A team of researchers in Japan has theoretically established a method for violating one of the most fundamental principles of classical physics, Newton’s third law of motion, within a solid material. The law, which states that every action has an equal and opposite reaction, governs systems in equilibrium. However, by irradiating a magnetic metal with precisely tuned light, the scientists demonstrated that it is possible to create non-reciprocal magnetic forces, where the action-and-reaction symmetry is broken, leading to a self-propelled, continuous rotation of magnetic layers within the material. This work challenges the boundaries of how matter can be controlled at the quantum level.
Published in Nature Communications by scientists from the Institute of Science Tokyo, Okayama University, and Kyoto University, the findings introduce a novel way to engineer non-equilibrium states in condensed matter. Led by Associate Professor Rio Hanai, the team showed that light could induce a torque between two magnetic layers, forcing them into a perpetual “chase-and-run” dynamic. This photoinduced effect not only provides a new tool for manipulating quantum materials but also forges a significant link between two distinct areas of physics: condensed matter and active matter. The principles demonstrated could pave the way for a new generation of technologies, including advanced spintronic devices and highly tunable oscillators.
Symmetry in Physics: Action and Reaction
Newton’s third law is a cornerstone of physics, describing a fundamental symmetry in the universe. In systems at thermal equilibrium, all forces are balanced, and for every force exerted on one object, an equal and opposite force is exerted back. This principle of reciprocity holds true for countless interactions, from the gravitational pull between planets to the electrostatic forces between charged particles. However, this symmetry is often broken in systems that are not in equilibrium, a category that includes all living things.
Many complex systems are inherently non-reciprocal. For example, the relationship between a predator and its prey is not symmetrical; one chases, and the other flees. Similarly, the intricate network of excitatory and inhibitory neurons in the human brain relies on directional, unbalanced interactions to function. These “active matter” systems, characterized by self-organizing and self-propelled components, have long been a subject of study in biology and soft matter physics. The Japanese research team sought to replicate these non-reciprocal dynamics, which are common in the biological world, within the highly structured and predictable realm of solid-state electronic systems.
Manipulating Magnetism with Light
The researchers’ breakthrough lies in their theoretical framework for controlling magnetic interactions inside a metal using light. Their method focuses on an established magnetic phenomenon known as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. In a typical magnetic metal, localized electron spins interact with free-moving electrons, and the RKKY interaction describes the resulting indirect coupling between the localized spins. Under normal, equilibrium conditions, this interaction is reciprocal; the force from spin A on spin B is equal and opposite to the force from spin B on spin A.
The team proposed a technique they term “dissipation engineering” to break this symmetry. By shining light of a specific frequency onto the material, it is possible to selectively activate decay channels for certain electron spins. This means the light energizes the system in a way that creates an imbalance, causing energy to dissipate unevenly. This engineered dissipation transforms the reciprocal RKKY interaction into a non-reciprocal one. The result is a persistent torque that one magnetic layer exerts on another, causing them to rotate continuously in a chase-and-run pattern without ever reaching a stable equilibrium.
A New Phase of Matter
The continuous, self-propelled rotation induced by the light represents a unique non-equilibrium phase transition. The researchers describe this state as a “chiral” phase, where the magnetization of the material exhibits a constant, dynamic rotation. This behavior is fundamentally different from that of conventional magnetic materials, which typically settle into a static alignment, such as in ferromagnets where spins align in parallel. The ability to create and sustain such a dynamic state opens up new possibilities for material design.
Crucially, the theoretical work suggests that this phenomenon is not just a scientific curiosity but is achievable with current experimental capabilities. The intensity of light required to induce the non-reciprocal phase transition is within the range of existing laboratory equipment. This practicality makes the discovery particularly exciting, as it provides a clear pathway for experimental physicists to verify and explore these findings in the real world. The ability to generate a controllable, persistent rotational force at the quantum level could have significant implications for nanoscale devices and sensors.
Forging Interdisciplinary Connections
One of the most profound contributions of this research is its role in connecting two previously disparate fields of physics. Condensed matter physics traditionally focuses on the properties of solids and liquids in or near thermal equilibrium. In contrast, active matter physics explores systems far from equilibrium, such as flocks of birds or bacterial colonies, which exhibit collective, self-driven behavior. This new work imports concepts from active matter—specifically non-reciprocity and self-propulsion—into the domain of condensed matter.
By demonstrating that light can be used to make electrons and their spins behave like an active system, the research opens a new frontier in materials science. It suggests that the principles governing biological systems might be engineered into solid-state devices. This convergence could lead to novel materials with life-like properties, capable of self-organization and autonomous motion. The long-term vision includes materials that can adapt, reconfigure, or perform complex functions in response to external stimuli like light.
Future Technological Frontiers
The potential applications of photoinduced non-reciprocal magnetism are vast and varied. In the field of spintronics, which utilizes electron spin to carry information, this technique could lead to new types of switches, memory, and logic devices that are faster and more energy-efficient. The ability to induce and control continuous rotation could also be harnessed to create frequency-tunable oscillators for communications technology and high-precision sensors.
Beyond these immediate applications, the research provides a foundational principle for controlling other quantum phenomena. The authors of the study suggest that similar dissipation engineering techniques could be applied to create other exotic states of matter, such as Mott insulators and novel forms of superconductivity. While the violation of Newton’s third law in this context is confined to the specific interactions within a non-equilibrium system—and does not upend the law of conservation of momentum when the electromagnetic field is considered—it represents a powerful new paradigm for material control. It transforms light from a mere probe into a tool that can actively sculpt the fundamental interactions that govern the properties of matter.
