A team of physicists set out to recreate a 60-year-old quantum mystery using a large water tank. While their experiment produced the expected results, it also revealed an unexpected behavior in the water.
The experiment centered on the Aharonov–Bohm effect, a strange quantum phenomenon where electrons respond to a magnetic field even when they never actually pass through it. First predicted back in 1959, this effect has been notoriously difficult to confirm because the changes it causes in electron waves are incredibly subtle.
Instead of using electrons and a particle accelerator, researchers from the Okinawa Institute of Science and Technology (OIST), along with collaborators in Oslo and Chile, recreated the effect using water waves, making an otherwise invisible quantum effect visible in a tabletop experiment. The team reported their findings in Communications Physics.
A 1980 Idea Gets a Modern Upgrade
In 1980, physicist Michael Berry showed that classical systems could model the Aharonov–Bohm effect. In this analogy, a vortex at the center of a water tank replaces the solenoid that creates a magnetic field in the quantum experiment. Water waves take the place of electrons. When these waves travel around the vortex, rather than through it, they acquire a measurable distortion. This appears as a pitchfork-shaped pattern in the wave fronts, showing that the vortex changes the phase of the waves without direct contact. The OIST team extended the approach by testing a new scenario.
“With waves traveling the opposite direction, you see a mirror image pattern,” says Jonas Rønning, co- author and former postdoc in the OIST unit. “The question for us was, what happens if you send waves from both directions at the same time? We thought that the patterns might cancel each other out, or both pitchfork-like patterns would be visible, but our intuition was completely wrong.”
Two Waves Walk Into a Vortex
The researchers constructed a custom tank, created a vortex at its center, and sent waves from opposite sides to meet in the middle. They used a high-speed camera to observe how the waves interacted on the water’s surface.
Without a vortex, the peaks and troughs remain in place. When the vortex is there, the standing wave breaks down. The vortex shifts the phase of the waves as they travel around it, altering the standing-wave interference pattern and creating rotating nodal lines. These are lines of momentarily flat water that spiral outward and turn in the opposite direction of the vortex.
“This was something new and unexpected,” says Aditya Singh, a PhD student in the Nonlinear and Non-equilibrium Physics Unit and co-first author of the study. “That’s what makes this fluid analogue system so valuable. It reveals topological effects — wave behaviors that occur across the whole system — that can’t be seen in quantum experiments.”
At first, the team suspected their equipment was causing the rotating lines. However, the same pattern also appeared in their computer simulations.
“When we first saw these lines, we thought they were an experimental artifact,” Singh says. “But when we also saw them in our simulations, we dropped everything and quickly worked out the mathematics underlying how they arise.”
More Speed, More Lines
As the strength of the vortex increased, more rotating nodal lines appeared, always moving in the direction opposite to the vortex. The study showed that the number of lines is quantized, meaning it is fixed to whole-number values determined by the vortex’s circulation. In other words, the topology of the vortex determines how many rotating nodal lines appear. This effect was observed across the entire wave field, not just near the vortex core as earlier models suggested.
This finding differs from Berry’s original experiment, which observed changes only near the vortex. In this case, the researchers discovered a non-local effect, a pattern that covers the entire system and depends on the overall setup. Instead of being limited to the area near the vortex, the rotating nodal lines appeared throughout the whole standing-wave pattern.
Although the practical uses of these newly discovered nodal patterns are not yet clear, the study shows that simple classical systems can reveal behaviors that are challenging to observe in quantum experiments. By making these effects visible, the researchers plan to explore more complex wave systems that could offer new insights into superconductivity and other quantum phenomena.
“One direction is to make the system more complex by introducing multiple vortices and arranging them into a lattice,” senior author Mahesh Bandi says. “That setup would mirror conditions in some superconducting materials, with the water waves behaving like a supercurrent. We don’t yet know what we’ll see — and that’s exactly what makes it worth doing.”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.
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