This is what makes the quantum world so strange and confusing

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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

Physics 25 August 2021

By Richard Webb

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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 …

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