A new probe for a changing constant

Water in distant galaxies is difficult to detect from Earth because the molecule lacks favorably placed spectral transitions. Astronomers seeking a proxy turn instead to water’s protonated form, hydronium (H₃O⁺), whose lowest vibrational transitions lie in the millimeter band. Now Vladimir Špirko of Charles University in Prague and his collaborators have identified another use for hydronium: as a probe to determine whether the proton-to-electron mass ratio, μ, varies. Given that gluons, not quarks, make up most of the proton’s mass, variations in μ could arise if the strong nuclear force changes through cosmic time, as some theorists have speculated. The energy of a vibrational state depends on the molecular mass and therefore on the proton mass and μ. When those vibrational states lie close to inversion and rotational states, the μ dependence strengthens. To determine how sensitive those combined states are to variations in...