In the early twentieth century, physics stood out as the dominant natural science--displacing biology and geology, which had held a similar position in the age of Darwin. In both cases the reasons for preeminence were the same. In the late nineteenth century, the biological sciences provided the metaphors and ways of thought--positive, determinist, material--that were congenial to the wider intellectual temper and seemed most readily applicable in other fields. In the twentieth century, the more abstract and indeterminate language of physics appealed to a society that was questioning nearly all the old certainties. Furthermore, a spurt of progress resembling that which biology had made in the second half of the nineteenth century was to give physics a special prestige in the first third of the century following.
At its earliest, the twentieth-century revolution in physics can be dated from 1895, when Professor Wilhelm Konrad R"ntgen of Munich discovered X rays. Thus began the atomic age, as one revelation followed another in quick succession. A year after R"ntgen's discovery, Henri Becquerel's experiments with uranium opened the way to an analysis of radioactivity, on which Pierre Curie and his wife were soon to be working, and in the next year, the identification of the electron as a negatively electrified corpuscle suggested an approach to explaining this whole series of new phenomena.
Initially, the explanations were presented in terms that combined the old Newtonian principles of motion with the nineteenth-century concept of electricity. Thus the physicists of the first decade of the twentieth century viewed electricity as the common property of all matter and pictured the atom as a miniature Newtonian solar system, in which the positively charged nucleus held in dynamic tension negatively charged electrons--varying in number from one in the case of hydrogen to ninety-two in the case of uranium I--that were circling in orbits around it. In this fashion, a physicist like Lord Rutherford (first in Montreal and Manchester, subsequently in Cambridge) in 1903 was able to ascribe radioactivity to an explosive disintegration of atoms of great weight--that is, which had a large number of electrons in orbit--and seven years later to make the basic discovery that identified the nucleus of the atom with its positive charge.
Many previously unfamiliar phenomena fitted conveniently into the new explanatory scheme. On the border line between physics and chemistry, it enabled scientists to bring to virtual completion the periodic table of the atoms by locating several theoretically possible elements that earlier had escaped detection. Meantime, however, two further threats to intellectual consistency had appeared. In different forms, the discoveries of Albert Einstein and Max Planck, both of Berlin, overturned the newly devised scheme of explanation and opened the major phase in the physical revolution that is still going on.
Einstein's work bore only tangentially on atomic theory, since it dealt chiefly with mechanics and astrophysics. As early as 1905, he had suggested that the notion of space and time as absolutes needed to be abandoned, that these were categories derived from metaphysics and should properly be viewed as always relative to the person measuring them. During the First World War, Einstein extended his theory to take account of the phenomenon of gravitation. This he explained in terms of a four-dimension continuum--in which time was the fourth dimension--and a "curved" universe that made possible the eventual return of light waves to their starting point. During the eclipse of 1919, Einstein's calculation of the deflection of light was confirmed by simultaneous astronomical observations from points on both shores of the South Atlantic.
Even for atomic physics, however, the implications of Einstein's theory of relativity were already clear: the hard, solid "matter" of traditional science-which men like Rutherford had dissolved into electricity--needed to be redefined still further in terms that made its particles no more than a "series of events in space-time." These conclusions were confirmed by the more directly relevant theories of Max Planck who, independently of Einstein, had almost simultaneously arrived at equally revolutionary conclusions.
Planck originally devised his "quantum theory" in 1901 to take account of certain jumps and discontinuities which he had observed in radiation phenomena. According to his new explanation, radiation did not come in continuous waves but rather in definite units or quanta. Indeed it was in terms of quanta, Planck argued, that energy in general and changes of atomic structure in particular should be viewed. At the start, physicists did not quite realize how novel this theory was, and the efforts of Niels Bohr of Copenhagen to fit Planck's quanta into the "solar-system" explanation of the atom seemed initially successful. In 1913, Bohr, working in Rutherford's laboratory, devised a way of combining the English physicist's theory of orbits with a concept of a series of "jumps" of electrons from one orbit to another.
For twelve years, this reconciliatory theory held the field. Then in 1925, the final and culminating phase of the revolution in physics began when it was discovered that Bohr's explanations did not account for all the phenomena observed in the hydrogen spectrum. Soon new theories of a bewildering diversity and complexity began competing for acceptance. On the one hand, Heisenberg argued for a complete discarding of physical hypotheses such as orbits--which he found unwarranted by the facts--in favor of the more abstract language of differential equations. On the other hand, Schr"dinger turned to the theory of wave mechanics that Prince Louis de Broglie had developed in France. Schr"dinger contended that a stream of electrons should be regarded as having certain properties of a wave as well as those of a series of particles. Meanwhile Bohr himself began to revise his earlier theories, and a large number of other physicists. British, Continental, and American, branched out into still newer hypotheses that the quantum theory had suggested.
A theoretical situation of unparalleled complexity resulted. Only on the ground of mathematics--where Heisenberg's and Schr"dinger's equations proved to be equivalent--could the new explanations meet. In terms of classical mechanics, unitary theory had broken down completely. Sometimes one spoke of particles; sometimes of waves. Physicists chose between the two on a pragmatic basis as one theory rather than the other seemed to fit particular experimental facts. Discontinuity, indeterminacy, and uncertainty replaced the earlier clear and unilateral explanations. Just as in the first stage of the revolution in physics, Rutherford and his colleagues had dissolved matter into motion and electricity, so in its second and third stages the notion of electricity itself began to break down, as the final explanations of science were resolved into either mathematics or mystery.
Mystery was the first of two contrasting implications that contemporaries drew from the twentieth-century revolution in scientific theory. If the physicists themselves--the acknowledged masters of abstract science--could not agree and were unsure of their conclusions, what was the mere layman to think? How was he to distinguish fact from fancy in the physical world? Thus the ordinary educated man who had picked up something of the new physics tended to turn either to skepticism or to religion. He might choose to live in a state of suspended judgment and philosophical pluralism. On the other hand he might decide to take a leap into religious faith--if the physical world was ultimately a mystery--that proved how right were the theologians who had always argued that this was so. The new self-doubt on the part of the natural scientists of the 1920's had not a little to do with the return to religion that was to be so striking a phenomenon in the two succeeding decades.
The second implication of the new discoveries was at first less troubling. However bewildering abstract scientific theory might appear around 1930, on the level of practical applications scientists were advancing from triumph to triumph. Two discoveries of the year 1919 inaugurated the period of applied atomic physics--the invention of the mass spectroscope, which made possible the identification of more than two hundred stable isotopes (i.e., variant forms of the basic atoms), and Rutherford's initial experiment with controlled atomic transformations. During further experiments of the latter sort, physicists discovered a whole new series of constituent particles of the atom comparable to electrons--positrons, protons, neutrons, and the like--until by 1944 seven in all had been identified. The most far-reaching of these discoveries was the identification of the neutron by Sir James Chadwick in 1932. It was with particles of this kind which carried no charge and hence could pass freely through the atoms in their paths, that the physicists began the intense bombardment of basic matter which was to culminate during the Second World War in the awesome discovery of the atomic bomb.