hereinafter QM

When an observer interacts with this wave function by taking a measurement, the wave function suddenly manifests as a particle with a position and speed to which numbers can be assigned. It ceases to be a quantum-mechanical phenomenon and becomes a “classical” one. This seems to give the observer’s consciousness a privileged position in our description of the world.
The fact that observer A takes measurement M is nothing to do with interaction between A and the universal wave function. They are already, and for ever, interacting — “entangled.” All that happened was that the universe split in two at the instant of observation (as it is always doing, observed or not) and the version of A that we know about was carried off willy-nilly on the branch associated with co-ordinate M. A copy of him, A2, was carried off on a different branch, with a different measurement. So were innumerable other copies. The universal wave function governs them all. Actually, it is them.

• This was the germ of what later became known as the Many Worlds Interpretation of QM.

There is also the small matter of evidence for the existence of these other universes. Are they in fact physically real? Everett, an atheist and a materialist, was as little inclined to metaphysics as his nemesis Bohr, though for different reasons