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Both laboratory molecular clocks and ancient molecular absorption in distant galaxies are used.","posed_since":"","precise":"Determine whether $\\mu = m_p/m_e$ varies, either as $d\\ln(\\mu)/dt$ today from comparisons of molecular (for example HD+, H2+, N2+, $\\mathrm{Sr_2}$) and atomic clocks, or as $\\Delta \\mu/\\mu$ at redshift $z > 1$ from $\\mathrm{H_2}$ and $\\mathrm{CH_3OH}$ absorption, at the $10^{-17}$ per year and $10^{-7}$ levels respectively.","problem_ref":"04418a37563b07155f3ab62212dc63ed0a14ca2db043c4a34c0a926de23f8e51","references":"","settled_by":"A molecular-clock to atomic-clock ratio tracked for several years with fractional uncertainty below $10^{-17}$, or a radio methanol absorption measurement with sub-$10^{-7}$ precision.","status_note":"","title":"Does the proton-to-electron mass ratio vary in time or 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