advanced data storage.
Over time, researchers have identified several types of time crystals with unique properties that may be useful for a variety of applications.
Time crystal floating with new sounds
Physicists at New York University have now created a new version of the time crystal. In this system, small particles float above a cushion of sound and interact by exchanging sound waves. During these interactions, the particles behave as if they violate Newton’s third law of motion. This law states that for every action there is an equal and opposite reaction (that is, forces always occur in balanced pairs). However, in this experiment, the particles do not follow that balance. Instead, they move in a non-reciprocal manner. That is, their interactions are uneven and unreflective.
The result is physical review letterpoints out new possibilities for using time crystals in technology and industry. Unlike many previous experiments, this system is visible to the naked eye and operates in a compact device about 1 foot tall that can be held in the hand.
“Time crystals are fascinating not only because of their potential, but because they seem so exotic and complex,” says physics professor David Greer, director of the New York University Center for Soft Matter Research and lead author of the paper. “Our system is great because it’s incredibly simple.”
Insights into biology and circadian rhythms
The research, conducted with NYU graduate student Mia Morell and NYU undergraduate Leela Elliott, could help scientists better understand biological timing systems such as circadian rhythms. Similar to these time crystals, some biochemical processes within the body involve non-interactions, such as the way the body breaks down food.
How sound waves suspend particles
The time crystal itself is made of small Styrofoam beads, similar to packaging material, and is held in place by sound waves. This setup acts as an “acoustic levitator” and allows beads to remain suspended in the air.
“Sound waves exert a force on particles, much like waves on the surface of a pond exert a force on floating leaves,” Morrell explains. “By immersing an object in a sound field called a standing wave, it is possible to suspend the object against gravity.”
When suspended beads interact, they do so by scattering sound waves from each other.
Uneven forces and broken symmetries
Larger beads scatter more sound than smaller beads. As a result, large particles have a stronger effect on small particles than small particles have on large particles. This creates an imbalance in the way they influence each other.
“Think of two ferries of different sizes approaching a pier,” Morrell says. “Each creates a wave of water that displaces the other, but the extent to which it does so varies depending on its size.”
Because these interactions are carried by sound waves, they are not limited by Newton’s third law. This causes the beads to automatically begin to vibrate while suspended in the air, creating a steady rhythm that reflects the unusual forces at work.
This research was supported by grants from the National Science Foundation (DMR-21043837, DMR-2428983).
