Non-Fiction Reviews
Quantum Drama
From the Bohr-Einstein Debate
to the Riddle of Entanglement(2024) Jim Baggott & John Heilbron, Oxford University Press, £25, hrdbk, x + 336pp, ISBN 978-0-192-84610-5
Richard Feyman famously remarked, in one of his lectures, “I think I can safely say that nobody understands quantum mechanics.” He was exaggerating, of course. Niels Bohr certainly thought he understood it – as expressing the fundamental limitation of classical concepts, such as position and momentum, energy and time. Einstein also thought he understood it but, contrary to Bohr, took it to be merely a stepping stone to some more fundamental account. In the debate between these two pillars of the physics establishment, Bohr was widely acknowledged as coming out the winner, with Einstein viewed as sadly having lost his grip on the fast-moving cutting edge of this new science. But then, thirty years later, a physicist at CERN by the name of John Bell showed how Einstein, together with his fellow critic Schrödinger, had actually put their fingers on something deeply peculiar about the theory. And that something – which Schrödinger labelled ‘entanglement’ – now lies at the core of a range of emerging, and potentially transformative, quantum technologies.
That, in brief, is the ‘drama’ presented here in four acts by Jim Baggott, an award-winning science populariser and John Heilbron, a historian of science who sadly died last year. The opener presents a summary of the history of quantum theory, leading up to Bohr’s crucial idea of ‘complementarity’: certain quantities, such as position and momentum, or time and energy, are to be understood as ‘complementary’ to one another in the sense that although both are necessary for a physical description of a system, the experimental set-up involved in measuring one excludes, in principle, that required to measure the other.
Einstein famously dismissed this as a ‘tranquilising philosophy’ and sought to demonstrate that there was something fundamentally wrong with the theory in the co-authored and now well-known ‘Einstein-Podolsky-Rosen’ (EPR) paper. What this was exactly, he later said had been ‘buried in erudition’ but it eventually emerged that it had to do with the way that quantum mechanics undermined the ‘separability’ of interacting systems – something that Einstein took to be an absolutely fundamental requirement to even do physics. This is covered in the second act of Baggott and Heilbron’s dramatisation, and here I felt it was a little unbalanced in not paying more attention to the development of Schrödinger’s related notion of ‘entanglement’, which comes to play such an important role later on.
In act three we are introduced to David Bohm, who, prompted by the EPR piece, stepped away from Bohr’s ‘Copenhagen orthodoxy’ and showed how the theory could in fact be understood in a completely different way – as describing particles moving along well-defined trajectories in space-time but guided by a kind of quantum ‘potential’ Most physicists were dismissive, if they paid any attention at all, mainly because this account failed to come up with any new testable consequences. Then, in the mid-1960s, John Bell presented his eponymous theorem which states that any such approach must be ‘non-local’, in the sense that Einstein and Schrödinger were grappling with, if it is to conform to quantum theory’s empirical successes.
Suitably simplified and re-stated, Bell’s Theorem opened up the possibility of experimental tests to decide between quantum theory and a broad class of local alternatives. The final act presents the extensive work involved in setting up these tests, all of which have come down on the side of quantum theory, together with the subsequent developments that have laid the basis for quantum computation, cryptography, teleportation and the like.
So, the book covers a lot of history and for the most part it is presented engagingly, with personalised vignettes sprinkled across the physics. Here and there, however, the mix of popularisation and serious historical exposition feels rather lumpy, as when the association of Einstein’s co-authors, Podolsky and Rosen, with communism is sketched on one page (p. 94) before plunging into the details of their paper on the next (p. 95; details that I have to say could be presented much more clearly, as they are in a number of undergraduate texts).
More fundamentally, when it comes to understanding the theory, the book feels a little lopsided. Bohr and Bohm are not the only options on the table and the ‘Many-Worlds’ interpretation, beloved of many science-fiction writers of course, is dismissed too quickly. Here I thought the authors might have drawn on recent work in the philosophy of physics, particularly that of David Wallace, for example, whose book The Emergent Multiverse (Oxford University Press, 2012), has significantly changed the fortunes of Everett’s approach (I should say that I have no skin in this particular game, although my PhD supervisor, the late Michael Redhead, is mentioned in passing). As this history itself illustrates, sometimes physicists, even Nobel prize winners, are not the best arbiters when it comes to ‘understanding’ this almost-centenarian theory.
Steven French
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