NYU Researchers Solve 143-Year Reverse Sprinkler Puzzle With Silly Sprinklers

Q: Why did Richard Feynman's famous experiment get the reverse sprinkler problem wrong?

Feynman’s experiment was wrong because it backed the wrong physical explanation. He accepted the long-standing view, associated with Ernst Mach, that a reverse sprinkler should end up with essentially no sustained rotation because the suction force and the reaction at the nozzle cancel out. The new NYU-led study found that this misses the real source of torque. Using specially designed “silly sprinklers,” the researchers showed that the decisive forces do not come from the outer arm flows that Feynman and earlier thinkers emphasized. Instead, the motion is controlled by momentum flux inside the device: water jets drawn inward collide within the sprinkler chamber and generate torque, acting like an “inside-out rocket.” That means Feynman’s famous result was not simply a matter of observing a slight tremor and then no motion; it was a limitation of the experimental setup and interpretation. His 1940s cyclotron-lab test did not isolate or measure the internal jet-collision effects that the new experiments were built to capture. The modern study reports that geometry strongly changes the outcome, which helps explain why earlier intuition and simpler tests were misleading. So the error was not that Feynman asked the wrong question. It was that his experiment, and the theory behind it, focused on the wrong mechanism. According to the new PNAS paper, reverse-sprinkler behavior is governed by internal momentum transfer, not by the external flow picture that shaped Feynman’s account.

Q: What modern tools helped solve a physics puzzle that stumped scientists for over 140 years?

The puzzle was cracked mainly with **custom-built modern experimental apparatus**: NYU researchers designed and tested “silly sprinklers” with different shapes and contours, then used precise measurements of **rotation, torque, and internal and external water flow** to see what was really happening. The news and supporting material do not identify a single branded technology as the breakthrough. Instead, the advance came from a **modern experimental setup** that let scientists vary sprinkler geometry and directly measure the forces in ways earlier generations could not. Those experiments showed that the key effect comes from **internal jet collisions** inside the sprinkler, supporting the team’s “momentum flux theory.” That matters because it overturned the long-accepted view associated with Ernst Mach and later Richard Feynman, who had concluded that a reverse sprinkler would largely not rotate except for a brief tremor. The new work found that geometry can produce reverse rotation, acting somewhat like an **inside-out rocket**. So, the modern tools were not a single gadget but a combination of **specially engineered sprinkler models and advanced fluid-dynamics measurement techniques** carried out in a contemporary lab setting.

3 articles · Updated · Ars Technica · Jul 13

A new PNAS paper from NYU’s Courant Institute reports experiments with “silly sprinklers” that resolve the reverse sprinkler problem, a fluid-dynamics puzzle dating to 1883.
The tests support Ernst Mach’s conclusion that a sprinkler sucking in water should show no sustained rotation in steady state because two opposing forces cancel at the nozzle.
The work also matches Richard Feynman’s later observation that the device can twitch briefly when flow or pressure first starts, then settle back and remain still.
Feynman helped popularize the question after Princeton debates in the 1940s, but the underlying thought experiment first appeared in Mach’s 19th-century mechanics textbook.