Newton’s Third Law, Bypassed: Japanese Physicists Trigger Unreciprocal Motion with Electric Fields

No Action, No Reaction? Electric Fields Break a Newtonian Dogma

According to НВ — Техно: August 10, 8:00 PM

Source: Interesting Engineering

At the Tokyo University of Science, researchers have engineered a suspension of over ten thousand colloidal particles that, when exposed to an alternating electric field, assemble into lopsided couples and travel through the liquid. This behavior appears to contradict Newton's third law-the principle that every action produces an equal and opposite reaction. The findings appear in Physical Review Letters.

How the Experiment Was Conducted

Professors Yutaka Sumino and Kiwamu Yoshii placed polystyrene spheres with radii of 1 and 1.5 micrometers in water sandwiched between transparent electrodes. For more than an hour, the team observed the particles under the alternating electric field. The field induced electrohydrodynamic flows around each particle, with the flow’s strength dependent on particle size. Bigger particles produced stronger currents, pulling smaller neighbors toward them with greater force than they received in return.

This imbalance caused particles of different sizes to form asymmetric pairs, each with a distinct front and back. No single particle could move on its own, but paired particles behaved as a self-propelling unit. These pairs then grouped into clusters that continually broke apart and reassembled. When all the particles were the same size, the interactions stayed balanced, and the particles simply arranged themselves into static crystals.

Yutaka Sumino noted: “This setup offers a controlled experimental case of non-reciprocal many-particle physics, where disrupting the equal-and-opposite balance triggers collective behavior.”

The Tokyo team’s work expands possibilities for probing collective phenomena in matter and its internal interactions, with implications for basic science and practical fields alike. In active-matter physics, such symmetry-breaking is a growing focus because it may underpin new behaviors in materials, biophysics, and robotics. Demonstrating this in an electric-field-driven system also reinforces the value of crossing disciplinary boundaries in physics research.

Sumino added: “The experiment shows that breaking action–reaction symmetry can be a universal route to spontaneously generating dynamic order in materials.”