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Observing one gives us new knowledge about the other, even if it is a long way away.Įinstein and Schrödinger argued that this meant that something is hidden inside these quantum systems prior to measurement – something not fully described by QM, and disallowed by Bohr’s view. The same is true for entangled particles. If the queen of hearts turns up in the hand on the left, say, then we know that it is not one of the hidden cards in the hand on the right. Moreover, when an additional card on one side is revealed, it changes our knowledge about the other hand.

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At least superficially, this looks like entanglement: a full quantum system can’t be described in terms of what’s known about its pieces.

The state of this game can no longer be described in terms of the known cards (the ones turned face up). To grasp the Einstein-Schrödinger argument, consider the two poker hands, now with some of the cards face down, hidden from view. Like Einstein before him, Schrödinger thought that entanglement proved Bohr wrong. Bohr argued that it was nonsense to think of quantum systems as having definite properties, before they were measured. This orthodox view was the so-called Copenhagen Interpretation, proposed by the Danish physicist Niels Bohr. He thought that this had important consequences, especially because it debunked what had become the orthodox view of what QM is telling us about the microworld. For him, its strangeness was the prohibition it imposed on describing a two-particle system by its parts. The full weirdness of entanglement wasn’t immediately obvious, and Schrödinger himself didn’t quite live to see it. Susskind says it is ‘the essential fact of quantum mechanics’, while in his Lectures on Quantum Mechanics (2013), Steven Weinberg writes that it is ‘perhaps its weirdest feature’. Schrödinger concluded elsewhere that entanglement is not ‘ one but rather the characteristic trait of quantum mechanics.’ Many physicists now agree. The full weirdness of entanglement wasn’t immediately obvious 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.’ Schrödinger said that, in general, the quantum description of the two particles is ‘entangled’, and the name stuck. What could be more obvious? But in QM, for some reason, the obvious thing doesn’t work. 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. 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.’

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.

Why does the quantum world behave this strange way? We think we’ve solved a central piece of this puzzle.Įntanglement was first clearly described, and named, in 1935, by the Austrian physicist Erwin Schrödinger. The strangeness has a name – it’s called entanglement – but it is still poorly understood. These days, this strangeness is familiar to physicists, and increasingly useful for technologies such as quantum computing. 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. Alongside Albert Einstein’s relativity theory, it became one of the two great pillars of modern physics. Through the 1920s, QM’s components were assembled by physicists such as Werner Heisenberg and Erwin Schrödinger.

This weird wunderkind was ‘quantum mechanics’ (QM), a new theory of how matter and light behave at the submicroscopic level. Now, just in time for its 100th birthday, we think we’ve found a simple diagnosis of its central eccentricity. Almost a century ago, physics produced a problem child, astonishingly successful yet profoundly puzzling.

Four quantum numbers