The Nobel Prize in Physics 1955
Willis E. Lamb, Polykarp Kusch
Presentation Speech by Professor I. Wailer, member of the Nobel Committee for Physics
Your Majesties, Your Royal Highnesses,
Ladies and Gentlemen.
The Nobel Prize winners in Physics of this year were both
employed shortly before the war at the Physics Laboratory of
Columbia
University in New York. Lamb was at first engaged in
theoretical research and published several important
investigations in this field. Kusch was soon one of the most
active collaborators of Professor Rabi, when the latter worked
out his resonance method, for which he was awarded the Nobel Prize in Physics of 1944. In this
method the spectra of the atoms are studied by radio waves and
the details of the spectra can thereby be investigated much more
accurately than before. Kusch and Lamb participated during the
war in the extensive work on radar technique which was then being
performed. Because of the great progress in this field the
resonance method could be much improved. It was used by Kusch
and, in an essentially modified form, by Lamb when in 1947 they
made their great discoveries as leaders of two separate research
groups. Not only were the discoveries made independently of each
other in the same laboratory and in the same year, but it was
also soon found that the explanation of both phenomena is the
same, namely the interaction of the electrons and the
electromagnetic radiation.
Lamb's discovery refers to the hydrogen atom, where one single
electron moves around the nucleus in one of a series of orbits,
each having a definite energy. These energy levels exhibit a fine
structure which means that they are arranged in groups of
neighbouring levels, the groups being widely separated. An
explanation of the fine structure which for a long time was
thought to be correct, was given in 1928 by the English physicist
Dirac, when he
proposed a theory of the electron based on the requirements of
the theory of relativity as well as the quantum theory.
Using optical methods many attempts were made during the next
decade to check the Dirac theory of the fine structure but no
definite results were obtained. Some investigations made it
probable, however, that the theory was not entirely correct and
the guess was made that there occur deviations which are similar
to those later found by Lamb.
Lamb was aware of the great importance that a careful check of
the Dirac theory would have. He began his experimental
investigations of the fine structure shortly after the war. His
technique was based on Rabi's resonance method which had to be
much modified, however. Lamb planned his difficult experiment
guided by a thorough theoretical analysis of the experimental
arrangements.
In 1947 his experiments were successful. He found that two fine
structure levels in the next lowest group which should coincide
according to the Dirac theory are in reality shifted relative to
each other by a certain amount which is now called the Lamb
shift. He succeeded in measuring this shift with great accuracy
and later made similar measurements on heavy hydrogen.
The discovery of Kusch refers directly to an important property
of the electron, namely its magnetic moment. It had been known
since long that the electron is a small magnet. The strength of
this magnet is measured by its moment. The magnitude of the
moment should be uniquely determined by the electron theory of
Dirac, mentioned before.
At the beginning of 1947 Rabi together with several collaborators
found that a property of the lowest hydrogen level (namely its
so-called hyperfine structure) does not entirely conform with
theory. It was suggested by the American physicist Breit that the
reason for this could be that the magnetic moment of the electron
is somewhat different from the value assumed until then which is
called a Bohr
magneton.
Starting from this idea Kusch made a series of very careful
investigations and found in 1947 that the magnetic moment of the
electron is larger than the Bohr magneton by about one part in a
thousand.
The effects discovered by Lamb and Kusch are exceedingly small.
They were revealed only with the help of a very refined
technique. As has happened before it was now found that the
discovery of minute deviations from existing theories can be
offer-reaching importance. The discoveries of Lamb and Kusch
resulted in a reshaping of the theory of the interaction of the
electrons and the electromagnetic radiation, the so-called
quantum electrodynamucs.
Lamb reported on his results at a physics meeting which was held
in the neighbourhood of New York in the early summer of 1947.
Many prominent theoretical physicists were present and among them
Professor Kramers from Holland, who died a few years ago. During
the discussion it was made probable that the Lamb shift could be
explained using certain general ideas of Kramers, the purpose of
which was to improve the theories just mentioned.
A rough estimate which agreed rather well with Lamb's
measurements was soon made and somewhat later Lamb himself and
many others carried out more accurate calculations. It was also
found by Professor Schwinger at
Harvard
University that the anomaly in the magnetic moment of the
electron found by Kusch could be similarly explained. In both
cases the measurements as well as the calculations have since
been considerably improved and agree now very well.
Professor Willis Lamb. Professor Polykarp
Kusch. Your discoveries which the Royal Swedish Academy of
Sciences wishes to recognize on this occasion have been made by
applying radiohrequency spectroscopy of the highest achievable
precision to the study of the properties of the electron. Your
work is marked not only by the beauty of your experiments but
equally by the profound significance of your results. It does not
often happen that experimental discoveries exert an influence on
physics as strong and invigorating as did your work. Your
discoveries led to a re-evaluation and a re-shaping of the theory
of the interaction of electrons and electromagnetic radiation,
thus initiating a development of utmost importance to many of the
basic concepts of physics, a development the end of which is not
yet in sight.
I now ask you to receive your Nobel Prize from the hands of His
Majesty the King.
From Nobel Lectures, Physics 1942-1962, Elsevier Publishing Company, Amsterdam, 1964
Copyright © The Nobel Foundation 1955
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