{"id":587456,"date":"2026-07-15T19:54:13","date_gmt":"2026-07-15T19:54:13","guid":{"rendered":"https:\/\/www.europesays.com\/ie\/587456\/"},"modified":"2026-07-15T19:54:13","modified_gmt":"2026-07-15T19:54:13","slug":"researchers-claim-to-have-solved-the-perplexing-reverse-sprinkler-problem-that-stumped-feynman","status":"publish","type":"post","link":"https:\/\/www.europesays.com\/ie\/587456\/","title":{"rendered":"Researchers Claim to Have Solved the Perplexing \u2018Reverse Sprinkler\u2019 Problem That Stumped Feynman"},"content":{"rendered":"

Scientists say they have solved in detail what may be the most famous lawn-care-related problem in all of physics.<\/p>\n

The so-called Feynman sprinkler problem is named after one of the best-known physicists of the 20th century, Richard Feynman<\/a>, even though he was not the first to pose it, and he was unable to solve it.<\/p>\n

The problem is this: Imagine a lawn sprinkler submerged underwater, with water being sucked into its arms instead of sprayed out. Does the sprinkler rotate in the same direction as before, spin in the opposite direction or not rotate at all?<\/p>\n

Scientists led by Leif Ristroph, an experimental physicist and applied mathematician at New York University, say they have cracked this question. \u201cI am confident we\u2019ve provided the experimental answer to the Feynman sprinkler problem,\u201d Dr. Ristroph said.<\/p>\n

The researchers report their latest findings in a paper published on Monday<\/a> in the Proceedings of the National Academy of Sciences.<\/p>\n

\u201cThis Feynman problem, as put by Feynman \u2014 this is solved,\u201d said Detlef Lohse, a professor at the University of Twente in the Netherlands who was not involved with the research.<\/p>\n

To understand the problem of the reverse, submerged sprinkler, first consider a normal S-shaped garden sprinkler that rotates around a central pivot and sprays water out. Water flows up through the center and flows out of nozzles at the ends of its two curved arms, producing thrust that spins the sprinkler. The physics to explain that motion \u2014 essentially like a water-powered rocket \u2014 is simple and uncontroversial.<\/p>\n

But reverse the flow, and the problem becomes complex.<\/p>\n

Even though the equations describing the motion of the water and the sprinkler are the same, there is no simple answer. Physicists have been arguing over it since Ernst Mach, an Austrian physicist, first described the problem in 1883.<\/p>\n

In his book, \u201cSurely You\u2019re Joking, Mr. Feynman,\u201d Feynman said it was a topic of argument among graduate students when he was at Princeton University in the 1940s.<\/p>\n

He offered an argument for how the reverse sprinkler could be spinning in one direction, and then made a conflicting argument for how it could be moving the other way.<\/p>\n

He did not reveal what he thought the answer to be, but he described how he had set up an experiment to settle the question. The apparatus exploded. \u201cSuddenly the whole thing just blew glass and water in all directions throughout the laboratory,\u201d Feynman wrote.<\/p>\n

Dr. Ristroph decided to tackle the quandary about five years ago. \u201cI was searching around for some good unsolved hard problems, and this one really caught my eye, just because it has such an infamous history,\u201d he said.<\/p>\n

One of the collaborators Dr. Ristroph recruited was Brennan Sprinkle, then a postdoctoral researcher just starting at N.Y.U. \u201cLeif said something very polite,\u201d Dr. Sprinkle said. \u201cA polite version of \u2018Hey, you\u2019ve got a kind of funny name \u2014 check out this experiment I got running.\u2019\u201d<\/p>\n

Dr. Sprinkle visited the laboratory and was intrigued, both because of his name and because of the science. \u201cMy first reaction was, yeah, this would be funny,\u201d he said. \u201cAnd then my second reaction was, \u2018Oh, actually, this is a deeply nonintuitive problem that is very cool.\u2019\u201d<\/p>\n

Theorists had made conflicting predictions about which direction the reverse sprinkler should be spinning. Experiments \u2014 even those that did not explode like Feynman\u2019s \u2014 were inconclusive.<\/p>\n

After conducting precise experiments, Dr. Ristroph and his colleagues at N.Y.U. reported two years ago that they had the answer<\/a>: the submerged sprinkler sucking in water spun in the opposite direction to a typical sprinkler expelling water.<\/p>\n

The spin was considerably slower, at about one-fortieth the usual speed.<\/p>\n

The researchers also had an explanation for why it spun in the opposite direction. The bends in the sprinkler arms created a force that shifted the two jets of incoming water.<\/p>\n

\u201cIf you\u2019re a car and you\u2019re turning right, you will feel a force, an inertial force, going in the opposite direction,\u201d Dr. Sprinkle said.<\/p>\n

In the fluid, the force shifted the jets so that they did not collide head on, creating a twisting force that rotated the sprinkler.<\/p>\n

Not everyone was convinced.<\/p>\n

\u201cI have never received so much hate mail,\u201d Dr. Ristroph said.<\/p>\n

Because the dynamics of flowing fluids are complex, the critics wondered, how could the researchers be sure that their experiment, which described one particular configuration, was true in general?<\/p>\n

A new series of experiments described in Monday\u2019s paper looked at what happened with \u201csilly sprinkler\u201d shapes, testing whether other factors might change the direction of the spin.<\/p>\n

For example, Feynman\u2019s argument for why the reverse sprinkler would move in the opposite direction, as Dr. Ristroph\u2019s experiments observed, was that suction at the ends of the arms would pull the sprinkler in that direction.<\/p>\n

If that were true, then adding a second bend to each arm so that the opening faced the other direction would conceivably change the direction of the spin.<\/p>\n

\u201cIt does nothing like it, just didn\u2019t matter,\u201d Dr. Ristroph said. The sprinkler with the extra bends rotated in the same direction as the simpler S-shape. So did other sprinkler variations. The most important factor was the amount of bending near the pivot, because that determined the offset of the two incoming jets.<\/p>\n

The alternative ideas are \u201cpretty definitively wrong from what we found,\u201d Dr. Ristroph said. That makes him more confident that the explanation they had offered in the 2024 paper was correct.<\/p>\n

Submerged sprinklers moving in the reverse direction do not have much obvious use in the real world, but the research could help with a general understanding of the complex and intertwined motions of a solid object immersed within the flows of a fluid.<\/p>\n

That knowledge could then be applied to design systems like those that harvest energy from ocean waves or winds.<\/p>\n

Dr. Ristroph said the Feynman sprinkler problem is not completely solved yet, because no one has yet succeeded in generating a computer simulation showing exactly how the water exerts pressure on the sprinkler. The problem is complex because of the high pressures within the sprinkler tubes and because everything is in continual motion.<\/p>\n

That is what Dr. Sprinkle, now a professor of applied mathematics and statistics at the Colorado School of Mines, is still working on.<\/p>\n

\u201cIt just winds up being a pretty numerically challenging setup,\u201d Dr. Sprinkle said.<\/p>\n","protected":false},"excerpt":{"rendered":"Scientists say they have solved in detail what may be the most famous lawn-care-related problem in all of…\n","protected":false},"author":2,"featured_media":587457,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_share_on_mastodon":"0"},"categories":[271],"tags":[18,249258,19,17,14998,452,142466,172,249259,133,447],"class_list":["post-587456","post","type-post","status-publish","format-standard","has-post-thumbnail","category-physics","tag-eire","tag-feynman","tag-ie","tag-ireland","tag-mathematics","tag-physics","tag-proceedings-of-the-national-academy-of-sciences","tag-research","tag-richard-p","tag-science","tag-water"],"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@ie\/116925750868614005","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts\/587456","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/comments?post=587456"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts\/587456\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/media\/587457"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/media?parent=587456"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/categories?post=587456"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/tags?post=587456"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}