Quantum Physics‘ Wave-Particle Duality Enigma Bleeds Into the Classical World

A group of Australian physicists has demonstrated for the first time that even atoms, relatively massive particles hundreds of million times larger than electrons (allowing for electrons to take up space at all, anyhow), exhibit the strangest properties of wave-particle duality—that is, when a particle acts as both a deterministic point and a probabilistic wave, depending on how and when we observe it. More specifically, the researchers performed a version of John Wheeler’s canonical „delayed choice“ thought experiment, which attempts to pin down when exactly a quantum particle „decides“ whether it’s going to act like a wave or point.

By Michael Byrne|MOTHERBOARD

The Australian group, led by optical physicist Andrew Truscott, describe their latest experiments in the current issue of Nature Physics.

So, the classic demonstration of quantum oddness goes like this. Particles are fired from an emitter at some barrier with two slits cut into it, side by side. Behind the slits is a surface that records where the particles arrive. In this basic configuration, the particles behave as waves rather than point-like, deterministic objects.

We know this because the final barrier, where the particles are detected, reveals an interference pattern characteristic of two waves meeting. As waves, individual particles don’t choose one slit or the other, they pass through both, as we’d expect. On the other side, the waves that passed through each slit recombine. The effect is in essence of a particle interfering with itself, which is really strange, and leads to the popular interpretation that a single particle is somehow in many places at once.

As Richard Feynman famously quipped, wave-particle duality is the „mystery that cannot go away.“

To really demonstrate wave-particle duality, the double-slit experiment is modified such that there’s a measurement apparatus placed on the emitter side of the slits. The particle is then measured before it goes through the slits, in which case the interference pattern vanishes and the particle just acts like a particle, registering in one place at one time, as expected. The conclusion is that measurement—observation, generally—forces the probabilistic particle-wave to „choose“ where it actually is, confining it to a discrete point. The nature of this choice is a deep question.

For example, when does a particle choose where it will be at a given time? Does it always know, or have some complete knowledge of the experiment beforehand? Or does it only know at the moment of measurement? How important is the observer, really? In Wheeler’s thought experiment, and the current IRL experiments, the decision of whether or not to measure the particle is made only after the particle is emitted.

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