This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Just as a leaf drifts along with a stream, objects in other moving fluids normally drift along with the flow. That is, unless they exert energy to move against it.
Although it may be less intuitive, light waves or photons work similarly. To move against a stream of light, an object or particle, like a photon, must either have an external force acting on it or actively use energy to move upstream. In a new study, published in Physical Review A, a team of physicists demonstrates how a beam of light can "swim" upstream in a quantum fluid of light by breaking action-reaction symmetry and reshaping how the surrounding forces affect the flow.
Matter is not in equilibrium when it is actively creating a force to oppose motion, like a motorized boat moving up a river. Newton's third law of motion states that for every action, there is an equal and opposite reaction. This is also known as action-reaction symmetry.
Action-reaction symmetry is not broken just because something is out of equilibrium, but this symmetry can be broken when particles exhibit non-reciprocal interactions. In these interactions, one object influences another differently than the other influences it. In non-reciprocal interactions, internal interactions can be converted into net momentum.
Scientists have proposed that these unusual interactions could create active behavior in quantum-like systems. Upstream motion has been demonstrated in previous experiments, but the mechanism involved vortex shedding, which imparts a net upstream recoil momentum and still obeys Newton's third law. The new study is the first to use non-reciprocal interactions, violating action-reaction symmetry.
The team involved in the new study combined theoretical scattering analysis, computer simulations and laboratory experiments. They sent two timed laser beams through a nonlinear crystal, with one representing the fluid and the other representing the swimmer. By tilting the fluid beam, they were able to control its transverse flow direction and speed.
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