Untangling entanglement: Explaining the quantum world in four easy steps

[Celerity]

Why does the quantum world behave in that strange, spooky way? Here’s our simple, four-step explanation (no magic needed)

https://aeon.co/essays/our-simple-magic-free-recipe-for-quantum-entanglement



Almost a century ago, physics produced a problem child, astonishingly successful yet profoundly puzzling. Now, just in time for its 100th birthday, we think we’ve found a simple diagnosis of its central eccentricity. This weird wunderkind was ‘quantum mechanics’ (QM), a new theory of how matter and light behave at the submicroscopic level. Through the 1920s, QM’s components were assembled by physicists such as Werner Heisenberg and Erwin Schrödinger. Alongside Albert Einstein’s relativity theory, it became one of the two great pillars of modern physics.

The pioneers of QM realised that the new world they had discovered was very strange indeed, compared with the classical (pre-quantum) physics they had all learned at school. These days, this strangeness is familiar to physicists, and increasingly useful for technologies such as quantum computing. The strangeness has a name – it’s called entanglement – but it is still poorly understood. Why does the quantum world behave this strange way? We think we’ve solved a central piece of this puzzle.

Entanglement was first clearly described, and named, in 1935, by the Austrian physicist Erwin Schrödinger. He pointed out that, after two quantum particles interacted, they could no longer be considered independent of each other, as classical physics would have allowed. As the contemporary US physicist Leonard Susskind puts it in the preface to Quantum Mechanics: The Theoretical Minimum (2014), ‘one can know everything about a system and nothing about its individual parts.’

Here’s a simple analogy. If we want to give a complete description of the present state of a two-handed poker game, for example, we just give a description of the two five-card hands. What could be more obvious? But in QM, for some reason, the obvious thing doesn’t work. Schrödinger said that, in general, the quantum description of the two particles is ‘entangled’, and the name stuck. As he puts it: ‘When two separated bodies that each are maximally known come to interact, and then separate again, then such an entanglement of knowledge often happens.’

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