When the Electron Falls Apart

It is ironic that, in the year when we celebrate the centenary of the discovery of the electron, the most exciting developments in the theory of electrons in solids have to do with the “fractionalization” of the electron—the discovery of particles that behave as though the electron had broken apart into three or five or more pieces each containing one‐third or one‐fifth of its charge, or into separate particles, one containing its charge and one its spin. (See figure 1.) No longer is the quantum theory of solids confined to the boring old electron; we now have a remarkable variety of fractional parts of electrons: composite fermions, composite bosons, spinons and holons, in addition to the heavy electrons, quasiparticles and small polarons of older stages of condensed matter theory.