Anderson , Carl David

(1905–1991) American physicist

Anderson, the son of Swedish immigrants, was born in New York City and educated at the California Institute of Technology where he obtained his PhD in 1930 and where he remained for his entire career, serving as professor of physics from 1939 until his retirement in 1978.

Anderson was deeply involved in the discovery of two new elementary particles. In 1930 he began to study cosmic rays by photographing their tracks in a cloud chamber and noted that particles of positive charge occurred as abundantly as those of negative charge. The negative particles were clearly electrons but those of positive charge could not be protons (the only positive particles known at the time) as they did not produce sufficient ionization in the chamber. Eventually Anderson concluded that such results “could logically be interpreted only in terms of particles of a positive charge and a mass of the same order of magnitude as that normally possessed by a free negative electron.” It was in fact the positron or positive electron, whose existence he announced in September 1932. In the following year his results were confirmed by Patrick Blackett and Giuseppe Occhialini and won for Anderson the 1936 Nobel Prize for physics.

In the same year Anderson noted some further unusual cosmic-ray tracks. As they appeared to be made by a particle more massive than an electron but lighter than a proton it was at first thought to be the particle predicted by Hideki Yukawa that was thought to carry the strong nuclear force and hold the nucleus together. The particle was initially named the ‘mesotron’ or ‘yukon’. However, this identification proved to be premature, as its interaction with nucleons was found to be so infrequent that it could not possibly perform the role described by Yukawa. From 1938 the particle became known as the meson, and the confusion was partly dispelled in 1947 when Cecil Powell discovered another and more active meson, to be known as the pi-meson or pion to distinguish it from Anderson's mu-meson or muon.