Coming to order

Modern physics began in the period 1895-1925; these years, in the words of George Gamow, were 30 years that shook physics. Until then classical physics seemed essentially complete: Newton's laws reigned supreme and electromagnetic phenomena were governed by Maxwell's equations. There were a few minor puzzles and anomalies, but it was expected that they would soon be tidied up. Certainly no one expected the fundamental changes that were so soon to come.

Yet a few years around the turn of the century Becquerel discovered radioactivity (1896), Planck obtained his black-body radiation formula and introduced the quantum (1900), Einstein formulated the special theory of relativity, the theory of the photoelectric effect and also that of the Brownian motion (1905), Rutherford discovered the atomic nucleus (1912), and Bohr proposed his quantum theory of atomic spectra (1913). Throughout this period, physicists struggled to understand what they were doing, with limited success.

The new theories fitted the experimental data, often extremely well, but the ideas on which they were based often contradicted classical mechanics, to say nothing of common sense. The ether had to be rigid to explain the propagation of light but must not offer any resistance to bodies passing through it because that would affect their motion. Relativity implied that moving clocks went slow and that velocities are not additive. There was no explanation of how electrons jumped from one orbit to the other and gave out radiation with a frequency related to the energy difference between the two orbits. It was not clear how the orbit could be defined, or why the electrons did not continuously radiate. Some of these difficulties were removed by the development of quantum mechanics in the late 1920s, but others are still with us today.

The standard accounts in textbooks are generally simplified and give no idea of the difficulties faced by the pioneers. Their experimental apparatus was often crude and unreliable and it was far from easy to be sure that it was working properly and measuring what the physicists wanted it to measure. The theories were often based on dubious conjectures, analogies and sometimes sheer guesses, and they learned which was correct by comparing their consequences with experiment. Even this is too simple an account: one of Einstein's predictions from relativity was contradicted by an experiment; he stuck to his guns and was vindicated when an error was found in the experiment.

Classical physics distinguished between particles moving in accord with Newton's laws and light waves obeying Maxwell's equations, but this clear distinction was destroyed by the new physics. Planck was forced, much against his will, to admit that light was propagated in discrete quanta, and this immediately explained the photoelectric effect. Further confirmation of the particle nature of light came when Compton showed that light quanta could collide with electrons and recoil just like billiard balls. On the other hand, de Broglie suggested that particles had wave properties, and this was subsequently confirmed experimentally. All this can be described mathematically to a high degree of accuracy, but we still do not understand just what is going on.

The story of these heroic years is very well told by Philip Stehle, a physicist who has already written texts on quantum mechanics and on elementary particles. He begins with the final years of classical physics and takes the story up to the threshold of quantum mechanics, when Heisenberg, Schrodinger, Born, Dirac and Pauli finally made the breakthrough that at last provided a consistent and successful method of calculating the results of a wide range of experiments.

Stehle's account has the great merit of bringing out the real difficulties faced by the physicists as the drama developed. He quotes freely from the original papers, and thus shows some of the misconceptions, false starts and blind alleys that were only gradually removed by later work. Additional mathematical details are given in boxed sections. It is hardly a book for the general reader, but it will be found most valuable not only by students of physics, but also by those lecturing to them and by historians of physics.

P. E. Hodgson is senior research fellow, Corpus Christi College, Oxford.