Physicists at New York University have built a time crystal powered entirely by sound waves, and lab tests suggest the particles can interact in a way that appears to defy Newton’s third law in the usual two-body sense. Tiny particles, levitated on a cushion of acoustic pressure, can push on each other with unequal effective force because their interaction is mediated by sound waves. As described in the team’s preprint and university press coverage, the work highlights how wave-mediated interactions can break simple “equal and opposite reaction” intuition for a particle pair even while total momentum is conserved when the wave field is included.
How Sound Waves Break Action-Reaction Symmetry
Newton’s third law states that when one object exerts a force on a second, the second pushes back with equal strength in the opposite direction. That rule holds perfectly when two objects interact through direct contact or through a static field. But the NYU experiment introduces a wrinkle: the particles do not touch each other. Instead, they communicate through wave-mediated interactions carried by sound. Because the acoustic waves themselves carry momentum and energy, the force that particle A exerts on particle B does not have to equal the force B exerts back on A. The “missing” momentum is absorbed or radiated by the wave field, so the total momentum of the entire system (particles plus waves) is still conserved. Newton’s third law, strictly speaking, applies to isolated pairs. When a wave field acts as a third participant, the pair-level symmetry can fail without violating deeper conservation principles.
This distinction matters because most everyday physics treats forces as instantaneous and pairwise. The NYU team showed that once you allow time-dependent wave coupling between objects, the effective forces become nonreciprocal. One researcher described the mechanism with a vivid analogy: “Sound waves exert forces on particles, just like waves on the surface of a pond can exert forces on a floating leaf.” That image captures the core idea: the medium itself is active, redistributing forces in ways that break the expected balance between any two particles.
From Nonreciprocity to a Classical Time Crystal
A time crystal is a state of matter that oscillates in a repeating pattern without consuming net energy from an external drive, breaking the time-translation symmetry that governs most physical systems. The concept was first proposed in quantum physics, and early experimental demonstrations relied on exotic setups involving trapped ions or superconducting qubits. What makes the NYU result distinctive is that it achieves time-crystal behavior in a purely classical, tabletop system. The levitating particles oscillate spontaneously once the nonreciprocal coupling kicks in, and the device is small enough to hold in your hand.
The connection between nonreciprocity and time-crystal formation is direct. In a reciprocal system, energy exchanged between two particles tends toward equilibrium, and sustained oscillation requires an external clock or periodic kick. When the forces are nonreciprocal, the asymmetry can channel energy into a persistent loop, driving continuous motion from the acoustic field without any additional periodic input. In NYU’s press coverage, the team described the result as “remarkable because it’s incredibly simple,” a pointed contrast with the cryogenic or laser-intensive rigs that prior time-crystal experiments often demanded.
Acoustic Levitation Has Deeper Roots Than You Think
The idea that sound waves can push solid objects is not new. A NASA technical report measured the acoustic radiation force on a rigid sphere inside a resonance chamber decades ago, finding results consistent with a theoretical framework developed by L.V. King in 1934. That early work established the quantitative relationship between sound intensity and the force on a suspended object, a relationship the NYU team now exploits to levitate and couple multiple particles simultaneously.
What changed is the leap from single-particle levitation to multi-particle interaction. When two or more objects float in the same sound field, each one scatters acoustic waves that then hit its neighbors. Those scattered waves arrive with phase delays and amplitude shifts that depend on geometry, frequency, and particle size. The result is a set of effective forces between particles that need not be symmetric. A thorough review in Nature Reviews Materials catalogs the mechanisms by which reciprocity can break in acoustic and elastic systems, including time-variance, momentum bias, and nonlinearity. The NYU experiment appears to rely on time-dependent (time-varying) wave coupling as one route to nonreciprocal interaction.
Does This Really “Defy” Newton?
Headlines claiming a violation of Newton’s third law deserve scrutiny. The law, as Newton wrote it, describes forces between bodies in an isolated system. The NYU particles are not isolated; they sit inside an active sound field that continuously mediates their interaction. A more precise statement is that the effective pairwise forces between the particles are nonreciprocal, while the total system, including the wave field, still obeys conservation of momentum.
This is not a semantic dodge. It reflects a real and important distinction that physicists have debated for years in the context of optical, hydrodynamic, and now acoustic systems. The Nature Reviews Materials survey explicitly defines nonreciprocity as a property of wave transmission between points, not as a breakdown of fundamental mechanics. When coverage says Newton’s third law “breaks,” it means the simplified two-body picture fails, not that the universe has stopped conserving momentum. For practical purposes, though, the distinction is enormously useful: engineers designing acoustic devices, metamaterials, or micro-robotic swarms can now treat nonreciprocal forces as a design tool rather than a paradox.
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*This article was researched with the help of AI, with human editors creating the final content.
