David Peat

From Object to Process 57late about atoms. They were used to explain the properties of gases: agas is made up of tiny balls constantly colliding with each other. Heatthe gas, and the balls move faster and for greater distances, and so thegas expands.The first real evidence for the actual existence of atoms came in1858 when Julius Plücker (1801–1868) noticed a curious radiationemitted as an electric current passed through a gas. Like light, these“cathode rays” traveled in straight lines but could also be deflected by amagnet. Scientists deduced that the radiation was composed of tinyelectrically charged particles. In 1897 the British physicist J. J. Thomsonsuggested that these “electrons” are components of every atom. Thusthe existence of hitherto hypothetical atoms was confirmed, and theywere simultaneously found to be composite entities rather than indivisibleunits.From Atoms to Elementary ParticlesIn 1902 Thomson and Lord Kelvin suggested that atoms are like Christmaspuddings, with the negatively charged electron “raisins” embeddedin a spherical “pudding” of positive charge. Then ErnestRutherford’s experiments showed that the atom is more like a miniaturesolar system with electrons orbiting around a central “sun” ornucleus. When two or more atoms share these orbiting electrons, atomscombine to form molecules.But what of the nucleus itself? Physicists soon discovered that it,too, was composite and contained elementary particles called protonsand neutrons. And what held this nucleus together? The Japanesephysicist Hideki Yukawa proposed a new type of particle, called a meson,that binds other elementary particles together. Soon scientists discoveredthat not one but several different types of meson existed.By the middle of the twentieth century there was an entire zoo ofvarious “elementary” particles. This situation was distressing to physicists,who prefer their world to be simple and elegant. A world composedof just three types of particles would be preferable to one madeup of a great many. And so the notion of the quark was proposed—