Researchers Resolve Feynman’s Reverse-Sprinkler Mystery

A multidisciplinary team from New York University’s Courant Institute of Mathematics, Computing, and Data Science experimentally solved the long-standing “Feynman Sprinkler Problem.” By constructing a series of custom sprinklers—including whimsical “silly” designs with loopy tubes—and testing them in both forward (water expelled) and reverse (water drawn in) modes, the researchers demonstrated that reverse sprinklers rotate opposite to conventional ones, but at roughly one-fiftieth the speed. The rotation arises from internal fluid jets that collide within the central hub, generating asymmetric pressure forces that drive the slow backward spin.

How Physicists Finally Solved the 140-Year-Old Reverse Sprinkler MysteryResearchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzleThe Debrief

The question dates to 1883 when Ernst Mach noted ambiguous rotation in a reverse sprinkler, and it was popularized by Richard Feynman in his 1985 memoir *Surely You’re Joking, Mr. Feynman!* Feynman’s own 1940s sink-based experiment failed, leaving the problem unresolved for more than a century. Earlier 2024 work by the same NYU group examined only conventional S-shaped sprinklers, finding reverse rotation 50 times slower than forward motion but leaving open whether alternative geometries might behave differently.

The Debrief

• Leif Ristroph, associate professor at NYU’s Courant Institute, senior author.

Researchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzle

• Brennan Sprinkle, assistant professor at Colorado School of Mines, co-author.

The Debrief

• Graduate students Jesse Smith, Mingxuan Zuo, and undergraduate Will Kuhlke (NYU) contributed to experimental design and data analysis.

Researchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzle

• Funding was provided by the National Science Foundation (grants DMS-2407787 and DMS-2407788).

Researchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzle

• Reverse sprinklers rotated counter-clockwise at approximately 1/50 the angular velocity of forward-spraying counterparts.

How Physicists Finally Solved the 140-Year-Old Reverse Sprinkler Mystery

• Torque measurements and flow-rate recordings across multiple sprinkler geometries consistently supported the momentum-flux mechanism.

The Debrief

• Experiments showed that external arm flows had no measurable effect on rotation, refuting the “outer-edge” theory attributed to Feynman.

How Physicists Finally Solved the 140-Year-Old Reverse Sprinkler Mystery

Understanding how flowing fluids exert forces on internal structures extends beyond a classroom curiosity. The momentum-flux framework can inform the design of turbines, tidal energy converters, and low-friction pumping systems, potentially improving efficiency in municipal water treatment and large-scale irrigation projects such as Kenya’s Galana Kulalu Food Security Initiative.

How Physicists Finally Solved the 140-Year-Old Reverse Sprinkler Mystery

Leif Ristroph emphasized that the 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.” Brennan Sprinkle added that the 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.” The team also noted that their momentum-flux theory “applies equally well for both reverse and forward modes and for all the differently shaped sprinklers,” suggesting broad applicability.

Researchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzleThe DebriefHow Physicists Finally Solved the 140-Year-Old Reverse Sprinkler Mystery

Earlier theoretical positions—Mach’s 1883 hypothesis that fluid and sprinkler swirl oppositely, and a later theory attributing rotation to outer-arm flows—were explicitly tested and found inconsistent with the observed torques and rotational directions.

The Debrief

• “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,” — Leif Ristroph

Researchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzle

• “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,” — Brennan Sprinkle

Researchers put “silly sprinklers” in reverse to further unravel decades-old physics puzzle

• “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.” — Leif Ristroph

The Debrief