The search for elementary particles with fractional electric charge and the philosophy of speculative experiments

All known elementary particles that can be isolated as individual particles have an electric charge that is equal in magnitude to the electron’s charge, q=1.6×10−19 C, or is zero. This includes the muon and tau charged leptons, the neutrinos, the photon, the nucleons, and the mesons. There have been many searches without confirmed success for isolated particles with fractional electric charge such as ±q/3, or ±q/2, or ±3q/2. The theory of the physics of elementary particles does not require the existence of such particles. In particular, current theory holds that quarks, whose charge is ±(1/3)q or ±2/3q, cannot be isolated. Despite past failures and current theory, we have been engaged for the past few years in a new search for isolated elementary particles with fractional electric charge. In the course of this paper we discuss models for fractional charge particles, summarize previous searches, and describe our experimental method; and because this is speculative research we will also present our thoughts on speculative experiments.