The coming of physics

For Lavoisier, heat was a substance, ‘caloric’; but by the 1830s the old theory that it was motion of particles was revived. James Joule in Manchester demonstrated this by heating water with a stirrer; and Hermann Helmholtz generalized this in 1847, arguing that heat, light, mechanical motion, electricity, chemical affinity and magnetism were all aspects of indestructible energy, could be quantitatively converted into each other, and be expressed in the same terms of space, time and mass. This great synthesis, the first law of thermodynamics, created the science of classical physics, more fundamental than chemistry: the task of physicists was to determine with great accuracy the exchange rates and constants such as the velocity of light, where the American Albert Michelson was prominent as the USA began to make its mark. The second law of thermo-dynamics, coming from the reflections of Sadi Carnot in France on the limits to the efficiency of steam engines, was generalized by Rudolf Clausius and by William Thomson to indicate that the universe had a direction to it: the availability of energy was decreasing (or ‘entropy’ increasing) with the passage of time. We should expect that the Sun will gradually cool, and that eventually the world will end in a ‘heat death’ of universal

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tepidity. This antidote to evolutionary, progressive optimism contributed to some fin de siècle gloom, as in H.G.WELLS’S Time Machine, 1895; though the calculations of Thomson (the first scientist to be made a peer, as Lord Kelvin) indicated that there were many million years to come, Kelvin’s calculations of the age of the solar system also indicated a relatively short geological past. Assuming that the Sun was made of the best coal and also fuelled by meteors and gravitational collapse, and that the Earth was a cooling sphere, he arrived at a maximum of one hundred million years. Huxley did his best to respond and cast doubt, but arguing with a confident and highly numerate physicist was not easy—though he was right, since Henri Becquerel’s discovery of radio- activity in 1896 revealed a new source of energy unsuspected by Kelvin.

This was one of a series of surprises that transformed classical physics. Faraday’s work on electromagnetism, put into mathematical form by Maxwell, introduced the idea of a field in place of Newtonian attractions and repulsions between point masses. Following from this, Heinrich Hertz observed radio waves. Crookes meanwhile was studying cathode rays, which J.J.Thomson at the Cavendish Laboratory in Cambridge (set up under Maxwell in 1871) demonstrated in 1897 to be a stream of negatively charged particles, which he first called ‘corpuscles’ but soon recognized to be the units of charge already called electrons. Those who had supposed that the outlines of physics were known, and only the details remained to be sketched in were wrong; and from 1897 Max Planck found that to account for the radiation from black bodies he had to suppose that energy came in lumps, or ‘quanta’, rather than continuously. Atoms were complex, and now matter and energy seemed indistinct.

The coming of physics