11 hours ago
(This post was last modified: 8 hours ago by C C.)
Physicists made light swim upstream and broke Newton’s third law?
https://www.zmescience.com/science/physi...third-law/
INTRO: 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. 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.
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... (MORE - details)
Finally: A proof that particles take all paths at once?
PRESS RELEASE: https://phys.org/news/2026-08-physicists...-path.html
SABINE HOSSENFELDER
https://youtu.be/A2ImadKWaHA
INTRO: Eighty years ago Richard Feynman said that a quantum particle takes every possible path at the same time. A team of physicists in China now says they have experimentally shown this is not just an interpretation but physical reality, and the headlines duly followed. I had a look at what they actually measured. [#9 on the BS meter]
Finally: A proof that particles take all paths at once? ... https://youtu.be/A2ImadKWaHA
https://www.youtube-nocookie.com/embed/A2ImadKWaHA
https://www.zmescience.com/science/physi...third-law/
INTRO: 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. 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.
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... (MORE - details)
Finally: A proof that particles take all paths at once?
PRESS RELEASE: https://phys.org/news/2026-08-physicists...-path.html
SABINE HOSSENFELDER
https://youtu.be/A2ImadKWaHA
INTRO: Eighty years ago Richard Feynman said that a quantum particle takes every possible path at the same time. A team of physicists in China now says they have experimentally shown this is not just an interpretation but physical reality, and the headlines duly followed. I had a look at what they actually measured. [#9 on the BS meter]
Finally: A proof that particles take all paths at once? ... https://youtu.be/A2ImadKWaHA