The Cavendish Laboratory, in the University of Cambridge, England, is arguably the most distinguished scientific institution in the world. Since it was established in the late nineteenth century it has produced some of the most consequential and innovative advances of all time. These include the discovery of the electron in 1897, the discovery of the isotopes of the light elements (1919), the splitting of the atom (also in 1919), the discovery of the proton (1920), of the neutron (1932), the unravelling of the structure of DNA (1953), and the discovery of pulsars (1967). Since the Nobel Prize was instituted in 1901, more than twenty Cavendish and Cavendish-trained physicists have won the prize for either physics or chemistry.1 Established in 1871, the laboratory opened its doors three years later. It was housed in a mock-Gothic building in Free School Lane, boasting a facade of six stone gables and a warren of small rooms connected, in Steven Weinberg's words, 'by an incomprehensible network of staircases and corridors'.2 In the late nineteenth century, few people knew, exactly, what 'physicists' did. The term itself was relatively new. There was no such thing as a publicly-funded physics laboratory-indeed, the idea of a physics laboratory at all was unheard-of. What is more, the state of physics was primitive by today's standards. The discipline was taught at Cambridge as part of the mathematical tripos, which was intended to equip young men for high office in Britain and the British empire. In this system there was no place for research: physics was in effect a branch of mathematics and students were taught to learn how to solve problems, so as to equip them to become clergymen, lawyers, schoolteachers or civil servants (i.e., not physicists).3 During the 1870s, however, as the four-way economic competition between Germany, France, the United States and Britain turned fiercer-mainly as a result of the unification of Germany, and the advances of the United States in the wake of the Civil War-the universities expanded and, with a new experimental physics laboratory being built in Berlin, Cambridge was reorganised. William Cavendish, the seventh duke of Devonshire, a landowner and an industrialist, whose ancestor Henry Cavendish had been an early authority on gravity, agreed to fund a laboratory provided the university promised to found a chair in experimental physics. When it was opened, the Duke was presented with a letter, informing him (in elegant Latin), that the laboratory was to be named in his honour.4 The new laboratory became a success only after a few false starts. Having tried-and failed-to attract first William Thomson, later Lord Kelvin, from Glasgow (he was the man who, among other things, conceived the idea of absolute zero and contributed to the second law of thermodynamics), and second Hermann von Helmholtz, from Germany (who had scores of discoveries and insights to his credit, including an early notion of the quantum), Cambridge finally offered the directorship to James Clerk Maxwell, a Scot and a Cambridge graduate. This was fortuitous. Maxwell turned into what is generally regarded as 'the greatest physicist between Newton and Einstein'.5 Above all, Maxwell finalised the mathematical equations which provided a fundamental understanding of both electricity and magnetism. These explained the nature of light but also led the German physicist Heinrich Hertz at Karlsruhe in 1887 to identify electromagnetic waves, now known as radio. Maxwell also established a research programme at the Cavendish, designed to devise an accurate standard of electrical measurement, in particular the unit of electrical resistance, the ohm. Because of the huge expansion of telegraphy in the 1850s and 1860s, this was a matter of international importance, and Maxwell's initiative both boosted Britain to the head of this field, and at the same time established the Cavendish as pre-eminent in dealing with practical problems and devising new forms of instrumentation. It was this latter fact, as much as anything, that helped the laboratory play such a crucial role in the golden age of physics, between 1897 and 1933. Cavendish scientists were said to have 'their brains in their fingertips'.6 Maxwell died in 1879 and was succeeded by Lord Rayleigh, who built on his work, but retired after five years to his estates in Essex. The directorship then passed, somewhat unexpectedly, to a twenty-eight-year-old, Joseph John Thomson, who had, despite his youth, already made a reputation in Cambridge as a mathematical physicist. Universally known as 'J. J.', Thomson, it can be said, kick-started the second scientific revolution, to create the world we have now. The first scientific revolution, it will be recalled from Chapter 23, occurred-roughly speaking-between the astronomical discoveries of Copernicus, released in 1543, and those of Isaac Newton, centring around gravity, and published in 1687 as Principia Mathematica . The second scientific revolution would revolve around new findings in physics, biology,
and psychology. But physics led the way. It had been in flux for some time, due mainly to a discrepancy in the understanding of the atom. As an idea, the atom-an elemental, invisible and indivisible substance-went back to ancient Greece, as we have seen. It was built on in the seventeenth century, when Newton conceived it as rather like a minuscule billiard ball, 'hard and impenetrable'. In the early decades of the nineteenth century, chemists such as John Dalton had been forced to accept the theory of atoms as the smallest units of elements, in order to explain chemical reactions-how, for example, two colourless liquids, when mixed together, immediately formed a white solid or precipitate. Similarly, it was these chemical properties, and the systematic way they varied, combined with their atomic weights, that suggested to the Russian Dimitri Mendeleyev, playing 'chemical patience' with sixty-three cards at Tver, his estate 200 miles from Moscow, the layout of the periodic table of elements. This has been called 'the alphabet out of which the language of the universe is composed' and suggested, among other things, that there were elements still to be discovered. Mendeleyev's table of elements would dovetail neatly with the discoveries of the particle physicists, linking physics and chemistry in a rational way and providing the first step in the unification of the sciences that would be such a feature of the twentieth century. Newton's idea of the atom was further refined by Maxwell, when he took over at the Cavendish. In 1873 Maxwell introduced into Newton's mechanical world of colliding miniature billiard balls the idea of an electro-magnetic field. This field, Maxwell argued, 'permeated the void'-electric and magnetic energy 'propagated through it' at the speed of light.7 Despite these advances, Maxwell still thought of atoms as solid and hard and essentially mechanical. The problem was that atoms, if they existed, were too small to observe with the technology then available. Things only began to change with Max Planck, the German physicist. As part of the research for his PhD, Planck had studied heat conductors and the second law of thermodynamics. This law was initially identified by Rudolf Clausius, a German physicist who had been born in Poland, though Lord Kelvin had also had some input. Clausius had presented his law at first in 1850 and this law stipulates what anyone can observe, that energy dissipates as heat when work is done and , moreover, that heat cannot be reorganised into a useful form. This otherwise common-sense observation has very important consequences. One is that since the heat produced-energy-can never be collected up again, can never be useful or organised, the universe must gradually run down into complete randomness: a decayed house never puts itself back together, a broken bottle never reassembles of its own accord. Clausius' word for this irreversible, increasing disorder was 'entropy', and he concluded that the universe would eventually die. In his PhD, Planck grasped the significance of this. The second law shows in effect that time is a fundamental part of the universe, or physics. This book began, in the Prologue, with the discovery of deep time, and Planck brings us full circle. Whatever else it may be, time is a basic element of the world about us, is related to matter in ways we do not yet fully understand. Time means that the universe is one-way only, and that therefore the Newtonian, mechanical, billiard ball picture must be wrong, or at best incomplete, for it allows the universe to operate equally in either direction, backwards and forwards.8 But if atoms were not billiard balls, what were they? The new physics came into view one step at a time, and emerged from an old problem and a new instrument. The old problem was electricity-what, exactly, was it?* Benjamin Franklin had been close to the mark when he had likened it to a 'subtile fluid' but it was hard to go further because the main naturally-occurring form of electricity, lightning, was not exactly easy to bring into the laboratory. An advance was made when it was noticed that flashes of 'light' sometimes occurred in the partial vacuums that existed in barometers. This brought about the invention of a new-and as it turned out all-important- instrument: glass vessels with metal electrodes at either end. Air was pumped out of these vessels, creating a vacuum, before gases were introduced, and an electrical current passed through the electrodes (a bit like lightning) to see what happened, how the gases might be affected. In the course of these experiments, it was noticed that if an electric current were passed through a vacuum, a strange glow could be observed. The exact nature of this glow was not understood at first, but because the rays emanated from the cathode end of the electrical circuit, and were absorbed into the anode, Eugen Goldstein called them Cathodenstrahlen , or cathode rays. It was not until the 1890s that three experiments stemming from