When fluid dynamics mimic quantum mechanics

Jul 29, 2013 by Larry Hardesty
When the waves are confined to a circular corral, they reflect back on themselves, producing complex patterns (grey ripples) that steer the droplet in an apparently random trajectory (white line). But in fact, the droplet’s motion follows statistical patterns determined by the wavelength of the waves. Credit: Dan Harris

In the early days of quantum physics, in an attempt to explain the wavelike behavior of quantum particles, the French physicist Louis de Broglie proposed what he called a "pilot wave" theory. According to de Broglie, moving particles—such as electrons, or the photons in a beam of light—are borne along on waves of some type, like driftwood on a tide.

Physicists' inability to detect de Broglie's posited waves led them, for the most part, to abandon pilot-. Recently, however, a real pilot-wave system has been discovered, in which a drop of fluid bounces across a vibrating fluid bath, propelled by waves produced by its own collisions.

In 2006, Yves Couder and Emmanuel Fort, physicists at Université Paris Diderot, used this system to reproduce one of the most famous experiments in : the so-called "double-slit" experiment, in which particles are fired at a screen through a barrier with two holes in it.

In the latest issue of the journal Physical Review E (PRE), a team of MIT researchers, in collaboration with Couder and his colleagues, report that they have produced the fluidic analogue of another classic quantum experiment, in which electrons are confined to a circular "corral" by a ring of ions. In the new experiments, bouncing drops of fluid mimicked the electrons' statistical behavior with remarkable accuracy.

"This hydrodynamic system is subtle, and extraordinarily rich in terms of mathematical modeling," says John Bush, a professor of applied mathematics at MIT and corresponding author on the new paper. "It's the first pilot-wave system discovered and gives insight into how rational might work, were such a thing to exist."

Joining Bush on the PRE paper are lead author Daniel Harris, a graduate student in mathematics at MIT; Couder and Fort; and Julien Moukhtar, also of Université Paris Diderot. In a separate pair of papers, appearing this month in the Journal of Fluid Mechanics, Bush and Jan Molacek, another MIT graduate student in mathematics, explain the fluid mechanics that underlie the system's behavior.

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Interference inference

The double-slit experiment is seminal because it offers the clearest demonstration of wave-particle duality: As the theoretical physicist Richard Feynman once put it, "Any other situation in quantum mechanics, it turns out, can always be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing.'"

If a wave traveling on the surface of water strikes a barrier with two slits in it, two waves will emerge on the other side. Where the crests of those waves intersect, they form a larger wave; where a crest intersects with a trough, the fluid is still. A bank of pressure sensors struck by the waves would register an "interference pattern"—a series of alternating light and dark bands indicating where the waves reinforced or canceled each other.

Photons fired through a screen with two holes in it produce a similar interference pattern—even when they're fired one at a time. That's wave-particle duality: the mathematics of wave mechanics explains the statistical behavior of moving particles.

In the experiments reported in PRE, the researchers mounted a shallow tray with a circular depression in it on a vibrating stand. They filled the tray with a silicone oil and began vibrating it at a rate just below that required to produce surface waves.

They then dropped a single droplet of the same oil into the bath. The droplet bounced up and down, producing waves that pushed it along the surface.

The waves generated by the bouncing droplet reflected off the corral walls, confining the droplet within the circle and interfering with each other to create complicated patterns. As the droplet bounced off the waves, its motion appeared to be entirely random, but over time, it proved to favor certain regions of the bath over others. It was found most frequently near the center of the circle, then, with slowly diminishing frequency, in concentric rings whose distance from each other was determined by the wavelength of the pilot wave.

The statistical description of the droplet's location is analogous to that of an electron confined to a circular quantum corral and has a similar, wavelike form.

"It's a great result," says Paul Milewski, a math professor at the University of Bath, in England, who specializes in fluid mechanics. "Given the number of quantum-mechanical analogues of this mechanical system already shown, it's not an enormous surprise that the corral experiment also behaves like quantum mechanics. But they've done an amazingly careful job, because it takes very accurate measurements over a very long time of this droplet bouncing to get this probability distribution."

"If you have a system that is deterministic and is what we call in the business 'chaotic,' or sensitive to initial conditions, sensitive to perturbations, then it can behave probabilistically," Milewski continues. "Experiments like this weren't available to the giants of quantum mechanics. They also didn't know anything about chaos. Suppose these guys—who were puzzled by why the world behaves in this strange probabilistic way—actually had access to experiments like this and had the knowledge of chaos, would they have come up with an equivalent, deterministic theory of , which is not the current one? That's what I find exciting from the quantum perspective."

Explore further: Duality principle is 'safe and sound': Researchers clear up apparent violation of wave-particle duality

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User comments : 6

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gmurphy
5 / 5 (1) Jul 29, 2013
Nice experiment but if you really want to emulate quantum mechanics, you'll have to address the delay choice quantum eraser conundrum
MrVibrating
1 / 5 (4) Jul 29, 2013
causality 1

indeterminacy 0
Noumenon
1.7 / 5 (33) Jul 29, 2013
Physicists' inability to detect de Broglie's posited waves led them, for the most part, to abandon pilot-wave theory.

Recently, however, a real pilot-wave system has been discovered, in which a drop of fluid bounces across a vibrating fluid bath, propelled by waves produced by its own collisions.


I think this statement is misleading. What they found was a physical analogy, not that the qm wavefunction is an observable entity in itself physically guiding a 'particle'. It's not. It is pointless imo to construct such 'epicycles'**, unless you can physically observe the 'epicycle'.

** I mean here a redundant and imagined physical device for purposes of intuitive explanation. That is not the role of physics at the qm scale.
Noumenon
1.8 / 5 (33) Jul 29, 2013
Experiments like this weren't available to the giants of quantum mechanics. They also didn't know anything about chaos. Suppose these guys—who were puzzled by why the world behaves in this strange probabilistic way—actually had access to experiments like this and had the knowledge of chaos, would they have come up with an equivalent, deterministic theory of quantum mechanics, which is not the current one?


Wouldn't have made a difference, nor would constructing a model out of popsicle-sticks, and imagining that there is therefore popsicle-sticks moving things. The wavefunction is still not an observable physical thing.
vacuum-mechanics
1 / 5 (8) Jul 29, 2013
"This hydrodynamic system is subtle, and extraordinarily rich in terms of mathematical modeling," says John Bush, a professor of applied mathematics at MIT and corresponding author on the new paper. "It's the first pilot-wave system discovered and gives insight into how rational quantum dynamics might work, were such a thing to exist."


It seems that this is not the first pilot-wave system discovered….We could easily see something analogous in nature, such as a water bug moving ( on water surface), could create a co-moving water surface wave around …..
http://www.vacuum...17〈=en
Noumenon
1.8 / 5 (32) Jul 29, 2013
The wavefunction is still not an observable physical thing.
So http://www.nature...F1.html, we already observed it directly? The trick is in the word "direct".


Nope. They're using quantum tomography which merely reconstructs or graphs out the wave-function, based on measurements not of the wave-function itself, but of repeated (weak) measurements of the photon position (and) or momentum.