Time crystals are unusual forms of matter made up of particles that “tick,” meaning they move back and forth in steady, repeating cycles. Scientists first predicted their existence and later confirmed them about a decade ago. Although practical uses 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 kinds of time crystals, each with unique properties that could be useful in different applications.
A New Sound-Levitated Time Crystal
Physicists at New York University have now created a new version of a time crystal. In this system, tiny particles float on a cushion of sound and interact by exchanging sound waves. During these interactions, the particles behave in a way that appears to break Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction (i.e., forces always occur in balanced pairs). In this experiment, however, the particles do not follow that balance. Instead, they move in a nonreciprocal way, meaning their interactions are uneven and not mirrored.
The results, published in Physical Review Letters, point to new possibilities for using time crystals in technology and industry. Unlike many previous experiments, this system is visible to the naked eye and operates on a compact device about one foot tall that can be held in your hand.
“Time crystals are fascinating not only because of the possibilities, but also because they seem so exotic and complicated,” says Physics Professor David Grier, director of NYU’s Center for Soft Matter Research and the paper’s senior author. “Our system is remarkable because it’s incredibly simple.”
Insights Into Biology and Circadian Rhythms
The study, carried out with Mia Morrell, an NYU graduate student, and Leela Elliott, an NYU undergraduate, may also help scientists better understand biological timing systems such as circadian rhythms. Similar to these time crystals, some biochemical processes in the body involve nonreciprocal interactions, including how the body breaks down food.
How Sound Waves Keep Particles Floating
The time crystal itself is made from small styrofoam beads, similar to packing material, that are held in place by sound waves. This setup acts as an “acoustic levitator,” allowing the beads to remain suspended and still in mid-air.
“Sound waves exert forces on particles — just like waves on the surface of a pond can exert forces on a floating leaf,” explains Morrell. “We can levitate objects against gravity by immersing them in a sound field called a standing wave.”
When the levitated beads interact, they do so by scattering sound waves between one another.
Uneven Forces and Broken Symmetry
Larger beads scatter more sound than smaller ones. As a result, a larger particle has a stronger effect on a smaller particle than the smaller particle has on the larger one. This creates an imbalance in how they influence each other.
“Think of two ferries of different sizes approaching a dock,” says Morrell. “Each one makes water waves that pushes the other one around — but to different degrees, depending on their size.”
Because these interactions are carried by sound waves, they are not limited by Newton’s Third Law. This allows the beads to begin oscillating on their own while floating in mid-air, producing a steady rhythm that reflects the unusual forces at play.
The research was supported by grants from the National Science Foundation (DMR-21043837, DMR-2428983).