Updated

Updated · BIOENGINEER.ORG · Jul 14

NYU Scientists Solve 100-Year Reverse Sprinkler Paradox, Finding Inward Jets Spin 50 Times Slower

Updated

Updated · BIOENGINEER.ORG · Jul 14

NYU Scientists Solve 100-Year Reverse Sprinkler Paradox, Finding Inward Jets Spin 50 Times Slower

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 · BIOENGINEER.ORG · Jul 14

Custom NYU sprinkler experiments showed angular momentum carried by fluid jets—not older swirl-based theories—sets the direction and strength of reverse-sprinkler rotation.
Measurements of torque, flow patterns and spin across multiple arm geometries backed a momentum-flux model in which inward jets collide inside the device and generate subtle rotational force.
Reverse sprinklers turned about 50 times more slowly than conventional ones because those internal collisions partly cancel the jets’ momentum while still leaving detectable torque.
Published in PNAS, the work challenges explanations associated with Ernst Mach and Richard Feynman and offers a broader framework for designing turbines and energy-harvesting devices.