Quantum and Nuclear

Anderson’s accident

Stories from Physics for 11-14 14-16 16-19 IOP RESOURCES

The first detection of the positron, at least according to Carl Anderson who is credited with its discovery, is another example of experimental good fortune. In 1928, in an attempt to reconcile quantum theory and relativity, Paul Dirac noted that an equation could have both positive and negative solutions, suggesting a positively charged partner for the electron. In 1932, Anderson was examining the tracks made by cosmic rays in a cloud chamber and noted that some tracks curved in the opposite direction from those made by negative electrons. He noted that the tracks couldn’t have been made by protons and, unaware of Dirac’s theoretical work, proposed the existence of positive electrons or ‘positrons’.

Anderson reported that his discovery was “wholly accidental”. He acknowledged that Chung-Yao Chao, a graduate student at Caltech, had collected inconclusive data three years earlier which hinted at the positron’s existence. Indeed, Frédéric and Irène Joliot-Curie, the daughter and son-in-law of Marie and Pierre Curie, and both future Nobel Prize winners, had evidence of positrons on photographic plates a few months before Anderson, but assumed the particles were protons. Anderson suggested that a wise physicist “had he [sic] been working in a well-equipped laboratory and had he taken the Dirac theory at face value he could have discovered the positron in a single afternoon”.



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