Congratulations to PHYSICS TODAY 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.
I enjoyed reading
I. I. Rabi's "Stories from the Early Days of Quantum Mechanics." Graduate students particularly
need to read stories like these to help them over the bumps they invariably encounter in their careers.
With respect to the "Pauli effect," I have a more recent story suggesting that the effect persisted
even after his death! At a 1961 meeting of the American Physical Society at the New Yorker Hotel in
New York City, Richard Feynman gave a talk to a packed conference hall on the quantization of the
gravitational field. He started out in typical Feynman fashion by saying, or actually shouting,
"Pretend Einstein never existed!" At this point, those of us in the hall heard a noise coming from
the ceiling: A loudspeaker had come loose, dangled for a moment from attached wires, and then finally
plunged to the floor. No one was injured, and amidst laughter after the shock wore off, Feynman continued.
Victor Weisskopf, who had also been a student of Pauli's, was heard to remark, "That was Pauli's
poltergeist." Feynman later elaborated on his talk to a huge audience at Columbia University,
but as far as I know, no further Pauli poltergeist activity was reported.
Featured Jobs
