Rutherford identified three distinct varieties of radioactive emanations and called them alpha, beta, and gamma rays. In quantum mechanics, certainty is replaced by chance, precise trajectories by probable paths. There is no subtle difference between them that explains which will live another billion years and which will decay. Two atoms of the same nuclear species are absolutely identical. Just which atom or atoms, if any, decay in that minute, and which survive for billions of years, no one can tell. In a one-microgram sample of 238U 92, an average of one atom decays per minute. Soon after radioactivity was found, Ernest Rutherford and the English radiochemist Frederick Soddy demonstrated that each radioactive species has a characteristic half-life. In general, A is the nearest integer to the atomic weight of an isotope Z is its atomic number or place in the periodic table. 3H 1, or tritium, is the radioactive hydrogen isotope made of one proton and two neutrons, 2 and 235U 92 is the fissile isotope of uranium containing 143 neutrons. A nucleus containing A nucleons and Z protons is designated AN Z, where N is the chemical symbol for the element. These two particles have about the same mass and are collectively called nucleons. 1 Neutrons, discovered in the same year by the British physicist James Chadwick, revealed that atomic nuclei, once regarded as composites of protons and electrons, were comprised of protons and their electrically neutral counterparts, neutrons. He had discovered positrons, antiparticles of electrons and the first known form of antimatter. In 1932, the American physicist Carl Anderson found positively charged electron-like particles among cosmic rays. In the beginning, the elementary particles comprised protons, photons, and electrons. Triumph though it is, for reasons both experimental and theoretical, the Standard Model is known to be incomplete. Its formulation, unlike the theories of Isaac Newton and Albert Einstein, depended on the work of thousands of physicists and engineers and the generosity of many governments. Together they form the Standard Model of particle physics, offering a complete, correct, and consistent description of all known elementary particle phenomena. The strong force results from a gauge theory based on an unbroken SU(3) symmetry called quantum chromodynamics, the weak and electromagnetic forces from a broken SU(2) × U(1) symmetry. Gravity seems to play no role in the subatomic world. In this part, I describe the Standard Model of particle physics, which encompasses three of the four forces of nature. Two forces of nature had been recognized, gravity and electromagnetism, but two more would needed to understand nuclear phenomena: a strong force to bind nucleons into nuclei, and a weak force to explain how they decay. This part begins with the challenges posed by the atomic nucleus. Parts one and two told of the development of the physical sciences from ancient times to the discovery of relativity and quantum mechanics in the first quarter of the twentieth century.

T his is the third and final essay in a three-part series.

Standard model particles