15 October 1979
The Royal Swedish Academy of Sciences has decided to
award the 1979 Nobel Prize in physics to be shared equally
between Professor Sheldon L. Glashow, Harvard University,
USA, Professor Abdus Salam, International Centre for
Theoretical Physics, Italy and Imperial College, Great Britain, and
Professor Steven Weinberg, Harvard University,
USA, for their contributions to the theory of the unified weak
and electromagnetic interaction between elementary particles,
including inter alla the prediction of the weak neutral
current.
Physics, like other sciences, aspires to find common causes for
apparently unrelated natural or experimental observations. A
classical example is the force of gravitation introduced by
Newton to explain such disparate phenomena as the apple falling
to the ground and the moon moving around the earth.
Another example occurred in the 19th century when it was
realized, mainly through the work of Oersted in Denmark and
Faraday in England, that electricity and magnetism are closely
related, and are really different aspects of the electromagnetic
force or interaction between charges. The final synthesis was
presented in the
1860's by Maxwell in England. His work predicted the existence of
electromagnetic waves and interpreted light as an electromagnetic
wave phenomenon.
The discovery of the radioactivity of certain heavy elements
towards the end of last century, and the ensuing development of
the physics of the atomic nucleus, led to the introduction of two
new forces or interactions: the strong and the weak nuclear
forces. Unlike gravitation and electromagnetism these forces act
only at very short distances, of the order of nuclear diameters
or less. While the strong interaction keeps protons and neutrons
together in the nucleus, the weak interaction causes the
so-called radioactive beta-decay. The typical process is the
decay of the neutron: the neutron, with charge zero, is
transformed into a positively charged proton, with the emission
of a negatively charged electron and a neutral, massless
particle, the neutrino.
Although the weak interaction is much weaker than both the strong
and the electromagnetic interactions, it is of great importance
in many connections. The actual strength of the weak interaction
is also of significance. The energy of the sun, all-important for
life on earth, is produced when hydrogen fuses or burns into
helium in a chain of nuclear reactions occurring in the interior
of the sun. The first reaction in this chain, the transformation
of hydrogen into heavy hydrogen (deuterium), is caused by the
weak force. Without this force solar energy production would not
be possible. Again, had the weak force been much stronger, the
life span of the sun would have been too short for life to have
had time to evolve on any planet. The weak interaction finds
practical application in the radioactive elements used in
medicine and technology, which are in general beta-radioactive,
and in the beta-decay of a carbon isotope into nitrogen, which is
the basis for the carbon-14 method for dating of organic
archaeological remains.
Theories of weak interaction
A first theory or weak interaction was put forward already in
1934 by the Italian physicist Fermi. However, a satisfactory
description of the weak interaction between particles at low
energy could be given only after the discovery in 1956 that the
weak force differs from the other forces in not being reflection
symmetric; in other words, the weak force makes a distinction
between left and right. Although this theory was valid only for
low energies and thus had a restricted domain of validity, it
suggested a certain kinship between the week and the
electromagnetic interactions.
In a series of separate works in the 1960's this year's Nobel
Prize winners, Glashow, Salam and Weinberg developed a theory
which is applicable also at higher energies, and which at the
same time unifies the weak and electromagnetic interactions in a
common formalism. Glashow, Salam and Weinberg started from
earlier contributions by other scientists. Of special importance
was a generalization of the so-called gauge principle for the
description of the electromagnetic interaction. This
generalization was worked out around the middle of the 1950's by
Yang and Mills in USA. After the fundamental work in the 1960's
the theory has been further developed. An important contribution
was made in 1971 by the young Dutch physicist van't Hooft.
The theory predicts among other things the existence of a new
type of weak interaction, in which the reacting particles do not
change their charges. This behaviour is similar to what happens
in the electromagnetic interaction, and one says that the
interaction proceeds via a neutral current. One should contrast
this with the beta-decay of the neutron, where the charge is
altered when the neutron is changed into a proton.
First observation of the weak neutral current
The first observation of an effect of the new type of weak
interaction was made in 1973 at the European nuclear research
laboratory, CERN, in Geneva in an experiment where nuclei were
bombarded with a beam of neutrinos. Since then a series of
neutrino experiments at CERN and at the Fermi Laboratory near
Chicago have given results in good agreement with theory. Other
laboratories have also made successful tests of effects of the
weak neutral current interaction. Of special interest is a
result, published in the summer of 1978, of an experiment at the
electron accelerator at SLAC in Stanford, USA. In this experiment the
scattering of high energy electrons on deuterium nuclei was
studied and an effect due to a direct interplay between the
electronmagnetic and weak parts of the unified interaction could
be observed.
Interaction carried by particles
An important consequence of the theory is that the weak
interaction is carried by particles having some properties in
common - with the photon, which carries the electromagnetic
interaction between charged particles. These so-called weak
vector bosons differ from the massless photon primarily by having
a large mass; this corresponds to the short range of the weak
interaction. The theory predicts masses of the order of one
hundred proton masses, but today's particle accelerators are not
powerful enough to be able to produce these particles.
The contributions awarded this year's Nobel Prize in physics have
been of great importance for the intense development of particle
physics in this decade.