Searching for Quarks: Perl's Philosophy of Speculative Experiments

Although most science courses and textbooks emphasize that the fundamental, indivisible unit of electric charge is the electron, physicists have been searching for fractional charges since the 1960s. Since the initial measurements of the electron charge a century ago, experimenters have faced the persistent question as to whether elementary particles exist that have charges that are fractional multiples of the electron charge. Millikan himself in his Nobel Prize acceptance speech in 1923 envisioned: “If the electron is ever subdivided it will probably be because man, with new agencies as unlike X-rays and radioactivity as these are unlike chemical forces, opens up still another field where electrons may be split up without losing any of the unitary properties which they have now been found to possess in the relationships in which we have thus far studied them” (Millikan, 1965, p. 61).

The objective of this chapter is to present Martin Perl's (Nobel Laureate in physics, 1995) strategy of speculative experiments in his search for quarks and its implications for understanding the Millikan—Ehrenhaft controversy (oil drop experiment, see Chapter 7) and cutting-edge experiments. This reconstruction is presented in the following sections:

• 1. Search for elementary particles with fractional electrical charge

• 2. Understanding scientific research methodology: contrasting Millikan and Perl

• 3. Conclusion: the role and importance of cutting-edge speculative experiments