Abstract
Physics has a long history, but more physics has been discovered in the twentieth century than in all previous eras together. That in itself would be a sufficient justification for a history of physics in the twentieth century, but the end of the previous century also marked a discontinuity, from Newtonian classical physics to relativity and quantum mechanics. If any single event marks the start of the process it is the discovery of x-rays in 1895, and Kragh's century spans from about 1895 to about 1995. It is, of course, too much for a single volume, even a large one, and Kragh recognizes from the outset that he has to be selective and concentrate on those subjects that define twentieth-century physics. For the early part of the century the author relies on carefully chosen secondary sources, to avoid the near-impossible task of absorbing a multitude of original papers. The recent period is more difficult, and the sources are articles, reviews, and the recollections of physicists.
The book is in three main sections, roughly to the end of World War I, to the end of World War II, and up to 1995, plus a retrospective summary. It deals with more than just discoveries in physics, looking also at physicists and institutions, and at their interactions with the rest of society. The broad outlines of many discoveries are often known to physicists who have no special interest in history, and Kragh is careful to point out where these conventional accounts are inadequate.
The first chapters set the scene at the end of the nineteenth century, acknowledging that there was a belief that all the grand underlying principles had been established, but also pointing out that there was a ferment of attempts to reinterpret physics in terms of concepts like vortices and hyperspaces. The history begins with the mould-breaking discoveries of x-rays, radioactivity and the electron. The chapters that follow look at theories about atomic structure, and at quantum physics, relativity and superconductivity, plus a glance at some practical applications of physics such as the beginnings of electronics. The interwar and World War II themes are quantum mechanics, the physics of the nucleus, the discovery of more fundamental particles and atomic fission, including the Manhattan project and nuclear weapons. Post World War II themes include nuclear physics and nuclear energy, particle physics, fundamental theories, solid-state physics, and some engineering physics - transistors, lasers and fibre optics. To show that physics is not a linear and uninterrupted process some unsuccessful ideas are included such as the concept of electromagnetic mass.
As fascinating as the physics is the development of the physics community. At the beginning of the century physics research was largely a European activity and Germany was the leading nation. Kragh, based at the University of Aarhus, in Denmark, is better placed than most historians writing in English to produce an even-handed account. He is also well placed to chronicle the role of the man who features so prominently in the middle period: Niels Bohr. It was not only Bohr's own work, but also the focus provided by his Copenhagen Institute for so many other contributions to atomic physics.
At the beginning of the century scientists considered themselves a supranational group. That illusion was shattered by World War I, when scientists showed themselves to be no less nationalistic than other groups. The ostracism suffered by German scientists after the war is something we ought to be more aware of. In part because science became a substitute for the political and military power much good work was done in Germany, at least until the Nazi regime began to sack non-Aryan scientists. As is well known, the loss of talent was enormous, and much of it went to America, which became the world power in physics as in so much else. The major role played by physicists in World War II, most obviously in the atomic bomb project, the largest research project ever undertaken, is discussed. Since World War II support from governments for physics has increased enormously, but the fortunes of many physicists have remained entwined with the military establishment and many others now work within megaprojects such as CERN. At the end of the century the numbers of physicists and of papers published have grown a hundred-fold, and funding by an even larger factor. Other aspects have changed less. Physics is still male-dominated and largely a North American and European enterprise. Latterly there has been some reduction in support. Physics, along with the rest of science, has also been under attack for the problems it has created. The beginning of the twenty-first century promises to be as interesting for physics as recent decades have been.
Comparison with the other recent work in this field, the three-volume Twentieth Century Physicsedited by Laurie Brown, Abraham Pais and Brian Pippard (Institute of Physics Publishing and American Institute of Physics Press, 1995), is unavoidable. They fill different niches. Twentieth Century Physicsis a massive and expensive work by some 30 leading physicists, destined mainly for the shelves of academic libraries. Quantum Generationsis a book by a professional historian covering much of the same physics, albeit more briefly, but written with a broader sweep that takes in more of the political and cultural milieu within which the physicists worked. References are given sparingly so as not to break up the text, but there are suggestions for further reading for each chapter, and there is an extensive bibliography. It is not a book for those with no background at all in physics - there are too many equations of nuclear reactions for that, and in any case such a book would be a superficial thing. Even many physicists will find some of the more esoteric ideas, such as the grand unified theories and superstring theory, heavy going. It is, however, accessible to a wide readership, and a book that can be read for the broad story, not just dipped into for specific details. It is likely to become the standard historical introduction to the history of physics in the twentieth century for scientists and historians, not to be superseded until well into the twenty-first century when the consequences of work that is still recent can be seen more clearly.