Researchers Solve Feynman's Sprinkler Problem After Decades-Long Quest

Researchers have made significant progress in solving a long-standing physics puzzle known as the Sprinkler Problem, originally posed by renowned physicist Richard Feynman. The problem revolves around understanding how a reverse sprinkler, which draws water inward rather than spraying it outward, would behave. In a recent paper published in the Proceedings of the National Academy of Sciences, scientists from New York University and Colorado School of Mines have provided experimental evidence to explain this phenomenon.

The research team, led by senior author Leif Ristroph, an associate professor at NYU’s Courant Institute, conducted experiments using various custom-designed sprinklers. These tests revealed that the angular momentum generated by water flow drives the rotation of the sprinkler in reverse. This finding not only answers Feynman's question but also offers new insights into fluid dynamics and its applications.

Feynman first attempted to solve this problem experimentally during the 1940s, though his efforts were unsuccessful. He later discouraged naming the problem after himself, noting that it was inspired by earlier work from physicist Ernst Mach in the late 1800s. The significance of the recent findings extends beyond theoretical physics; they could guide future engineering and technological advancements for devices like turbines that convert fluid flows into energy.

In their previous research published in 2024, the team discovered that a reverse sprinkler operates similarly to a conventional one but at approximately 50 times slower. Unlike traditional sprinklers, which function as water rockets, Feynman's reverse sprinkler acts more like an inside-out rocket, directing water towards the center of the device. The researchers observed that the water jets do not collide head-on in the central chamber, leading to a reverse rotation.

To further explore this concept, they fabricated and tested various sprinkler designs with different arm shapes, measuring torque, flow rate, and rotational motion both forward and backward. Their experiments disproved earlier theories by Mach and Feynman himself, confirming that the shape of the sprinkler arms controls water jet flow and thus drives rotation.

According to Ristroph, these findings address a long-standing open problem in fluid physics and provide practical knowledge about how such devices function and their efficiency. The paper titled "Geometry Controls Momentum Flux in the Sprinkler Problem" was published on July 13, 2026, marking a significant milestone in solving one of Feynman's most intriguing challenges.

This breakthrough not only resolves an academic debate but also opens new avenues for understanding fluid dynamics and its practical applications in engineering.