To the inquiring eye of the curious scientist, the structure of physical science in the 1890s was not quite perfect. There were a few loose bricks, a few things that did not quite fit into place. In 1856, Urbain Leverrier in France had calculated with great care the influence of the other planets on the motion of Mercury. He found that the elliptical orbit traced out by Mercury should itself rotate slowly about the Sun, the point of Mercury’s closest approach to the Sun—its perihelion—advancing at 527 seconds of arc per century, or once around in about 2,500 centuries. (There are 60 seconds of arc in one minute, 60 minutes in one degree, and 360 degrees in a full circle.) But Mercury’s orbit was known to shift around by 565 seconds per century. The tiny but significant discrepancy of 38 seconds per century lurked unexplained in the background of science for sixty years. Improved measurements and calculations had succeeded only in making the discrepancy grow slightly, to 42 seconds, when in 1916 Einstein predicted from the general theory of relativity an extra rotation of exactly 42 seconds.
Another puzzle of physical science at the end of the nineteenth century was the amazing shyness of the ether, which refused to show itself to its most persistent pursuers. To Albert Michelson and Edward Morley, as well as to several of their contemporaries with alternative stratagems of their own, the ether refused to reveal its presence. It is not the first time in history that what was difficult to conquer took on a special fascination, and some of the most inspired scientific work of the period 1890 to 1905 was dedicated to explaining how the ether could exist yet defy detection. Hendrik A. Lorentz in Holland and Henri Poincaré in France developed some of the ideas of relativity in this period from the desire to explain the ether’s anonymity. But it took the genius of Einstein to reject the ether altogether and to appreciate the revolutionary nature of the vacuum left by its departure.
Still another puzzle whose solution led to revolutionary new ideas at the turn of the century had to do with light waves and energy. A drama of physics was played out in what seems to be a most uninspiring arena, just an empty container, or hohlraum. Within the hohlraum, the classical theories of mechanics, thermodynamics, and electromagnetism should have met and united in a final triumph of the classical description of nature. The mechanical vibrations of molecules in the wall and the electromagnetic vibrations of waves in the cavity should have shared energy according to the equipartition theorem of thermodynamics. But they refused to cooperate. The high-frequency radiation received less than its fair share of the energy. In the first years of this century, Max Planck saved the equipartition theorem, but he had to introduce the energy quantum and topple a good deal of the structure of classical physics to do it.
Another puzzle of the late nineteenth century: The atom was beginning to act up. Few scientists doubted the existence of atoms. The role of atoms in chemistry was established, and a good deal about the “external” properties of atoms—how they interacted with each other and with the environment—was known. The existence of characteristic line spectra suggested also a complex inner structure for the atom, but so far there was no evidence that this structure would not yield to analysis with the tools of classical physics. When Henri Becquerel discovered in Paris in 1896 that certain atoms, which we now call radioactive, emit penetrating radiation, the inside of the atom forced itself upon our attention in a more insistent way, and we have not been able to ignore it since. At about the same time, in Cambridge, J. J. Thomson learned that the negative electricity which can be torn from atoms consists of small swift particles, which suddenly made atoms seem large and clumsy. So was discovered the first and most important fundamental particle, the electron, whose existence revealed the presence of a new and unknown world within the atom. Since then the electron has gone on to independent glories of its own, now doing most of the brain work and some of the manual labor in the world’s billions of electric and electronic devices.
From our present vantage point in history, it is easy to select these examples of weak spots and gaps in classical physics at the end of the nineteenth century. But it should be borne in mind that to most observers these appeared as rather small and inconsequential flaws in a magnificent and well-tested structure. To Einstein, Planck, Lorentz, Poincaré, Rutherford, and the relatively small group of scientists throughout the world who fixed their attention on the flaws without being blinded by the beauty of the rest of the structure do we owe the creation of the new structure of twentieth-century physical science. Now we find ourselves in the twenty-first century, waiting, hopes so far unfulfilled, for a possible new theory as grand and all-encompassing as relativity and quantum theory.