“Let me add a final word of warning. In a formulation of quantum theory called quantum electrodynamics, developed in the 1950s and 60s by Richard Feynman together with Julian Schwinger and Sin-Itiro Tomonaga, the path that a quantum particle takes as it travels through space takes into account not just straight-line trajectories but every route possible. That’s to say, the equations of quantum electrodynamics contain terms that correspond to every path, however tortuous and crazy. However, when you add all these terms up, most of them cancel out – the wavefunction has essentially zero amplitude throughout most of space. So it’s sometimes said that quantum electrodynamics really does show that an electron or a photon goes through both slits in the double-slit experiment – because it takes every path ‘at once’. However, this picture is just a metaphor for the mathematics. You can think of the particle taking all possible paths if you like, but you can never show that they do. To interpret quantum electrodynamics this way is to attempt to tell a classical story about quantum mechanics. The electron or photon does not take all possible paths. To imagine that it does is not just mistaken; it is fundamentally the wrong way to think about quantum mechanics.”