The flaws that floored

The Origin of the Concept of Nuclear Forces
March 13, 1998

Laurie Brown and Helmut Rechenberg's book describes the history of fundamental theories of the nuclear force between 1930 and 1950 and illustrates beautifully how the path towards a proper understanding of the world around us is littered with many a blind alley and wrong turn. Their story begins at the turn of the century by examining the experimental evidence that hinted at the existence of forces other than electromagnetism and gravity, such as the puzzles of radioactivity and penetrating high-energy radiation later identified as originating from cosmic rays. It is fascinating to learn about the long-forgotten competing theories of the time developed from the newly emerging ideas of quantum mechanics and experimental evidence. This then makes it possible to appreciate why the flawed theories seemed very reasonable and did not lead to immediate acceptance of the correct theory.

One of the best examples given is the interpretation of nuclear structure following Chadwick's discovery of the neutron in 1932. At that time, alpha and beta particles produced in the radioactive decay of some elements had been identified as helium nuclei and electrons. It was therefore not unreasonable to assume that beta decay involved the ejection of an electron from the nucleus, and that the neutron was made from a proton and an electron rather than existing as an elementary particle in its own right. This then led to the electron being thought of as the carrier of the force binding the neutrons and protons together.

Despite the difficulties with this concept, including the violation of energy conservation and quantum mechanics, many preferred to believe this was evidence of the fundamental laws of physics breaking down at the nuclear level rather than an indication of a flaw in theory.

Brown and Rechenberg then concentrate on the next major breakthrough in thinking proposed by the Japanese physicist Hideki Yukawa in 1934. His revolutionary idea was to introduce a new massive particle to carry the nuclear force between the protons and neutrons. Yukawa realised that his new particle, called the meson, had to have a mass 200 times greater than the electron to explain the very short distance over which the nuclear force operates. He recognised that such a massive particle could be produced only from very high-energy particles accessible via cosmic rays.

Yukawa's initial confidence that his theory had been substantiated began to fade when the predicted properties of his meson failed to be reproduced by experiment. By 1941 there were serious doubts whether the cosmic ray mesotron and Yukawa's meson were indeed the same particle. But the situation remained murky, as small changes to the theoretical assumptions and experimental limitations meant that the discrepancy could not be unambiguously identified. The puzzle was not resolved until 1947 when the pion was discovered by physicists from Bristol, reminding us that experiment is as, if not more, important than theory in the progress in scientific research. The paradox was resolved - the meson and mesotron were the pion and muon respectively.

One of the most fascinating themes of the book is the impact of the second world war on nuclear physics research. During the war, many nuclear physicists joined the effort to develop radar and the atomic bomb. It is ironic that two weeks before the atomic bomb was dropped on Hiroshima, Yukawa reported his belief that no country had harnessed the power of the atom for military use. At the end of the war, most of the equipment used for nuclear research in Germany and Japan was destroyed. The American nuclear physicists, by contrast, were regarded as heroes and rewarded with power, glamour and funding that kept them at the forefront of nuclear research for two decades.

This book provides a fascinating insight into the scientific and sociological development of nuclear physics research. But the large sections quoted from scientific journals of the time means the book is aimed more at the professional nuclear or particle physicist rather than at the general reader.

Valerie Noyes is a researchfellow, Particle and Nuclear Physics Laboratory, University of Oxford.

The Origin of the Concept of Nuclear Forces

Author - Laurie M. Brown and Helmut Rechenberg
ISBN - 07503 0373 5
Publisher - Institute of Physics Publishing
Price - £49.50
Pages - 392

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