In physics, the proton-to-electron mass ratio (symbol μ or β) is the rest mass of the proton (a baryon found in atoms) divided by that of the electron (a lepton found in atoms), a dimensionless quantity, namely: The number in parentheses is the measurement uncertainty on the last two digits, corresponding to a relative standard uncertainty of 1.7×10−11.
Astronomical searches for time-varying μ have typically examined the Lyman series and Werner transitions of molecular hydrogen which, given a sufficiently large redshift, occur in the optical region and so can be observed with ground-based spectrographs.
Reinhold et al. (2006) reported a potential 4 standard deviation variation in μ by analysing the molecular hydrogen absorption spectra of quasars Q0405-443 and Q0347-373.
Bagdonaite et al. (2013) used methanol transitions in the spiral lensing galaxy PKS 1830-211 to find ∆μ/μ = (0.0 ± 1.0) × 10−7 at z = 0.89.
Using three methanol lines with similar frequencies to reduce systematic effects, Kanekar et al. (2015) obtained ∆μ/μ < 4 × 10−7.