Einstein's Unfinished Revolution - Lee Smolin ****

You wait years for a book on the interpretations of quantum theory and then they appear in droves. In reality though, it's a good thing - each of them has brought its own slant and approach, and never more so than in Einstein's Unfinished Revolution from theoretical physicist Lee Smolin.

Although Smolin takes us on a tour of the best-known interpretations, he sets out his intention from the beginning - he is a realist and wants an interpretation that incorporates realism. That might not seem much of an ask - surely everyone in science wants realism? - until you realise that the long-prevailing approach, the Copenhagen interpretation is anti-realist. This is because we're dealing with a particularly theoretical physics view of the world. It's not that those who have held the Copenhagen interpretation don't think the world as a whole is real. Rather that the requirement that, say, a quantum particle has no location, just probabilities, until it interacts with something means that quantum theory is not a description of something 'real' in the usual sense.

The lack of realism was one of the reasons that Einstein became so uncomfortable with the way quantum theory was developing, making his famous remarks about playing dice, and Smolin plays this up as a defender of Einstein's position, particularly in the preface to the book. I found the way this was done a trifle disingenuous, as Einstein was also unhappy with non-locality, the idea that, say one quantum particle can influence another instantly at any distance - and it's impossible to have realism in quantum theory without non-locality. But this is only a passing irritation.

Smolin comes down firmly on the side of realism, more it seems for intuitive reasons than anything else, and accordingly looks at the possible approaches that embrace this. He gives an excellent overview of the deBroglie-Bohm pilot wave interpretation, which he clearly likes - in it, particles are real, always having a specific location - but is honest about issues with it, notably his dislike of the way that particles are steered by the pilot waves, but the particles can't influence the waves. He then takes us through the Many Worlds approach, which he impressively shreds (rather beautifully referring to it as 'magical realism'), before introducing one or two more esoteric possibilities.

Up to this point, Smolin has done what others have done, and does it very well, managing all this without any mathematics and with clear, approachable language. But then he faces the problem square on. No approach is totally satisfactory. And, for that matter, no one has managed to bring general relativity and quantum theory together despite decades of trying. He suggests, then, that we need to take a step back and start from scratch, giving us an overview of some, at the moment very toy, approaches to doing this.

This section is much harder to follow than the rest. When Smolin outlines his current ideas involving networks* of 'nads', what strikes me is both fascination and sadness. Fascination to see how such an edifice is constructed, but sadness because, for me, what he constructs seems far further from any idea of a comprehensible worldview than even the Copenhagen anti-realism. This is the only place where I got the feeling of a theoretical physicist detached from the rest of us, except when Smolin makes the assertion that the anti-science efforts of climate change deniers and some religious extremes reflects a lack of trust in science because of anti-realist quantum interpretations. This seems a very parochial worldview that, dare I say it, suggests theoretical physics has more influence in the world than it really has.

That isn't the end, though. Although very inward looking, it's well worth persevering with Smolin's short epilogue in which he explains the difficulties of making the leap away from the standard approaches at a fundamental level and starting over within the confines of the academic system, and ponders whether to make the leap - I found this genuinely moving.

Overall, while more effort was needed to make the nads comprehensible (and particularly seeming anything like reality), the book is an excellent contribution to thriving debate on what to do about the elderly enfant terrible of physics, quantum theory.

* Purists: I know they're not strictly networks, but it's alliterative. 

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Review by Brian Clegg