Electromagnetic mass, relativity, and the Kaufmann experiments

This paper presents the theoretical background for and the detailed analysis of Kaufmann’s 1901–1905 experiments to determine the e/m ratio for fast electrons. Far from providing the first experimental confirmation of Einstein’s special theory of relativity, as is often claimed in physics textbooks today, these data were initially interpreted as confirming Abraham’s classical model of a rigid spherical electron and as providing evidence against special relativity. Only in 1906–1907, upon Planck’s subsequent reanalysis of Kaufmann’s 1905 data, did these experiments become evidence marginally in favor of relativity over classical models of the electron. This particular issue, of the superiority of special realtivity over classical theory in providing a fit to e/m determinations, was not definitely settled until 1914 with new extensive and accurate data obtained by Neumann. The entire episode provides another example that science does not proceed by a strict falsificationist methodology. It shows rather that a great scientist such as Einstein at times gives more weight to a theory that has a certain beauty and produces equations simple in form than he does to experimental results that apparently conflict with such a theory.