From a scientific point of view, the theories of relativity and quantum mechanics are often considered the 20th century’s most renowned and profound discoveries. But the past 100 years have also seen many other significant advances in science: from the discovery of penicillin to the structure of DNA, from continental drift to the Big Bang, and even that of information theory, which set the basis for today’s hi-tech society. However, there is an often forgotten but nevertheless crucial discovery in physics that, in my opinion, surpasses all the others. By that I mean the pioneering work done by physicist John Bell on “local hidden variables” of quantum mechanics, which ultimately led to his ideas of “non-locality” or Bell’s inequalities.

In John Stewart Bell and Twentieth-Century Physics: Vision and Integrity, fellow physicist Andrew Whitaker tells the story of Bell’s life and his revolutionary discovery that not everything in physics can be explained using only local variables. Back in 1935 Albert Einstein, Boris Podolsky and Nathan Rosen realized that two quantum particles can be in a state such that a measurement on one particle instantaneously affects the other – no matter how far apart they may be. This effect, more commonly referred to today as entanglement, upset the trio because such “spooky action at a distance” would require information to travel faster than the speed of light. We now know than entanglement emerges thanks to correlations between measurements made on the two particles, and that entangled particles have much stronger correlations than are allowed in classical physics. But it was Bell’s breakthrough in 1964 that laid the groundwork for this phenomenon, when the Northern Irish physicist calculated an upper limit on how strong these correlations could be, if they were caused by local physics alone. Bell reasoned that correlations stronger than this limit would occur only if the particles were entangled and this is Bell’s inequality.

Whitaker, a physics professor at Queen’s University, Belfast, tells the story of Bell’s main discovery, but the book also goes beyond that. Bell was no one-discovery-wonder and, peculiarly, pursued quantum mechanics as a “hobby” in his spare time. Indeed, he was a very successful high-energy theoretical physicist, spending most of his career at the CERN particle-physics laboratory in Geneva. The book sets the stage with Bell as a student at Queen’s, and then follows his dual career – from the early 1950s to his “decade of great success” in the 1960s – including the publication of his seminal paper in 1964, which he wrote while in the US on sabbatical from CERN.

Through the book, one reads a lot about Bell’s character and the many people with whom he interacted including Alain Aspect, Abner Shimony, Reinhold Bertlmann and even myself. Interestingly, despite the fact that Bell seemed to discuss his ideas with a number of fellow scientists, he had very few joint publications on his work on quantum foundations. It is also remarkable how few papers Bell published in refereed journals. It seems he didn’t quite like the referee reports he must have received about his fundamental work, which was initially ignored and did not truly gain favour until the 1970s.

Bell died unexpectedly at the relatively young age of 62, from a cerebral haemorrhage, which Whitaker describes as the “final tragedy”. The book continues with the far-reaching implications of Bell’s discovery, including brief descriptions of many of today’s active researchers. Bell’s inequalities are now experimentally testable and his concept of non-locality is gaining momentum. Violating a so-called Bell inequality shows that an experiment is truly quantum in nature and there are no “local hidden variables” at play.

Today, Bell’s non-locality is also being exploited for futuristic applications in a new field that would never exist without Bell’s seminal discovery – namely, “device-independent quantum information processing”. The idea is that a quantum protocol would be completely independent of the internal workings of the devices being used, which would therefore eliminate the risk of a quantum cryptographic system being hacked. That is because the protocol looks merely at the statistics of any measurement made, without the need to understand in any detail how the data were collected; it suffices to know that they were produced at separate locations that couldn’t communicate. The National Institute for Standards and Technology in the US has already tapped into this idea and has created a free, public random number generator that you can access online. Large sets of truly random numbers are difficult to produce, but they are used in a variety of applications today, including in unpredictable sampling and secure authentication methods.

I truly enjoyed reading this very informative book. Moreover, it is nicely illustrated with many pictures of John, his wife Mary and others such as Michael Horne, Daniel Greenberger and Artur Ekert. This is not a book to learn about physics, but to get to know a bit about the man who made one of the most profound, if not the most profound, discoveries of the 20th century.

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