100 Years of Planck’s Quantum Ian Duck and E. C. G. Sudarshan World Scientific, River Edge, N.J., 2000. $86.00 (545 pp.) ISBN 981-02-4309-X
Ian Duck and E. C. G. Sudarshan’s 100 Years of Planck’s Quantum is an unusual hybrid of a reprint volume and a running commentary on the papers being reprinted. The authors, who are “interpreters” as much as editors, believe that one should have as a basis the actual words of the scientists as they went through their creative processes. At the same time, they are aware that history has selected the important from the unimportant, changing the emphasis from what the creators might have thought was important to what seems important in light of subsequent events. In some cases of course, the originators had it exactly right!
Duck and Sudarshan believe that their selection requires a continuing commentary to place the papers within a modern historical context. Their aim is clearly not to provide the reader with a historical document as such. Rather, they want to provide the flavor of the original papers, most of which are excerpted rather than reprinted in full, and to help the reader interpret the papers within a context that was not apparent at the time the papers were written.
As a consequence, one gets a rather uncommon blending of original works colored strongly by a layer of interpretation, sometimes quite insightful and sometimes historical. It is rather like one of those music appreciation courses in which the pianist plays a passage of a famous piece and then dissects it; one ends up with less than a full picture of the whole, but generally with a much better idea of how the piece was put together and how its various parts interrelate.
Of course, within the framework of 100 years of the quantum, there is a lot of selecting to do. For this purpose, the authors make a conscious decision to limit themselves to nonrelativistic and non-field-theoretic aspects of the subject. They divide the book into four parts. The first deals with work that precedes modern quantum theory. Here they emphasize the work of Max Planck, Albert Einstein, Arthur Compton, and Niels Bohr. But they also include a prescient paper by Hendrik Kramers and Werner Heisenberg, and an explanatory paper on action variables by Karl Schwarzschild.
Part two is devoted to the classical development of the formalism of quantum theory, including Heisenberg’s matrix formulation, the famous “dreimännerarbeit” of Max Born, Heisenberg, and Pascual Jordan, explaining Heisenberg’s breakthrough in terms of orthodox matrix techniques and commutation relations, and P A. M. Dirac’s independent reformulation of the theory, as well as Erwin Schrödinger’s wave interpretation. They also discuss some of the early interpretational battles Heisenberg and Bohr had with Schroedinger. Part three is devoted to some classic interpretational questions, including Born’s work on interpreting the magnitude of the wave function in terms of probability. It also includes Heisenberg’s introduction of the uncertainty principle and Bohr’s complementarity. Finally there is a discussion of the famous EPR (Einstein-Podolsky-Rosen) “paradox,” and Bohr’s answer. During the discussions, they manage to bring out the various personality conflicts involved.
The last part of the book includes modern developments such as John Bell’s demonstration that one could distinguish experimentally between the EPR and Bohr points of view, the derivations of Bell’s inequalities, and some of the now classic experiments such as Alain Aspect’s, which tested these results and thus lent their weight to the conventional quantum interpretation. The authors also introduce the Feynman path integral as an especially important modern development.
The rest of the chapter is the only really debatable part of the book, so far as the authors’ choice of material is concerned. It includes some modern attempts at alternative interpretations of the subject, and many readers would quibble at their selection. Included are papers by James Hartle, Hartle and Murray Gell-Mann, and Bryce DeWitt, on versions of decoherence and consistent-histories interpretations, because these offer a possibility of interpreting the concept of a wavefunction for the entire universe, necessary for a cosmological extension of quantum mechanics. This inclusion reflects the authors’ own interest in particle physics, and although they are rather admirably constrained in their comments concerning an ultimate “theory of everything,” they still seem to be somewhat infected by this particular bug, an occupational hazard for particle physicists. Other types of interpretations, such as Bohm’s trajectories or many world interpretations, don’t impress them as very relevant.
Finally, Duck and Sudarshan have very positive things to say about an informational interpretation of the wave function, and they discuss David Deutsch’s introduction of quantum mechanical computers. However, whenever they discuss the “conventional” interpretation, they incorporate a density matrix approach and do not seem to believe that the wavefunction makes sense for an individual system. That would be the case for a frequency interpretation of probability. But one of the great strengths of an informational approach is that it gives meaning to the wavefunction for an individual system, because a single experiment can provide information about such a system. In contrast, a frequency interpretation only gives probabilities of occurrence within ensembles of systems. So their comments don’t seem to me to take full advantage of the possibilities opened up by an informational approach.
At the end of the book, Duck and Sudarshan have some fun speculating about the future of quantum mechanics, although they are fully aware of the unreliability of such projections. They do have some interesting things to say about physicists themselves being partly responsible for the general lack of interest in the subject by nonphysicists.
When you have finished the book, you will have read parts of some papers that you probably would not have otherwise read, and you will have been given a guided tour through confusing territory by some wise and knowing guides. A different tour might have covered different landmarks, but you will have gotten your money’s worth. Can’t ask for more than that!
