Imagine shattering one of the most fundamental laws of physics—Newton's third law of action and reaction—with nothing but a beam of light shining on a metal. Sounds like science fiction, right? But here's the groundbreaking reality that's got physicists buzzing.
In a fascinating leap forward, a team of researchers from Japan has outlined a theoretical approach that could make non-reciprocal interactions—those that defy the usual give-and-take of forces—a reality in solid materials, all thanks to carefully tuned light. Picture this: by blasting a magnetic metal with light at just the right frequency, they can create a torque that sets two magnetic layers into an endless, self-sustaining spin, like a cosmic game of tag where one magnet chases the other in perpetual motion. This innovative work is pushing the boundaries of non-equilibrium materials science and hints at exciting new ways to manipulate quantum materials with light control.
To grasp why this is so revolutionary, let's take a step back. In everyday, balanced systems—what physicists call equilibrium states—everything follows the principle of action and reaction, minimizing free energy to keep things stable. Think of how magnets attract or repel each other equally; it's all about symmetry. But in the wild world of non-equilibrium setups, like living organisms or active materials, things get asymmetric. These systems thrive on interactions that break that symmetry, effectively flouting Newton's law.
And this is the part most people miss: it's not just theoretical fluff. For example, in your brain, neurons don't interact in a perfectly balanced way—some excite while others inhibit, creating the complex thoughts and decisions you're having right now. Or consider nature's drama: a predator chases prey, but the prey isn't symmetrically pushing back in kind; it's all about survival and evasion. Even in the lab, tiny colloids (those microscopic particles) floating in special optically active fluids show these lopsided interactions. So, the big question emerges: Could we bring this kind of asymmetry into the rigid world of solid-state electronics, where everything is usually predictable and reciprocal?
Enter the research team led by Associate Professor Ryo Hanai from the Department of Physics at the Institute of Science Tokyo (IST), teaming up with Associate Professor Daiki Ootsuki from Okayama University and Assistant Professor Rina Tazai from Kyoto University. They've answered with a resounding yes, unveiling a light-based method to introduce these non-reciprocal effects in solids. Their findings, published in Nature Communications on September 18, 2025 (DOI: 10.1038/s41467-025-62707-9), detail how light can transform ordinary back-and-forth spin interactions into something fundamentally one-sided.
"Our approach offers a universal strategy to convert standard reciprocal spin interactions into non-reciprocal ones via light," Hanai explains. "To illustrate, we demonstrate how a classic phenomenon in magnetic metals—the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, which governs how spins communicate through electron pathways—can become asymmetric when hit with light tuned to activate decay for specific spins, leaving others untouched." For beginners, think of RKKY as the invisible handshake between magnetic moments in a metal, usually fair and even. Light disrupts this by favoring some spins, like picking sides in a game.
Inspired by how non-reciprocal behaviors pop up everywhere in nature—from animal migrations to cellular dynamics—the team crafted a "dissipation-engineering" strategy. This involves using light to selectively turn on decay pathways in magnetic metals, which have fixed spins and roaming electrons that exchange spin information. By creating an energy imbalance—some spins lose energy faster than others—it leads to these provocative non-reciprocal magnetic forces.
But here's where it gets controversial: Applying this to a two-layer ferromagnetic setup (where magnets naturally align), they foresee a dramatic non-equilibrium shift called a non-reciprocal phase transition. Previously explored by one of the authors in active matter studies (as in phys.org/news/2021-05-physicists-reveal-motion-frustration.html), this transition flips the script: one magnetic layer tries to match the other's direction, while the other pushes to oppose it—all under light's influence. The result? A spontaneous, unending rotation of magnetization, dubbed a "chiral" phase, embodying a persistent chase-and-run cycle. It's like two dancers locked in a mesmerizing whirl, never stopping.
What makes this chiral phase stand out is its defiance of action-reaction symmetry, a hallmark of non-reciprocal worlds. And get this—the light power needed? Totally achievable with today's tech, meaning we're not talking pipe dreams.
"This breakthrough not only equips us with a fresh way to steer quantum materials using light but also connects ideas from active matter (like self-propelled particles) and condensed matter physics," Hanai adds. It could extend to exotic states like Mott insulators in strongly interacting electrons, multi-band superconductors, or even superconductivity boosted by light-activated phonons (vibrational quanta in solids). On the practical side, envision new spintronic gadgets—devices leveraging electron spin for data processing—and oscillators that tune to different frequencies on demand.
In wrapping up, this study illuminates how non-reciprocal interactions can thrive in solid-state systems, paving the way for cutting-edge technologies that might redefine computing and materials engineering.
But let's stir the pot: Is violating Newton's law in this way a step toward harnessing chaos for good, or does it risk undermining the very foundations of physics as we know them? What do you think—could this lead to ethical dilemmas in technology, like creating materials that 'think' asymmetrically? Share your thoughts in the comments; I'd love to hear if you're team symmetry or embracing the imbalance!
For more details: Ryo Hanai et al, Photoinduced non-reciprocal magnetism, Nature Communications (2025). DOI: 10.1038/s41467-025-62707-9 (https://dx.doi.org/10.1038/s41467-025-62707-9)
Citation: Photoinduced non-reciprocal magnetism effectively violates Newton's third law (2025, November 4) retrieved 4 November 2025 from https://phys.org/news/2025-11-photoinduced-reciprocal-magnetism-effectively-violates.html
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