Light Breaks Newton's Third Law

Japanese Physicists Use Light to Break Newton's Third Law — A Leap Toward Non-Reciprocal Quantum Materials

A Groundbreaking Twist — Light-Induced Violation of Newton's Third Law

In a discovery that challenges one of physics' most established principles, researchers in Japan have demonstrated how light can induce non-reciprocal interactions in solids—effectively breaking Newton's third law within a controlled environment.

By findings, published on 18 September 2025 in Nature Communications, mark a revolutionary step in non-equilibrium physics, opening pathways to light-driven quantum materials and energy-efficient spintronic technologies.

Related reading: Light-Induced Magnetism — The Future of Quantum Devices

From Action and Reaction to Chase and Run

Under ordinary conditions, all physical systems obey the law of action and reaction, maintaining symmetry through the principle of free energy minimization. However, in non-equilibrium systems—such as living cells, neurons, or active matter — this balance often collapses.

Examples abound in nature:

• The asymmetric interplay between neurons in the human brain.
• Predator-prey dynamics in ecological systems.
• Colloidal motion in optically active fluids.

These phenomena hint a non-reciprocity, where one entity's influence on another is not equally returned.

This raised a profound question in physics:

Could solid-state materials, governed by quantum mechanics, exhibit the same non-reciprocal behaviour seen in living systems?

A team led by Associate Professor Ryo Hanai (Institute of Science Tokyo), Associate Professor Daiki Ootsuki (Okayama University), and Assistant Professor Rina Tazai (Kyoto University) has now provided the theoretical framework that says — yes.

How Light Turns Reciprocal Spins into Non-Reciprocal Motion

"Our research presents a general framework for transforming conventional reciprocal spin interactions into non-reciprocal ones using light," said Dr Ryo Hanai, the study's lead author.

The team examined the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction—a fundamental quantum mechanism through which local magnetic spins in metals communicate via conduction electrons.

By shining light of a specific resonant frequency, the researchers showed that it is possible to activate certain spin decay channels while leaving others unaffected. This selective activation creates an imbalance of energy flow among spins, breaking the usual action-reaction symmetry.

(Also read: Breaking Symmetry in Quantum Systems—A New Frontier in Physics)

Engineering Dissipation—The Power of Controlled Imbalance

Inspired by active matter systems—where continuous energy input drives motion—the team proposed a dissipation-engineering strategy that uses light to control decay processes inside a magnetic metal.

In such materials, localized spins interact with mobile conduction electrons, exchanging angular momentum. By using laser light to tune which decay paths remain open or closed, the researchers manipulated the internal flow of spin energy.

This engineered imbalance generated non-reciprocal magnetic interactions, where one layer's torque is not equally opposed by the other.

In their theoretical model, when the technique was applied to a bilayer ferromagnetic system, a striking new phase emerged—a non-reciprocal chiral phase.

The Birth of a "Chiral" Magnetic Phase

The Endless Chase of Two Magnetic Layers

In the chiral phase, one magnetic layer aligns while the other resists, producing a continuous rotation of magnetization under constant illumination.

Unlike ordinary magnets, which reach a stable configuration, this light-induced system never settles —  the two layers continuously chase and evade each other.

This behaviour represents a spontaneous violation of Newton's third law in a microscopic setting— the very principle that dictates every push must have an equal and opposite pull.

"The non-reciprocal motion we predicted is a hallmark of broken action-reaction symmetry," the authors wrote.

Remarkably, the light intensity required to achieve this state lies within the range of current experimental technology, meaning laboratories could soon verify these effects in real materials.

(Discover more: Quantum Chiral Materials  — Where Physics Breaks the Rules)

A Bridge Between Active Matter and Solid-State Physics

This breakthrough doesn't just push the boundaries of condensed matter theory— it also connects quantum materials research with concepts from biophysics and soft matter.

Dr Hanai's team notes that non-reciprocal interactions are a universal feature of active systems, from cell migration to chemical pattern formation. Translating these ideas into solid-state materials could yield entirely new classes of non-equilibrium matter.

"The framework unifies how we understand activity and dissipation," Dr Hanai said. "It allows physicists to approach solids as systems that can host self-organizing, non-reciprocal dynamics— just like living matter."

Potential Applications— From Spintronics to Quantum Control

Beyond theoretical elegance, the implications of this discovery are technologically profound.

The ability to induce and control non-reciprocal interactions using light could revolutionize several fields:

• Spintronics: enabling directional spin currents and frequency-tunable magnetic oscillators.
• Quantum computing: controlling spin coherence through light rather than electric current.
• Superconductivity: exploring light-tuned Mott-insulating and multiband superconductors.
• Non-equilibrium devices: designing optically driven circuits that mimic biological motion.

Inside the Study — Japan's Collaborative Effort in Quantum Theory

The project united three major Japanese Institutions:

• Institute of Science Tokyo (Lead) —  theoretical framework and quantum modelling.
• Okayama University — magnetic structure simulations.
• Kyoto University — active-matter dynamics and chiral phase analysis.

Together, they formed a multidisciplinary bridge connecting condensed matter physics, optical engineering and non-equilibrium thermodynamics.

The paper's publication in Nature Communications cements Japan's reputation as a global leader in light-matter interaction research.

The Next Frontier — Non-Equilibrium Materials and Beyond

The researchers emphasize that their framework could be extended to complex quantum systems, including Mott insulators and superconductors, where electrons behave as strongly correlated particles.

By adjusting light frequency and intensity, scientists could one day program material responses, switching between reciprocal and non-reciprocal modes on demand.

This would represent a paradigm shift — from passive materials governed by equilibrium to active solids driven by controlled dissipation.

As Dr Hanai concludes:

"Our findings not only introduce a new way to manipulate quantum materials with light but also unify the understanding of non-reciprocal processes across physics and biology."

Conclusion —  When Light Bends the Rules of Physics

This research redefines what's possible in condensed matter physics. By proving that light can break Newton's third law within magnetic systems, scientists have opened a portal to a new class of active, intelligent materials.

These findings may soon transform quantum electronics, superconducting technologies, and even artificial intelligence hardware, all driven by non-equilibrium design principles.

As light continues to blur the boundary between matter and motion, one truth remains clear:

The next revolution in physics may not come from smashing particles—but from shining light upon them.

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