What is quantum theory?

For much of the twentieth century, it was widely believed that the interpretation of quantum theory had been essentially settled by Bohr and Heisenberg in 1927. But not only were the ‘dissenters’ of 1927 – in particular de Broglie, Einstein and Schrödinger – unconvinced at the time: similar dissenting points of view are not uncommon even today. What Popper called ‘the schism in physics’ (Popper 1982) never really healed. Soon after 1927 it became standard to assert that matters of interpretation had been dealt with, but the sense of puzzlement and paradox surrounding quantum theory never disappeared.

As the century wore on, many of the concerns and alternative viewpoints expressed in 1927 slowly but surely revived. In 1952, Bohm revived and extended de Broglie's theory (Bohm 1952a,b), and in 1993 the de Broglie–Bohm theory finally received textbook treatment as an alternative formulation of quantum theory (Bohm and Hiley 1993; Holland 1993). In 1957, Everett (1957) revived Schrödinger's view that the wave function, and the wave function alone, is real (albeit in a very novel sense), and the resulting ‘Everett’ or ‘many-worlds’ interpretation (DeWitt and Graham 1973) gradually won widespread support, especially among physicists interested in quantum gravity and quantum cosmology.