Remembering Rabi: A challenge and a ghost story

Congratulations to Physics Today August 2006, page 36 for publishing the fascinating lecture by I. I. Rabi, “Stories from the Early Days of Quantum Mechanics” (August 2006, page 36). I was particularly interested in Rabi’s statement, “During the first period of its existence, quantum mechanics didn’t predict anything that wasn’t also predicted before…. The results that came out of quantum mechanics had to a large degree been previously anticipated.” This goes directly against the view, frequently expressed in later years, that quantum mechanics was accepted because all of its predictions were confirmed. Presumably that means predictions in advance: deducing results that were not known before quantum mechanics was proposed in 1925–26. (Physicists and other scientists sometimes use the word “predict” to mean “deduce” a known fact, so the phrase “in advance” is needed to exclude that situation.)

But perhaps Rabi was right. After all, quantum mechanics was accepted by 1928, so there wasn’t much time to carry out and publish new experimental tests of its predictions. My impression from Rabi’s lecture and other sources is that quantum mechanics was quickly accepted by physicists primarily because it allowed one to derive in a very direct way, from a small number of postulates, all the correct results of the old quantum theory. Getting both the discrete (negative energy) and continuous (positive energy) states of the hydrogen atom from the same equation was especially impressive. The old theory could yield those results, but sometimes only by using an inconsistent collection of ad hoc assumptions—for example, the anomalous Zeeman effect. In addition, quantum mechanics gave correct results for phenomena—notably the spectrum and ionization potential of neutral helium—that had completely stumped practitioners of the old quantum theory.

Quantum mechanics did of course produce many “predictions in advance.” My question is, did their confirmation have any significant role in persuading physicists to accept the theory by 1928?

A possible candidate for a confirmed prediction is electron diffraction: The Davisson–Germer and G. P. Thomson experiments of 1927 are often cited as confirmations of Louis de Broglie’s wave theory. But there is some doubt about whether those experiments revealed a completely new phenomenon, theoretically predicted in advance. In any case, quantum mechanics went far beyond de Broglie’s theory in its range of applications and philosophical consequences. So I propose a challenge to readers of Physics Today: Find evidence that the confirmation of any prediction in advance, other than electron diffraction, led any physicist to accept quantum mechanics before 1928.

Rabi also said in his lecture that John Van Vleck was “very unfortunate” in publishing a book on the old quantum theory just before it became obsolete. I don’t think we should feel too sorry for Van Vleck. His expertise in the old quantum theory helped him become, very quickly, a first-rate practitioner and expositor of the new theory. His review, “The New Quantum Mechanics,” published in Chemical Reviews in 1928, is an excellent resource for anyone who wants to know what achievements helped persuade scientists to accept quantum mechanics at that time. And he did, after all, eventually win the 1977 Nobel Prize for his research.