A team at Harvard University has made a new precise measurement of the electron magnetic moment, which in turns allows the fine structure constant to be determined with an uncertainty 10 times smaller than previously attained.
Gerald Gabrielse and colleagues have measured the value of the constant g of the electron, which relates its magnetic moment to the Bohr magneton, ehbar/2m, where e is the size of the charge on the electron, and m is the electron’s mass. For a Dirac point particle of spin 1/2, g should have a value of 2, but quantum electrodynamics (QED) predicts a value slightly higher, owing to vacuum fluctuations and polarizations effects.
To measure g more precisely than before, the Harvard team has resolved the cyclotron and spin energy levels of an electron confined for several months in a cylindrical Penning trap cooled to 100 mK (Odum et al. 2006). The value they obtained is g/2 = 1.00115965218085(76); the uncertainty of 0.76 ppt is nearly six times lower than the most recent accepted value, measured nearly 20 years ago (Van Dyck et al. 1987).
Working with Cornell University and RIKEN in Japan, Gabrielse and colleagues have used this new value of g with a prediction from QED involving 891 eighth-order Feynman diagrams, to determine a new value for the fine structure constant, α. They obtain α–1 = 137.035999710(96), that is, with an uncertainty of 0.70 ppb – an uncertainty that is about 10 times smaller than for any rival method to determine a (Gabrielse et al. 2006).
Further reading
G Gabrielse et al. 2006 Phys. Rev. Lett. 97 030802.
B Odum et al. 2006 Phys. Rev. Lett. 97 030801.
R S Van Dyck et al. 1987 Phys. Rev. Lett. 59 s26.
