Physicists Provide Experimental Solution to the Feynman Sprinkler Problem

A team of experimental physicists led by Leif Ristroph at New York University has reported a definitive experimental answer to the long-standing “Feynman sprinkler” puzzle. Their findings appear in a recent paper in *Proceedings of the National Academy of Sciences*. The experiment examined a lawn-sprinkler model submerged underwater, with water drawn into its arms rather than expelled, and measured the resulting torque to determine whether the device rotates in its original direction, the opposite direction, or remains stationary.

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The question, popularly linked to Richard Feynman, asks how a reverse-flow sprinkler behaves when water is sucked in. Although Feynman discussed the problem, he never resolved it, and it has persisted as a classic thought experiment in fluid dynamics and physics education.

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• Leif Ristroph – experimental physicist and applied mathematician, New York University, principal investigator of the study.

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• Detlef Lohse – professor of physics at the University of Twente in the Netherlands, external commentator on the results.

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Ristroph expressed confidence that the experiment settles the debate, noting that the measured torque aligns with theoretical predictions for reverse-flow rotation. Lohse, while not involved in the work, affirmed that the problem as posed by Feynman is now resolved, highlighting the significance of the experimental verification for fluid-mechanics theory.

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• “I am confident we’ve provided the experimental answer to the Feynman sprinkler problem,” — Dr. Ristroph

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• “This Feynman problem, as put by Feynman — this is solved,” — Detlef Lohse, a professor at the University of Twente in the Netherlands

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