In quantum mechanics, small bits of matter have the mysterious behavior of sometimes seeming to behave as particles, sometimes as waves. But the dominant explanation of this mystery, assumed for the last century, may be wrong, researchers say.

That explanation has been known as the "Copenhagen interpretation," which posits that any single particle is actually a wave that is smeared out in space and which will collapses down to an actual location -- as a discrete particle existing at a single point -- only when it is observed.

However, in the early days of quantum theories some pioneers in the field, among them Louis de Broglie, considered an alternative explanation, which they dubbed "pilot-wave theory," holding that quantum matter always exists as particles but are carried along by some sort of waves that influence them so they seem to exhibit wave-like behavior.

Prompted by recent results from work by French physicists, MIT mathematics professor John Bush says he believes the pilot-wave theory, long an also-ran in quantum physics circles, should be given another look.

In the French effort, researchers at the University of Paris Diderot discovered a pilot-wave system that, although operating at the macroscopic scale, displays statistical behavior under certain circumstance that is similar to that ascribed to quantum systems.

In Paris, the researchers created a basin of fluid kept vibrating at frequencies just below a point at which waves could begin to form on its surface. Onto this basin a single drop of the identical fluid was dropped, causing waves to propagate outward from where it hit.

The droplet was carried across the basin, moving on the very same waves it created.

"This system is undoubtedly quantitatively different from quantum mechanics," Bush says. "It's also qualitatively different: there are some features of quantum mechanics that we can't capture, some features of this system that we know aren't present in quantum mechanics. But are they philosophically distinct?"

The orthodox Copenhagen interpretation ignores the conundrum of calculating quantum matter's trajectory by denying that it exists as a particle until and only during the time it is under observation.

When a measurement is made on a quantum particle it causes the wave form to collapse, but the determinate state the particle assumes is completely random, the interpretation holds, meaning its existence is a statistical construct, not rooted in reality.

Bush says that's not good enough.

"The key question is whether a real quantum dynamics, of the general form suggested by de Broglie and the walking drops, might underlie quantum statistics," he says. "While undoubtedly complex, it would replace the philosophical vagaries of quantum mechanics with a concrete dynamical theory."

Other experts say they find themselves in agreement.

The work in Paris and Bush's interpretation "provides the possibility of understanding previously incomprehensible quantum phenomena, involving 'wave-particle duality,' in purely classical terms," says Keith Moffatt, a professor of mathematical physics at Cambridge University in Britain.

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