Physicists have debated which way a submerged sprinkler sucking in water would spin. Careful experiments provide an answer.

This week in science and technology news we’re covering at The Debrief: For decades, physicists have argued about Richard Feynman’s Sprinkler Problem, involving the question of how a reverse sprinkler would operate. Now we may finally know the answer… elsewhere, new studies reveal how breathwork may trigger psychedelic-like states, emotional breakthroughs, and psychological insights without drugs. Meanwhile, a stone sphere unearthed at an archaeological site in Azerbaijan is reportedly unlike anything previously documented in the South Caucasus.

Also, in an exclusive feature, University of Toronto bioethicist KM Bowman, Ph.D., writes for The Debrief about his firsthand experience with uncontacted indigenous tribes: “Through the haze stood a man. Barely clothed, carrying what appeared to be a weapon of some form, he remained motionless for only a few seconds before silently disappearing back into the forest.” In his essay, Bowman shares his experience and asks whether governments can protect “the uncontacted.”

Finally, here’s a look at more stories The Debrief is tracking right now…

For decades, physicists have debated Richard Feynman’s famous Sprinkler Problem—the question of how a reverse sprinkler would behave—and now researchers may have finally found the answer.

Made famous in Feynman’s 1985 memoir, Surely You’re Joking, Mr. Feynman!, the problem concerns the spin direction of a sprinkler-like device inside a pool that draws water inward instead of spreading it out. The researchers revealed their experimental results, based on tests with a variety of custom sprinklers, in a recent paper published in the Proceedings of the National Academy of Sciences, adding new life to the long-standing physics debate.

Feynman Experiments

“This work provides the experimental answer for Feynman’s Sprinkler Problem by showing, across several sprinkler types, how the angular momentum of water flows drives sprinklers’ rotation,” said senior author Leif Ristroph, an associate professor at New York University’s Courant Institute School of Mathematics, Computing, and Data Science.

The significance of the work extends well beyond an esoteric question about a hypothetical device, providing new insights into the physics of fluid dynamics. Feynman famously attempted to investigate the problem experimentally during the 1940s, though his apparatus reportedly failed dramatically before producing a definitive result. During his lifetime, Feynman discouraged naming the problem after himself, noting that it built upon ideas first explored by physicist Ernst Mach in the 1880s.

“Our findings provide a firmer understanding of how components respond to fluid flows—knowledge that can guide future engineering and technological advances for devices, such as turbines, that convert these flows into energy,” said co-author Brennan Sprinkle, an assistant professor at Colorado School of Mines.

Exploring Feynman

This isn’t the team’s first work on Feynman’s Sprinkler Problem, as it builds on what they reported in an earlier 2024 paper. Unlike Feynman himself, the researchers managed to make their reverse sprinkler rotate, leading to intriguing new physics discoveries. In that work, they discovered that a reverse sprinkler would operate much like a conventional one, though at about 50 times the slower rate.

While traditional sprinklers act like a rocket of water, the Feynman sprinkler is comparable to an inside-out rocket, shooting towards the junction of the arms at the device’s center. Inside that central chamber, the team discovered something interesting: the water jets do not collide head-on, which produces the sprinkler’s reverse rotation, in what the researchers dubbed the momentum flux theory.

That 2024 study was limited to traditional sprinklers with S-shaped arms, leaving the researchers to ponder how alternate shapes would impact the water flow. After fabricating their own series of sprinkler designs, the researchers tested water flow in both forward and reverse directions, measuring torque, flow rate, and rotational motion.

Testing Theories

One of the theories they tested was Ernest Mach’s 1883 theory that the fluid and sprinkler would swirl in opposite directions, but they found it did not align with the reverse rotations and torques observed in their experiments. They also tested a theory originally attributed to Feynman and later refined by others, which suggested that water flow at the outer edges of the sprinkler arms drove rotation; however, the team’s observations showed this is not the case.

Their observations supported the team’s existing momentum flux theory in both the forward and reverse directions, across all sprinkler shapes. According to their work, the shape of the sprinkler arms controls the flow of the water jet.

“By showing that momentum flux is the answer to Feynman’s Sprinkler Problem,” Ristroph concluded, “our findings address a long-standing open problem in flow physics and provide useful knowledge about how these devices work and their effectiveness.

The paper, “Geometry Controls Momentum Flux in the Sprinkler Problem,” appeared in the Proceedings of the National Academy of Sciences on July 13, 2026.

Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.

Watering your lawn in the summer can be both pragmatic and fun with so-called "silly sprinklers," designed to create amusing loops and spirals of water jets. And there's some fascinating physics at work to boot. Researchers at New York University's Courant Institute conducted a series of experiments with different silly sprinkler designs to find the answer to a longstanding problem in fluid dynamics, according to a new paper published in the Proceedings of the National Academy of Sciences.

As previously reported, the reverse sprinkler problem is associated with physicist Richard Feynman because he popularized the concept, but it actually dates back to a chapter in Ernst Mach’s 1883 textbook The Science of Mechanics (Die Mechanik in Ihrer Entwicklung Historisch-Kritisch Dargerstellt). Mach’s thought experiment languished in relative obscurity until a group of Princeton University physicists began debating the issue in the 1940s.

Feynman was a graduate student there at the time and threw himself into the debate with gusto, even devising an experiment in the cyclotron laboratory to test his hypothesis. One might intuit that a reverse sprinkler would work just like a regular sprinkler, merely played backward, so to speak. But the physics turns out to be more complicated. “The answer is perfectly clear at first sight,” Feynman wrote in Surely You’re Joking, Mr. Feynman (1985). “The trouble was, some guy would think it was perfectly clear [that the rotation would be] one way, and another guy would think it was perfectly clear the other way.”

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