Download Otto Stern and the discovery of space quantization

Z. Phys. D - Atoms, Molecules and Clusters 10, 119-120 (1988)
Otto Stern and the discovery of space quantization
I.I. Rabi as told to John S. Rigden
As a beginning graduate student back in 1923,
I was greatly influenced but not convinced by the
quantum theory. I suppose that is not surprising. If
one tries to think logically about atomic phenomena on the basis of an undergraduate education
in classical physics, the quantum theory seems
like Reaganomics: It doesn’t make too much
sense. Neither did the phenomena for that matter.
But I hoped that with ingenuity and inventiveness
I could and ways to fit the atomic phenomena into
some kind of mechanical systems. After all, J.C.
Maxwell had done it with the mechanical ether: It
was intricate, with all sorts of machinery, gears
and pulleys. Of course, further use of Maxwell’s
equations so habituated people to the ether that it
became natural for them to consider the ether as a
mechanical object.
My hope to explain atomic phenomena
mechanically died when I read about the SternGerlach experiment. In this experiment a stream
of silver atoms was sent trough an inhomogeneous
magnetic field. The silver atom is supposed to
possess angular momentum, that is, a magnetic
moment. Now there is nothing in physics to suggest that these magnetic moments and angular
moments would line up in a magnetic field in any
coherent fashion. Because the angular moments
could point in any direction, there would be a big
concentration equatorially around the zero projection.
The results of that experiment were astounding, although they were hinted at by quantum theory. The separation of the beam of silver atoms
into two components occurred as if these moments pointed either one way or the opposite way.
There was no mechanism that would orient them
in one way or another since on leaving the source
they were arranged quite statistically. There was
no way by which those in the negative direction
could gain or lose energy. In fact, the whole thing
was a mystery. Here was something that could not
be explained by any mechanism that could think
of.
This convinced me once and for all that an ingenious classical mechanism was out and that we
had to face the fact that the quantum phenomena
required a completely new orientation. This new
orientation did come with quantum mechanics. Of
course, this resolved one mystery, but it substituted another: that is, the basic mystery of quantum mechanics.
I first met Stern in the fall of 1927. I had been
in Copenhagen at the Niels Bohr Institute of
Theoretical Physics and Bohr made arrangements
for Yoshio Nishina and me to go and work with
Wolfgang Pauli at the University of Hamburg.
When I got there, I was pleased to find that Stern
and his associates were engaged in very exciting
molecular beam experiments. While my prime
interest was with Pauli in theory, I spent time in
Stern’s laboratory talking with Ronald Fraser, a
Scotsman, and John Taylor, an American. I came
to understand the subtleties of the molecular beam
experiments and recognized that the components
of an atomic beam could be separated with a homogeneous magnetic field. I explained the idea to
Stern and he suggested I do the experiment. I was
told what an honor it was to be invited by Stern to
do an experiment in his laboratory. I had no job
and I had a wife to support so I was in no position
to refuse an honor. My experiment was a success
and when it came time to write up the results, I
sow a demonstration of Stern’s generosity, his
fairness, and his pride. “First, publish a letter in
Nature”, said Stern. “If you publish it first in German, they’ll think it’s my thing, and it’s yours.”
My paper, in Zeitschrift für Physik, became the
twelth paper in Stern’s U.z.M. (“Untersuchungen
zur Molekularstrahlmethode”) series.