Einstein's Unfinished Revolution

Subtitled 'the search for what lies beyond the quantum', this is a brilliant exposition of the field and its development, exploring in particular the case for a realist rather than anti-realist interpretation of quantum physics and criticising the hegemony of the Copenhagen interpretation. Realism is the position that the natural world exists independently of our minds and that the properties of matter can be comprehended and described by us: 'they in no way depend on our knowledge or perception of it', as is the common (neo-Kantian) understanding of observation in quantum mechanics and the collapse of the wave function. Smolin's starting point is that current models only allow us to know half of what we need to know, as reflected in Heisenberg's uncertainty principle with respect to waves and particles. After explaining the development of quantum mechanics, he goes on to discuss what quantum mechanics does not explain and the triumph of anti-realism with Niels Bohr's notion of complementarity.

The three central figures of the second part are Louis de Broglie, John Bell and David Bohm. De Broglie extended the notion of the wave and particle to the whole of matter, which was then mathematically formulated in Schrödinger's wave equation. He presented his pilot wave theory to the Solvay conference in 1927; this theory 'predicts everything quantum mechanics does, but explains a good deal more' through the influence of the wave on the particle. However, his contribution was ignored by Copenhagen textbooks, and this line of thinking was dormant until taken up by David Bohm's hidden variable theory in the early 1950s, which Smolin sees as different versions of the same idea. Ironically, seemingly through the influence of a now discredited paper by John von Neumann, even de Broglie himself thought that Bohm's theory was wrong, like his own previous proposal. Einstein's reaction was also discouraging, and continued neglect of Bohm's contribution continued throughout his life – he was only elected a Fellow of the Royal Society in his 70s. Smolin is somewhat disparaging about Bohm's mystical and philosophical quest, although he does acknowledge his courage and vision in taking this step. Just this morning, by coincidence, I was reading the conclusion of an article by his collaborator Basil Hiley who argues that these 1952 papers referred to 'were not an attempt to find a deterministic, mechanical account of the quantum formalism', but rather a step towards a deeper understanding of a particle as 'some quasi stable semi-autonomous feature of the unfolding process.' One needs to understand his philosophy explained in Wholeness and the Implicate Order as a point of departure.

The third part is devoted to a survey of current developments in developing an approach based on common sense realism. Smolin reviews historical attempts in this respect and explores ways of going beyond pilot wave theory and collapse models. His approach begins with principles (though as Nicholas Maxwell argues, one should in fact begin with presuppositions), moving on to hypotheses that must satisfy the principles, models illustrating their partial implications, and only then complete theories. His five principles are background independence, the relationality of space and time, causal completeness, reciprocity and the identity of indiscernibles, all of which he regards as aspects of a single principle, namely Leibniz's principle of sufficient reason. This then leads on to three hypotheses, that time, in the sense of causation, is fundamental, that it is irreversible and that space is emergent. Smolin then elaborates a theory of relational hidden variables as a 'simultaneous completion of quantum mechanics and general relativity', describing some recent developments in this respect. General readers can just about keep up with the narrative even if some technical details will elude them. The elephant in the room is referred to only in the preface: 'let us tiptoe past the hard question of consciousness to simpler questions.' However, it is clear that consciousness plays a very different role in a realist account of quantum mechanics, even though not for David Bohm - there is plenty more thinking to be done, but this book is a highly readable and stimulating contribution.