IMAGINE that someone asks you how to distinguish consciousness from unconsciousness – a difficult task indeed. If consciousness has a physical basis, can the same be said about unconsciousness?

You might have expected psychologists to have tackled this question, but perhaps not physicists. After all, physics is concerned with the study of matter and radiation. But you would be wrong. Wolfgang Pauli, winner of the 1945 Nobel prize in physics for his work on quantum mechanics, did just that. He proposed that the interaction between consciousness and unconsciousness is analogous to one of the central ideas in quantum physics, called complementarity – that it is impossible to distinguish between the behaviour of an atomic object and its interaction with the instrument observing it.

Pauli's analogy deals with two totally different scales of enquiry: the quantum scale, which is the realm of atoms, electrons and photons, and the macroscopic ...