Physical laws bear some constants, which are deemed natural and for the moment lacks an explanation. It is believed that, such constants might be derivable from more fundamental laws, which remain to be discovered yet. If so, it is expected that, the value of such constants should be determined by the initial conditions of the universe and might evolve in time. Given this, how do we measure such change ?
A paper in PRL [Phys. Rev. Lett. 106, 100801 (2011)] suggests one method. It looks at the e/p mass ratio. See the synopsis on this [http://www.nature.com/nphys/journal/v7/n4/full/nphys1982.html?WT.ec_id=NPHYS-201104]: "Methanol is among the simplest molecules that undergo internal rotation. It is also one of the most abundant molecules and is responsible for prominent radio emission lines generated by astrophysical masers. Paul Jansen and colleagues have found that transitions that convert energy from internal rotations of the methyl (CH3) and hydroxyl (OH) groups of methanol to gross rotation of the molecule as a whole depend strongly on the value of the proton-to-electron mass ratio. If this ratio were to change by a fraction, their calculations indicate that the emission frequency of this transition should change by 50 times the fraction."
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