Presentation Speech by Professor I. Waller, member of the Nobel Committee for Physics
Royal
Highnesses, Ladies and Gentlemen.
For many ages, an important aim
of science has been to explain the phenomena we observe by the properties of fundamental
particles. In modern physics this problem is of first importance. During the last
decade, fundamental particles called "mesons" have turned out to be particularly
interesting. The mesons are particles heavier than the electrons but lighter than
the nuclei of the hydrogen atoms, i.e. the protons.
The mesons were
entirely unknown until Hideki Yukawa in 1934 predicted their existence on the
basis of a theoretical investigation of the nuclear forces. This is the achievement
which has now been rewarded by the Nobel Prize in Physics.
From earlier
research by Heisenberg and others one knew that an atomic nucleus, i.e. the central
core of an atom, is composed of protons and of other particles which have the
same mass as the protons but no charge. These building elements of the atomic
nuclei are called "nucleons" and are held together by the so-called nuclear forces.
Attacking the problem of the nature of the nuclear forces, Yukawa used
the electromagnetic field as a model. He found that this field could be modified
so as to give forces which like the nuclear forces have a short range. He therefore
assumed that the new field corresponds to the nuclear forces. Each field of force
is, according to modern theories, associated with some kind of particles. Yukawa
discovered that there is a simple relation between the range of the forces and
the mass of the corresponding particles. He estimated the range from known experimental
data and found that the new particles should be about 200 times heavier than the
electrons. The name of mesons for these particles was not introduced until later.
According to Yukawa's theory, the nuclear forces can be traced back to an exchange
of mesons between the nucleons. These are continually emitting and absorbing mesons.
Yukawa also studied the important question of whether the mesons can appear
outside the nuclei. He found that the mesons can be created during the interaction
of nucleons if these can deliver a sufficient amount of energy. Therefore, mesons
cannot be created in ordinary nuclear reactions. Yukawa emphasized, however, that
they can be expected to appear in the cosmic radiation, in which particles of
great energy are found.
Yukawa assumed that mesons can have both
positive and negative charge and that the magnitude of the charge is the same
as that of the electron. A theory of Fermi, which had been proposed some years
earlier, led Yukawa to the assumption, that a meson can be transformed into an
electron and a light particle without charge called "neutrino". As was pointed
out later, free mesons could therefore be supposed to exist only for a very short
time, some millionth of a second or less.
As Yukawa had suggested,
the study of the cosmic radiation gave the first experimental evidence of the
existence of mesons. This evidence was given in 1937 by Anderson and Neddermeyer
and other American physicists. Since that time, the mesons in the cosmic radiation
have been very much studied. These investigations have been guided by the theory
of Yukawa. A new period in meson research began about three years ago. The British
physicist Powell and his collaborators then found that there exist two kinds of
mesons. The mesons of one kind are those found in 1937, whereas the mesons of
the other kind are somewhat heavier and different also in other respects. Mesons
can now be produced in the large cyclotron in Berkeley, California. This has greatly
increased the possibilities of studying them.
These experimental
investigations have shown, that the masses of both kinds of mesons agree with
Yukawa's prediction as far as the order of magnitude is concerned. The heavier
mesons, but not the lighter ones, have an interaction with the nucleons about
as strong as Yukawa had postulated. The fact that particles of this kind have
been found experimentally provides a brilliant vindication of Yukawa's fundamental
ideas. The electric charge of both kinds of mesons agree with Yukawa's prediction.
It has also been experimentally confirmed, that the mesons can exist only for
a very short time. A heavy meson lives only for about one hundredth of a millionth
of a second and is then transformed into a light meson and probably a neutrino.
The light meson disappears after a few millionths of a second, and electrons are
then created and probably also neutrinos.
After experimental evidence
of the existence of mesons had been given the interest in Yukawa's theory rose
quickly. Much effort was expanded in developing the theory and investigating its
consequences. In this work Yukawa and his Japanese collaborators took the lead.
Among other things, they found theoretically that neutral mesons exist besides
the charged ones.
It has not yet been possible to give a theory for
the nuclear forces, which yields results that are in good quantitative agreement
with the experiments. Yukawa's theory has, however, led to many important qualitative
results about the nuclei. The theory has also proved to be of great value in cosmic-ray
research. It was e.g. possible to understand, that mesons can be created in the
upper layers of the atmosphere by the primary cosmic radiation falling on the
earth.
The research on mesons will probably lead to new discoveries.
The meson theory may develop into other forms. By having predicted the existence
of the mesons and many of their essential properties Yukawa has accomplished pioneering
research of utmost importance. His ideas have proved to be an enormous stimulus
to the research in theoretical as well as experimental physics.
Professor
Hideki Yukawa. In 1934, when you were only 27 years old, you boldly predicted
the existence of new particles, now called "mesons", which you anticipated to
be of fundamental importance for the understanding of the forces acting in the
atomic nucleus. Recent experiments have provided brilliant support for your essential
ideas. These ideas have been exceedingly fruitful and are a guiding star in present-day
theoretical and experimental work on atomic nuclei and on cosmic rays. You have
also contributed much to other problems in basic theory and you have played a
great role in bringing your country to its very high position in modern physical
research.
On behalf of the Royal Swedish Academy of Sciences, I wish
to congratulate you on your ingenious work, and I now ask you to receive your
Nobel Prize from the hands of His Royal Highness the Crown Prince.
From Nobel Lectures, Physics 1942-1962, Elsevier Publishing Company, Amsterdam, 1964
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