Are 'Ghost Waves' Behind Quantum Strangeness?

The concept in question is called "wave-particle duality." Simply, beyond a certain very, very, very, very small threshold, objects cease to behave in deterministic ways. Instead, they become probabilities; you don't say a quantum object is "here," you say it's "likely here" or "maybe there" or "both." These probabilities exist as smooth waves describing a quantum existence that is always everywhere between "here" and "there." That's Schrödinger's cat, existing in its closed box not just alive or dead but in an infinity of states in between the two. This really the heart of all quantum oddness.

What this might mean for reality is unsettled, and new experiments in fluid dynamics, described in the Annual Review of Fluid Mechanics, would seem to point to one possible reality that isn't quite so odd after all: ghost waves.

First, understand that the wave-ness of quantum-scale objects (particles) is physically real, verified by classic experiments that produce effects making it appear as though a single, indivisible particle can travel through two openings at once. But the particle-ness is real too; if one were to take a precise measurement of a particle's location, they would find this system of probabilities collapses into just one reality. The particle is no longer everywhere in between but just precisely here and only here. If we want an exact position for the particle, we have to throw out all the other information.

That's just life, it seems. But the duality is something vexing for physicists because, how can this seemingly indivisible thing go back and forth between everywhere and just here? The popular interpretation, known as the "Copenhagen interpretation" holds that the wave-ness of a particle is sort of just real in that it's a statistical observation, a limitation of our own perspective that basically makes statistical reality into lived reality.

That's not quite satisfying, however, and a number of physicists have proposed otherwise, including one theory in particular that supposes that both the wave and particle are "real" and distinct entities; quantum physics pioneer Louis de Broglie was one of the leading thinkers behind the theory. In a way, particles surf along some hidden wave structure and when we make our seemingly conflicting observations about waves and particles, we're really observing two different things. Problem solved?

Well, no. A separate wave has never been detected and every indication is that waves and particles are very much the same thing, with the prior just being a limitation of observation. It's also unclear as to how such a set-up would even work. This is where fluid dynamics comes in. John Bush, the MIT professor behind the current study, has conducted experiments using fluids that demonstrate the possibility of so-called pilot-waves, the separate (yet the same) physical entities that carry particles along.

The experiment goes like this. Some fluid bath is energized via slight vibrations just to the point right before it would begin to generate waves. It looks smooth, but the fluid is packed with energetic potential. Next, the experimenter lays a small droplet of the fluid on this energized liquid. The result are waves, as just the little bit of added energy pushes the energized fluid over the edge. The droplet then rides along on top of these waves, which are, in a quite real sense, its own waves. This is then a model (the first experimental model) of what one might expect in a quantum pilot-wave scenario.

"This system is undoubtedly quantitatively different from quantum mechanics," Bush said in a statement. "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 radiating waves and surfing particle of Bush's experiment behave in a way that has a bit to do with chaos theory, or how a very small disruption (the droplet falling on the energized fluid bath) can cause big, deeply-felt disturbances. Because a thing appears random, like a waveform collapsing into a particle (or a cat living or dying), doesn't mean that it's not fundamentally deterministic.

"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," Bush said. "While undoubtedly complex, it would replace the philosophical vagaries of quantum mechanics with a concrete dynamical theory."