insider brief

• Researchers have demonstrated that two particles suspended by a stationary sound wave can enter into a self-sustaining, repetitive motion known as a classical time crystal, without external timing or quantum effects.
• The system relies on non-reciprocal acoustically mediated forces between slightly different particles, allowing energy from a static sound field to balance friction and sustain long-lived vibrations.
• The results of this study show that the symmetry of time transformations can be broken in ordinary physical systems, opening a potential avenue for miniature oscillators and sensors based on classical physics.
• Image: Stop-motion image showing pairs of millimeter-scale beads forming time crystals over about a third of a second. The colors represent the beads interacting at different stages during this period. (New York University Soft Matter Research Center)

A pair of small particles suspended by a sound wave can be locked into perpetual motion without external timing, offering a new way to build self-sustaining oscillators and challenging assumptions about how time-ordered systems should work.

This is the central finding of a new study by researchers at New York University, who report that two millimeter-scale particles suspended in an acoustic standing wave can enter into a stable, repeating cycle of motion (known as a time crystal) without friction or periodic drive. This study shows that time crystals, once thought to be primarily quantum phenomena, can arise in ordinary classical systems through carefully structured interactions.

This study physical review lettershow that non-reciprocal forces, i.e. interactions with unequal action and reaction, allow the system to continuously extract energy from a static sound field and convert it into long-lived motion. Under certain conditions, vibrations break time and translation symmetries. This means that the system changes over time even if the governing rules do not change.

The university says the results expand on time crystal systems and expand the possibilities these crystals offer for technology and industry, including use in sensing, signal generation, and mechanical timing devices.

The researchers added that these time crystals are visible to the naked eye and suspended in a foot-tall device that can be held in the hand.

“Time crystals are fascinating not only because of their potential, but also because they seem so exotic and complex,” said physics professor David Greer, director of New York University’s Center for Soft Matter Research and lead author of the study, in the paper. University release. “Our system is great because it’s incredibly simple.”

Breaking the symmetry of time with sound

A little background: Time crystals are a system that repeats in time, much like regular crystals repeat in space. Rather than atoms arranged in a fixed pattern, time crystals move in a regular rhythm. The decisive feature is that this movement is not forced by an external clock, but appears on its own.

In the New York University experiment, the researchers used an acoustic levitator, a device that traps small objects with pressure nodes created by standing sound waves. Each node acts like a shallow bowl, holding lightweight beads in place. When two beads are confined close to each other, they interact by scattering sound waves back and forth.

These scattered waves generate forces between the particles, according to the research team, which includes New York University graduate student Mia Morell and New York University undergraduate Leela Elliott. Importantly, the researchers added, the force is not always equal in both directions if the particles differ slightly in size. This asymmetry allows the pair to extract energy from the surrounding sound field.

In most cases, the friction from the air quickly dampens the motion and the particles stop moving. However, the researchers found that for certain size combinations, the energy gained from these asymmetric interactions exactly balances the energy lost to drag. Once that happens, the particles settle into a steady rhythm of movement.

Depending on the parameters, the particles can move together synchronously or oscillate in opposite directions, like two blobs connected by an invisible spring. In some regions, the oscillations break both spatial and temporal symmetries, satisfying the formal definition of a continuous-time crystal.

This system does not require external timing signals. The sound waves that hold particles in place are static, and the vibrational frequency emerges from the dynamics of the particles themselves.

How researchers built the system

As mentioned earlier, this experiment uses a relatively simple setup. The flotation device operates at 40 kilohertz, well above the range of human hearing, and creates a line of pressure nodes a few millimeters apart. Expanded polystyrene beads, which are lighter than air but stiff enough to scatter sound, are placed at adjacent nodes.

High-speed cameras track the movement of the beads over long periods of time, sometimes hundreds of seconds at a time. Researchers analyze motion by separating it into collective modes and measuring the frequency content of each.

Alongside their experiments, the team developed a theoretical model that captures how sound-mediated forces act between particles. The equation describes how restoring forces, drag forces, and nonreciprocal interactions combine to determine whether motion increases, decays, or stabilizes.

This model predicts a sharp boundary between passive state, normal oscillator, and time crystal behavior. The experimental measurements are in close agreement with predictions, including the frequency and stability of the observed oscillations.

One notable result is consistency of behavior. In some cases, the vibrations last for several hours, much longer than the time it would take for friction alone to stop the vibrations. This persistence is a key requirement for practical timing and sensing applications.

Why this matters for technology

This discovery suggests a new way to design compact oscillators and detectors that do not rely on electronic feedback or external clocks. Since the oscillation frequency comes from the system itself, such devices may be inherently stable to certain types of noise.

More broadly, this study shows that energy dissipation, or loss of energy to the environment, does not necessarily destroy order. Under the right conditions, dissipation can help stabilize. This idea runs counter to traditional engineering intuition, which treats friction and losses as problems to be minimized.

This study also highlights the role of non-interaction as a source of sustained activity. Similar effects have appeared in optics, mechanics, and electronics, suggesting that the underlying principles may be applicable to other platforms.

The authors point out that larger arrays of particles can exhibit even richer behaviors, such as transitions between wave-like motion and localized activity. Such systems could serve as testbeds for studying how order emerges in driven nonequilibrium systems.

What does this mean and what does it not mean for quantum technology?

Time crystals are often associated with quantum systems because they were first proposed in quantum systems. However, the researchers stress that the study is completely classic. Particles follow normal laws of motion and do not involve quantum coherence or entanglement.

This research does not advance quantum computing, at least not directly, nor does it provide new ways to store or process quantum information. But those in the quantum industry may see this research as a way to advance quantum by revealing which aspects of time crystal behavior depend on quantum mechanics and which do not.

This is valuable for quantum engineers, as quantum time crystals face major challenges from noise and heat generation. This acoustic system shows how dissipation and nonreciprocal coupling can be used to stabilize symmetry-breaking motion. These tools also exist in quantum hardware such as microwave circuits and photonic networks.

This work provides insight into how timing and vibrational phases are designed in more fragile quantum systems by providing a clean and controllable classical example. It also helps separate the benefits of real quantum from the behavior that classical physics can already reproduce.

Limits and next steps

This system relies on carefully tuned particle properties. If the particles are too similar, the interactions become interactions and the oscillations disappear. Effectiveness also depends on operating within a narrow range of size and coupling strength.

Scaling a system beyond a small number of particles introduces additional complexities, such as disorder and competing modes of motion. Understanding how these elements interact is key to practical applications.

Future studies may investigate other wave-based platforms, including optical and mechanical systems, to test whether similar principles apply. Researchers may also investigate whether such time crystal behavior can be used to sense weak forces or changes in the environment.