The Age of Entanglement: When Quantum Physics Was Reborn

  • Louisa Gilder
Knopf: 2008. 464 pp. $27.50 1400044170 9781400044177 | ISBN: 1-400-04417-0

Quantum: Einstein, Bohr, and the Great Debate about the Nature of Reality

  • Manjit Kumar
Icon Books: 2008. 480 pp. £20 9781848310292 | ISBN: 978-1-8483-1029-2

The nearly simultaneous publication of two popular histories of quantum mechanics, each highly readable and basically reliable, speaks well about the growing public interest in modern physics and about the commitment of major publishers to respond.

Louisa Gilder and Manjit Kumar take different approaches and emphasize different stages in the fascinating history of quantum physics. Kumar writes a conventional narrative history, focusing on the long-running debate between Niels Bohr and Albert Einstein, which took place from the mid-1920s through to the mid-1950s, over the adequacy of the quantum theory as a framework for fundamental physics. Gilder writes a delightfully unconventional history in the form of conversations — real or reconstructed — among the physicists themselves. She emphasizes the recent history of Bell's theorem, which concerns correlations between the quantum properties of separated elementary particles, its experimental tests and the subsequent exploitation of quantum entanglement in quantum computing, quantum information theory and quantum teleportation.

Critical of quantum theory, Einstein (right) forced Bohr (left) and others to refine their thinking. Credit: SCIENCE MUSEUM/SCIENCE & SOCIETY PICTURE LIBRARY

Gilder's is, on balance, the better book, partly because of the conversational format, which brings the scientist actors to life as complex personalities with interesting lives. Especially enjoyable are the portraits of the less famous physicists who, starting in the 1960s, put entanglement to the test and taught us how to engineer with it, starting with John Bell and including Abner Shimony, John Clauser, Alain Aspect and Anton Zeilinger.

Gilder has done her homework. Kumar relies uncritically on an old generation of specialist histories — such as Max Jammer's classic The Conceptual Development of Quantum Mechanics (McGraw-Hill; 1966), or the multivolume history of quantum physics (Springer; 1982–1987) by Jagdish Mehra and Helmut Rechenberg — but he does not recognize where those histories have been questioned or eclipsed by more recent work. Gilder, by contrast, has read the newer specialist literature. She has also done her own research, such as tracking down an old physics colleague of Boris Podolsky, and eliciting from him the striking memory that Podolsky said he and Nathan Rosen never asked Einstein for permission before putting his name on the classic 1935 Einstein, Podolsky and Rosen paper, 'Can Quantum Mechanical Description of Physical Reality Be Considered Complete?'.

One must, however, be cautious about reconstructing conversations. Even the participants' own memories are not always to be trusted, the conversations recalled in Werner Heisenberg's Physics and Beyond (Harper and Row; 1971) being a notorious case in point. And when an author gathers quotations from letters, memoirs or published papers covering a modest span of years and reassembles them as one exchange, there is as much fiction as fact in the product. Gilder's imagined contents of the lively chat between Bohr and Einstein that, time and again, kept them missing their tram stop during Einstein's Copenhagen visit in 1923 is wonderful as stagecraft. Yet I ask myself how soon I'll see a student presenting it in a term paper as if it were a verbatim transcript.

Two larger reservations must be recorded about both books. The first concerns their treatment of Bohr and what is commonly termed the Copenhagen interpretation of quantum mechanics. Both Kumar and Gilder take as fact much of the folklore and mythology surrounding the Copenhagen interpretation. Legend has it that, from 1927 onwards, the entire Copenhagen community spoke with one voice about questions of interpretation. In fact, from the beginning, Bohr and Heisenberg disagreed about many fundamental points. Bohr never endorsed such famous bits of alleged Copenhagen orthodoxy as wave-packet collapse, a privileged role for the subjective observer, or a distinction between quantum and classical levels of description based solely on a system's scale. Heisenberg, who invented the term Copenhagen interpretation much later in 1955, asserted all of these claims. Bohr did not. Differences between Heisenberg and Bohr go almost unnoticed by both Kumar and Gilder, with unfortunate consequences for such crucial issues as understanding what really was at stake in the Bohr–Einstein debate and the consequences for Bohr of experimental tests of Bell's theorem. These tests provide strong evidence for quantum entanglement — the entwined states of interacting quantum systems — that was the basis of Bohr's own interpretation of quantum mechanics.

The second reservation concerns the consequences of experimental tests of Bell's theorem for another major interpretive project — David Bohm's construction in 1952 of a 'non-local hidden variables theory'. The theory posited unknown and perhaps inaccessible degrees of freedom of quantum systems, the values of which fix the behaviour of such systems. But quantum indeterminism is overcome at the price of allowing 'non-local' or 'faster-than-light' communication among those hidden variables, which might conflict with Einstein's theory of relativity. Gilder devotes long discussions to the career and ideas of Bohm. All the more puzzling, then, that when she tells us again and again about experiments confirming violations of the Bell inequality, she leaves the reader with the impression that this is a hands-down victory for quantum mechanics over hidden variable theories, when it is only local hidden variable theories that are put in jeopardy. One might reject Bohmian mechanics for other reasons, but these tests alone do not afford them. Kumar's hasty resumé of the same results leaves one still more confused. He suggests that Einstein's reservations about the completeness of quantum mechanics are somehow vindicated by experiments confirming the predictions of standard quantum mechanics.

Both authors would have been helped by a closer look at the well-known re-analysis of Bell's argument, introduced in 1984 by Jon Jarrett and refined by Shimony. Jarrett showed that the original Bell locality condition is a conjunction of two logically independent conditions that Shimony called outcome independence and parameter independence. The former is akin to a denial of quantum entanglement, the latter to relativistic locality constraints. Experimental violations of the Bell inequality can now be traced to violations of one or the other of these two conditions. That there are two independent routes to violations of the Bell inequality makes it clear how orthodox quantum mechanics, which presumes entanglement, and a Bohm-type hidden variables theory, which assumes relativistic non-locality at the microlevel, can both claim to have been vindicated by the Bell experiments. But these gentle criticisms should not deter the interested reader from enjoying two welcome additions to the popular history of twentieth-century physics.