The magnetic moment of the muon worries theorists
F. J. M. Parley · Europhysics news · 2001
The magnetic moment of the electron is g times the natural unit which is its angular momentum h/4p multiplied by the Larmor ratio e/2mc. Dirac theory predicted g = 2 but we now realise that this is modified by quantum fluctuations. In quantum field theory the electron is surrounded by a thin soup of evanescent particles which wink in and out of existence in fractions of a second. This soup is slightly magnetic so it increases the magnetic moment to g = 2 (1 + a). The small correction a (about 0.12%), called the anomalous moment, but often referred to as ig-2i is a pure quantum effect entirely due to the soup of virtual particles around the electron. Its detection and measurement with gradually increasing accuracy inspired the theory of quantum electrodynamics (QED) and the present spectacular agreement [2] with calculation to 24 parts per billion (ppb) in a is one of the main pillars of the theory. (The measurement is more accurate, but independent knowledge of the fine structure constant a limits the comparison).