A new experimental limit to the possible time variation of the proton gyromagnetic ratio times the electron-to-proton mass ratio has been obtained from the frequency comparison data between Cs and Mg atomic beam standards. The measurements, made over a period of one year, lead to an upper limit for the rate of change of ${\mathit{g}}_{\mathit{p}}$(${\mathit{m}}_{\mathit{e}}$/${\mathit{m}}_{\mathit{p}}$) of 5.4×${10}^{\mathrm{-}3}$/yr. This result, based only on the validity of the quantum mechanical theory of the atomic spectra, together with astrophysical data regarding analogous products of atomic fundamental constants, leads also to a new limit for the time stability of the fine structure constant α (2.7×${10}^{\mathrm{-}13}$/yr).

• Received 21 April 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.2364