The Latest in Wave-Particle Duality

Light. It’s been the center of debate and controversy in the physics world for centuries. The main battle centers around the very nature of the stuff- is light composed of particles or is it a wave?

Isaac Newton and company were all about light being particles. But others, like James Clerk Maxwell (noted for his theory of electromagnetism), were sure it was composed of waves. Back and forth the battle raged, and just when it seemed like wave theory was pulling ahead, out from the metaphorical shadows stepped…


Albert Einstein.

The year was 1905, and Einstein was about to rock the light debate. Einstein was the one to prove that there were quantas of light (photons) in his work on the photoelectric effect (work that earned him his Nobel Prize).

So, while light had repeatedly been proven to act a wave (thank you, Maxwell), Einstein had also proven it acted like a particle. And this is the core of wave-particle duality. Particles (like photons) have the ability to act as both waves and particles (see the double slit experiment for more information on particles exhibiting their dual natures).

Like much of quantum mechanics, wave-particle duality seems to fly in the face of reason. And it doesn’t help that we are only able to observe a photon acting as a particle or as a wave- never both at once.

At least, we weren’t able to until recently.

A group of physicists from the University of Bristol have created a whole new measuring apparatus that can actually detect wave and particle behavior at the same goddamn time.


The device is powered by quantum nonlocality, a quantum effect observable in entangled particles. Quantum nonlocality is, at its core, the ability of a particle to determine (and take) the state of its entangled partner instantly, regardless of distance and with absolutely no regard for the speed of light. Dubbed by Einstein as “spooky action at a distance”, quantum nonlocality/entanglement is a bizarre (and lovely) bit of physics.

Unfortunately, I was unable to read the full paper, so the particulars of the experiment remain unknown to me. But I know the basic concept. The experiment revolves around a modification of a delayed choice experiment (which is itself a variation on the double slit experiment). In a delayed choice experiment, the delayed choice of the observer impacts the outcome, regardless of the fact that the choice occurs after the event being measured. In the Bristol experiment, their measurement device replaces that delayed observer choice with quantum nonlocality. By observing strong nonlocal correlations, the physicists proved the genuine quantum nature of the photons (proving they were behaving as both particle and wave simultaneously).

As Dr. Peruzzo, of the Centre for Quantum Photonics, said, “This represents a strong refutation of models in which the photon is either a wave or a particle.”

Fascinating stuff, no?

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