Richard Feynman was arguably one of the greatest intuitive minds in modern physics. He helped build the atomic bomb, rebuilt quantum electrodynamics, and could explain complex particles using simple scribbles. But one deceptively simple puzzle got under his skin so deep that he actually blew up a laboratory glass carboy trying to solve it.
It is called the reverse sprinkler problem.
For eighty years, physicists argued about what happens when you submerge a common lawn sprinkler in a tank of water and suck water in instead of blowing it out. Does it spin forward, backward, or just sit there vibrating like a confused piece of plumbing?
A team of applied mathematicians finally put this mystery to bed. Using custom-built, ultra-low-friction rigs and high-speed imaging, they proved that the reverse sprinkler does indeed spin. But the physics behind why it spins is wilder than Feynman ever anticipated. It turns out that everyone was looking at the wrong part of the sprinkler.
The Simple Physics of the Forward Sprinkler
To understand why the reverse version is such a headache, we have to look at how a regular lawn sprinkler works.
When you hook up a standard sprinkler to a garden hose, water rushes out of the bent nozzles. As the water sprays outward, it pushes back against the nozzle. This is basic action-reaction physics. The water goes one way, and the metal arm goes the opposite way.
You can easily calculate this using Newton's third law of motion. It is intuitive, predictable, and keeps golf courses green.
But reverse the flow. Imagine you place that exact same sprinkler inside a deep pool of water. Instead of pumping water through the hose, you hook up a vacuum and suck water into the nozzles.
If the water is now entering the nozzles instead of exiting, does the sprinkler spin in reverse? Does the incoming momentum pull the sprinkler forward? Or do the forces cancel each other out, leaving the device completely motionless?
Why Feynman Blew Up a Princeton Lab
During his graduate student days at Princeton in the 1940s, Feynman became obsessed with this question. The physics department was split. Some of the brightest minds of the era insisted the sprinkler would rotate in reverse. Others swore it would not budge.
Feynman decided to settle the debate with an experiment. He set up a large glass carboy, filled it with water, ran a tube inside to act as a reverse sprinkler, and hooked it up to the university's compressed air line to create suction.
It did not go well.
As Feynman increased the pressure to pull more water through, the air pressure built up inside the glass vessel. The carboy shattered violently, flooding the Princeton basement and ending the experiment. Feynman never published an official paper on his findings, though he briefly mentioned the puzzle in his famous Lectures on Physics, hinting that the sprinkler did not move at all once a steady state was reached.
For decades, the physics community took Feynman's word as gospel, or they published conflicting mathematical proofs that arrived at completely different conclusions. The problem was that building a truly frictionless experiment in a fluid medium is incredibly difficult. Gravity, buoyancy, and the drag of the water itself almost always skewed the results.
Inside the NYU Breakthrough
The breakthrough came from researchers at New York University's Courant Institute of Mathematical Sciences. Led by Leif Ristroph, the team spent years designing an experimental setup that eliminated the errors of previous attempts.
Instead of heavy metal pipes, they used a lightweight, custom-engineered rotary device. They suspended it on ultra-low-friction bearings. To measure the water flow without interfering with the movement, they put tiny, light-reflecting micro-particles into the water and illuminated them with green lasers.
When they turned on the suction, they saw something remarkable.
The reverse sprinkler actually rotates. But it does not rotate in the same direction as a normal sprinkler, nor does it behave like a simple mirror image. It spins in "reverse"—meaning the nozzles lead the way, rotating in the direction of the incoming water.
Even more surprising was the speed. The reverse sprinkler rotates much slower than a forward sprinkler. It is also highly unstable. If you pull the water too slowly, nothing happens. You need a certain threshold of flow to get the rotation started.
The Secret is on the Inside
So why does it spin? The answer lies inside the sprinkler head itself, a detail that most theorists completely ignored for decades.
When a normal sprinkler operates, the water sprays out into the open air. The reaction force happens right at the nozzle tip. The air outside does not exert any significant force back on the sprinkler.
When you suck water in, the story changes. The water does not just magically disappear once it enters the nozzle. It forms a high-speed internal jet inside the central chamber of the sprinkler.
Think of it this way. When water enters the bent arm of the reverse sprinkler, it has to turn a corner. As it turns that corner, it exerts a centrifugal force on the outer wall of the tube. That force wants to push the sprinkler in one direction.
But as that water shoots out of the nozzle pipe and enters the central reservoir, it forms a jet. That jet of water slams into the opposite internal wall of the sprinkler chamber.
You have two competing forces:
- The force of the water entering the nozzle, which pulls the nozzle forward.
- The force of the internal water jet slamming into the back wall of the chamber, which pushes the nozzle backward.
In a perfectly symmetrical, theoretical world, these forces would cancel out perfectly. This is why many physicists argued the sprinkler would remain stationary. But in the real world, fluids do not behave symmetrically.
The NYU team showed that the internal jets of water collide and create a complex, chaotic flow pattern inside the central hub. The momentum of these colliding internal jets is not fully transferred back to the structure. Some of it is lost to fluid friction and turbulence.
Because the internal jet force is slightly degraded by this internal chaos, the forward pull at the nozzle wins the tug-of-war. The result is a slow, steady rotation where the nozzles lead the movement.
What Most People Get Wrong About This Problem
The biggest mistake people make when thinking about the reverse sprinkler is assuming that fluid dynamics are reversible. They look at time-reversal symmetry in physics equations and assume that if you reverse the arrow of time, or the arrow of fluid flow, the mechanical output simply flips.
Fluids do not work that way.
When you blow water out of a nozzle, you create a tight, directed stream that shoots far into the distance. Think of a garden hose. The jet retains its shape and momentum for a long time.
When you suck water into a nozzle, you do not get a nice, directed stream from across the room. Instead, the water is drawn in uniformly from all directions, like a sink drain. It is a completely different flow geometry.
Because outgoing flow is a directed jet and incoming flow is a diffuse sink, the external forces on the outside of the sprinkler are completely asymmetric. You cannot just slap a minus sign on the equations and call it a day.
Your Next Steps to Mastering Fluid Mechanics
If you want to truly understand how these counterintuitive forces work in real life, do not just stop at reading about the reverse sprinkler. You can see these principles in action using items you have at home.
- Observe the Sink Effect: Turn on your kitchen faucet and watch the water hit the bottom of the sink. Notice how it creates a smooth, spreading sheet of water that eventually breaks into turbulent chaos. That transition from smooth (laminar) to chaotic (turbulent) flow is exactly what happens inside the reverse sprinkler hub.
- Test the Asymmetry: Take a straw and blow through it hard. You can easily blow out a candle from a few inches away because the air forms a directed jet. Now, try to suck out a candle from the exact same distance. It is impossible. The air enters the straw from all directions around the tip, destroying the directed force. This simple test shows why the reverse sprinkler cannot just be a simple inverse of the forward sprinkler.
- Read the Source Material: If you want to dive deeper into the actual mathematics, look up the 2024 paper published by Leif Ristroph and his team at NYU. They break down the exact Reynolds numbers and flow velocities required to trigger the rotation.
Feynman did not have access to high-speed digital cameras, laser-illuminated particle tracking, or precise 3D-printed components. If he had, he probably would have skipped the exploding glass carboy and solved the mystery in an afternoon.
Sometimes, even the greatest minds in history just need better tools.
