Some laws of physics are so fundamental that they’ve served as the baseline for science for centuries. But with technological advancements, physicists can—albeit temporarily—create small worlds that evade these laws. And when they do, strange things occur.
In a recent study published in Physical Review Letters, Japanese physicists created a system of more than 10,000 particles that defied Newton’s third law of motion for an hour. According to this law, passive particles exert equal forces on each other, so none can just suddenly push itself along on its own. But in devising this system, the team subjected particles to an alternating electric field that caused passive particles to spontaneously form pairs and “chase” each other in a liquid environment.
“This research demonstrates that the breaking of action-reaction symmetry is a fundamental principle that generates new collective motions and self-organization of matter,” Yutaka Sumino, the study’s co-author and a physicist at the Tokyo University of Science in Japan, said in a statement.
A physical standstill
To quote Isaac Newton himself, the third law of motion mandates that “mutual actions of two bodies upon each other are always equal and directed to contrary parts.” The key here is symmetry—a reciprocal relationship between action and reaction.
Scientists have also found ways to engineer this principle to their benefit. For instance, the thrust of a rocket engine comes from the action of the hot gas flowing out from the back, which produces a reaction in thrusting force in the opposite direction.
Beyond the norms
More recently, researchers have been looking into systems that break this symmetry, according to the paper. For the latest study, the team suspended microscopic colloidal particles in water, then confined them between specially fabricated electrodes. Then, the researchers applied an alternating electric field. Initially, the larger particles developed a larger electrical flow, attracting the smaller particles.
This introduced an imbalance that led the particles to move as self-propelled units through the suspension, the team explained in the paper. For comparison, the team tried something similar for suspensions containing particles of only one size. However, the results were very different, as the particles exhibited reciprocal interactions and gradually organized into a crystal.
“To put it more simply, particles that attract each other, such as sand, powder, or raindrops, generally continue to gather over time, growing into larger clumps,” Sumino said. The system with differing sizes, on the other hand, did cluster but never clumped together permanently, “instead gathering and then splitting,” he added.
The team suspects that similar interactions take place in biological systems, such as cell colonies and animal groups. If true, the mechanism studied here could even inspire programmable materials and microrobotic systems, it noted.
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