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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+, Sr2) and atomic clocks, or as $\\Delta \\mu/\\mu$ at redshift $z > 1$ from H2 and CH3OH absorption, at the $1e-17$ per year and $1e-7$ levels respectively.","problem_ref":null,"references":"","settled_by":"A molecular-clock to atomic-clock ratio tracked for several years with fractional uncertainty below 1e-17, or a radio methanol absorption measurement with sub-1e-7 precision.","status_note":"","title":"Does the proton-to-electron mass ratio vary in time or 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