Brandt - The harvest of a century: discoveries of modern physics in 100 episodes

The Harvest of a Century

Discoveries of Modern Physics in 100 Episodes

Siegmund Brandt

Physics Department
University of Siegen

Great Clarendon Street, Oxford OX2 6DP

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Siegmund Brandt 2009

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First Published 2009

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Printed in Great Britain
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ISBN: 9780199544691

10 9 8 7 6 5 4 3 2 1

To Martin

The intention of this book is to give an impression of the advances of physics in the twentieth century by retracing important discoveries. In this period physicists focussed their interest primarily on two topics, the microscopic structure of matter and the structure of space and time. Sometimes an exact date is attributed to the beginning of modern physics, 14 December 1900, when Planck presented his radiation law, featuring the quantum of action. Half a century later Einstein stated that this discovery [of the quantum of action] became the basis of all twentieth-century physics and has almost entirely conditioned its development ever since. That is still true, now that the whole century has passed. In spite of this, our tale begins a little earlier with important experimental discoveries, those of X rays and the electron, and those of radioactivity, radium, and alpha, beta, and gamma rays before we touch on Plancks work.

The book is conceived as a string of a hundred episodes, covering both experimental and theoretical physics. This structure is meant to allow readers to follow their own particular interests, picking topics of their choice. Still, episodes are arranged in chronological order and readers with no or little previous knowledge in modern physics may want to follow that order, since they will become familiar with the development of the field and its concepts as they go along. Each episode is a short story of the scientists involved, their time, and their work; each is self-contained, with references to others where required. The text is intended for easy reading. Occasionally, a more thorough discussion of experimental set-ups and theoretical concepts is presented in special boxes for readers interested in more detail.

In contrast to science itself, history of science is not well ordered; it develops in twists and turns with ramifications and blind alleys. In our collection of episodes, which can be seen as a mosaic with wide spaces between its stones, many historical details are left unmentioned. The year, given in the title of each episode, is that in which the corresponding discovery was made public, usually by submission of a paper to a learned journal. Often, it took quite some time until discoveries were generally accepted. This, in particular, holds for new theories like Plancks or Einsteins. Again, we scarcely dwell on the reception of the discoveries we describe.

The selection of topics is, of course, to some extent arbitrary. On the other hand, many of those chosen definitely had to be included. It is sincerely hoped that every reader will find at least half of her or his own favourite hundred. The episodes refer to atomic, nuclear, and particle physics, to physics of condensed matter, to relativity, and to physical properties of the universe.

An important starting point for atomic physics was the development of optical spectroscopy in the nineteenth century. Spectroscopic measurements became the basis of Plancks work; when the electron and the nucleus had been found, they allowed the construction and the refinement of atomic models. The culmination point was the formulation of quantum mechanics in the 1920s, a theory which, in extended form, seems to govern all microscopic physics.

Nuclear physics began with the discovery of radioactivity which provided projectiles of rather high energy for scattering and collision experiments. In the early 1930s higher energies, reached by accelerators, and neutrons as new projectiles brought this field to the foreground. A peak was reached with the discovery of nuclear fission and the construction of nuclear reactors in the late 1930s and the 1940s, respectively. Nuclear physics experiments gradually made it clear that there were two additional forces in nature besides the gravitational force (which binds planets to a star) and the electromagnetic force (which binds electrons to a nucleus). These new forces were called strong and weak, respectively. The strong force was needed to keep the nucleus together, the weak one to explain a particular type of its decay. Nuclear physics brought some progress in the understanding of the weak force but no lasting success with the strong.

Particle physics is an off-spring of nuclear physics. It makes use of the very high energies provided by cosmic radiation and modern accelerators and studies collisions of particles simpler than nuclei (except for the proton, the simplest nucleus). Such experiments led to the discovery of many new particles coupled to the strong force (hadrons) and of a few showing only weak and, at most, electromagnetic interactions (leptons). In the 1960s it was found, that hadrons are composed of more fundamental particles, called quarks, and later on that there are as many quarks as there are leptons. On the basis of quarks and leptons, successful theories of the strong and the weak interaction were developed (the latter even was unified with the electromagnetic one) and confirmed by experiment.

From the diversified field of condensed matter physics, episodes are presented on unusual and unexpected phenomena such as superconductivity, superfluidity, and the quantum Hall effects, all understood later on the basis of quantum mechanics. But quantum mechanics also helped to design novel materials and devices. In that context we describe the development of the band model of semiconductors and the construction of transistors and semiconductor laser diodes.