‘Feynman and followers, you guys are off’: Physicists disprove decades-old Richard Feynman theory on ‘silly’ sprinklers
Richard Feynman was a serious physicist who liked to answer non-serious questions. What can math tell you? How to order the best lunch? Can a human’s tracks smell like a Bloodhound’s? smelling my own footprints? Now, researchers have taken up one of Feynman’s most curious unanswered questions by looking at the physics of the “foolish” sprinkler.
A typical lawn sprinkler with an S-shaped nozzle rotates in a certain direction as it sprays water from its two holes. But if you throw that sprinkler into a swimming pool and hook it up to a vacuum so that it sucks water in instead of spraying it out, which direction will it spin – the same direction as before, or the opposite?
feynman raised that question As a Princeton graduate student in the 1940s. According to Feynman, he rigged a glass sprinkler in his university laboratory. It gave a brief “shudder”, then barely budged even when pressure was increased. He repeated this process until the glass broke, obscuring the answer. Since then, decades of follow-up experiments have provided every possible answer – the sprinkler rotating one way, rotating the other, rocking back and forth, or not rotating at all – depending on how carefully the tests were conducted.
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In 2024, a team at New York University led by an applied mathematician and experimental physicist leif restroff I attacked the question for the first time and found that reverse sprinkler rotates Opposite of forward sprinkler. But that explanation was only tested on common S-shaped sprinklers, and it had not yet been pitted directly against the two other leading theories.
One, which dates back to Austrian physicist Ernst Mach, posits that the total angular momentum of water rotating inside the sprinkler’s arms must be balanced by the opposite spin of the sprinkler. The second, linked to Feynman himself, focuses on pressure and suction effects on the external nozzle.
So Ristroff’s team asked, what if the sprinkler wasn’t exactly shaped like a sprinkler? What if its arms spiraled, twisted in the wrong direction, or turned back on themselves?
In a new study published July 13 in the journal PNASThe researchers deliberately created seven “silly” sprinklers with unusual arm geometry to pit the leading theories against each other. It consists of a sprinkler with arms that spiral multiple times to maximize the angular momentum of the water, and another with a counter-bend at the nozzle.
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Sprinkler designs were studied, with the rotation direction observed in forward (red arrow) and reverse (blue) modes.
In the process, he demolished both ideas. If Mack was correct, the spiral design should have rotated dramatically differently. If Feynman was correct, reversing the nozzle turn should have changed the direction of the sprinkler. That didn’t happen either.
“We were forced to say, ‘Feynman and the followers, you guys are gone,'” Ristorff told Live Science.
The spiral-armed sprinkler built to test Mack’s theory was equally adamant. Even though the fluid inside had a lot of angular momentum, “the solid barely cared,” Ristroph said.
All three measurements in the seven designs point to the central hub of the sprinkler, where the arms meet. There, the incoming water collides and rotates, generating a flux of angular momentum inside the device that is pushed back by the solid structure. The discovery shows that the reverse sprinkler is nothing more than an inside-out version of the forward sprinkler, driven by the same physics running on opposite ends of the arms.
Ristroph emphasized that this result was made possible because of Jesse Smith, who completed his physics doctorate at NYU while working on the project with a small team of Ristroph’s own students and Ristroph’s longtime collaborator. Brennan SprinkleA computational fluid dynamics expert at the Colorado School of Mines, whose surname is a coincidence.
Now, the team is building computer simulations to test whether the momentum-flow model holds beyond previously studied flow conditions, and they hope to eventually derive it from the fundamental fluid dynamics equations.
Ristroph said this “silly” problem has real-world applications: Understanding how curved channels convert fluid flow into rotational force could inform the design of turbines and other devices that harvest energy from air and water currents.
“If we could do something that would help engineers design equipment to make better use of the enormous amounts of wind and water energy that are all around us,” he said, “that would definitely be a fantastic thing.”
Smith, J.E., Zuo, M., Kuhlke, W., Sprinkle, B., and Ristroff, L. (2026). Geometry controls momentum flow in the sprinkler problem. Proceedings of the National Academy of Sciences, 123.https://doi.org/10.1073/pnas.2537479123
