Particles in many places at once, spooky influences and cats that are dead and alive at the same time – these are the phenomena that earned quantum theory its reputation for weirdness
25 August 2021
THE pleasure and pain of quantum theory began when an “or” became an “and”. Are the fundamental components of material reality – the things that make up light, matter, heat and so on – particles or waves? The answer came back from quantum theory loud and clear: both. At the same time.
Max Planck started the rot back in 1900, when he assumed, purely to make the maths work, that the electromagnetic radiation emitted by a perfectly absorbing “black body” comes in the form of discrete packets of energy, or quanta. In 1905, Albert Einstein took that idea and ran with it. In his Nobel-prizewinning work on the photoelectric effect, he assumed that quanta were real, and all electromagnetic waves, light included, also act like discrete particle-like entities called photons. Work in the 1920s then reversed the logic. Discrete, point-like particles such as electrons also come with a wavelength, and sometimes act like waves.
Physicist Richard Feynman called this “wave-particle duality” the “only mystery” of quantum physics – the one from which all the others flow. You can’t explain it in the sense of saying how it works, he wrote; you can only say how it appears to work.
How it appears to work is often illustrated by the classic double-slit experiment. You fire a stream of single photons (or electrons, or any object obeying quantum rules) at two narrow slits close together. Place a measuring device at either of the two slits and you will see blips of individual photons with distinct positions passing through. But place a screen behind the slits and, over time, you will see a pattern of light …