Physicists Made Light Swim Upstream and Broke Newton’s Third Law

A passive optical swimmer moved against a flowing light fluid without powering itself.

23 Sep 2026, 18:22 by Tibi Puiu · ZME Science

A leaf dropped into a river goes wherever the current takes it. To travel upstream, something has to push — a motor, a paddle or a swimming animal burning energy.

This is all covered by Isaac Newton’s third law of motion, which is one of physics’ most familiar rules: for every action, there is an equal and opposite reaction.

But physicists in China have now built a “river” out of light and watched another sliver of light move upstream without propelling itself.

The trick was to engineer a nonreciprocal interaction, in which the effective force one part of the system exerts on another is not matched by an equal-and-opposite force in return. Between the two interacting beams, in other words, the usual action–reaction symmetry breaks down.

“For me, the biggest takeaway is that active behaviour does not necessarily require an intrinsically active particle or swimmer,” Yi Hu, a physicist at Nankai University and an author of the study, told New Scientist.

A swimmer made from light

The researchers didn’t send photons swimming through an actual liquid. Instead, they built an optical analogue of a quantum fluid, using light propagating through a nonlinear crystal.

They sent two laser beams through the crystal. One was broad and played the role of the flowing fluid. By slightly tilting this beam, the researchers could set the direction and speed of its transverse flow.

The second beam was much narrower and acted as the “swimmer.” It was shaped into a stable solitary wave and sent through the optical fluid.

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Under ordinary reciprocal interactions, the experiment behaved much as intuition suggests: the flowing optical fluid carried the swimmer downstream. Then the researchers changed the way the two beams interacted.

They engineered the system so that the swimmer affected the surrounding optical fluid differently from how the fluid affected the swimmer. This kind of interaction is called nonreciprocal.

The uneven light distribution produced a force on the swimmer pointing against the direction of the flow. Instead of being swept downstream, the beam began moving upstream.

In an ordinary interaction, the forces between two objects come in equal and opposite pairs. Here, the effective forces between the swimmer and the optical fluid did not balance that way, allowing the swimmer to move against the current without supplying its own propulsion.

Intriguingly, turning everything up to maximum did not produce the strongest effect. The swimmer travelled upstream most effectively at intermediate fluid speeds and densities.

That does not mean physicists have discovered that Newton was wrong. Newton’s third law applies straightforwardly to isolated mechanical interactions. Effective forces can break action-reaction symmetry in open, nonequilibrium systems, where other parts of the environment participate in the exchange of momentum. Scientists have previously studied such nonreciprocal effective forces in plasmas, colloids and living systems.

A different way to swim against light

Remarkably, this isn’t the first time physicists have made something move upstream through a fluid of light.

Earlier this year, another team demonstrated an optical swimmer that propelled itself upstream by shedding pairs of tiny quantum vortices behind it. Like a squid expelling water, the swimmer received recoil from what it sent downstream, though this mechanism still respected the usual action-reaction symmetry.

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The new experiment achieves the same broad outcome by a fundamentally different route. No vortex wake is required to provide the push.

Mathias Albert, a physicist at Côte d’Azur University who worked on the earlier experiment but not the new study, told New Scientist that the result fits into a rapidly developing area of physics.

“This question is very much in the air at the moment, with several groups exploring how the concepts of active matter can be combined with quantum fluids and photonic systems,” Albert said.

Researchers have already begun extending nonreciprocal active-matter ideas deeper into quantum physics. In 2025, for example, physicists proposed quantum spins that interact nonreciprocally, producing collective dynamics resembling one group chasing another. Earlier in 2026, Hu and colleagues also demonstrated an unusual reversal of momentum in nonreciprocally interacting optical waves.

Next, Hu’s team hopes to build more elaborate systems, including experiments with several swimmers interacting at once.

Ultimately, controlling where packets of interacting light travel could prove useful in photonic or quantum-information devices. More immediately, the experiment reveals an unusual route to motion: sometimes an object does not need its own motor. It just needs the world around it to push back differently.