A new paper has looked into the question of whether the masses of particles change with time. That this may be an issue may have not occurred to people (at least it had not occurred to me) but when you think about it, it is a valid concern: Are the masses of particles the same now as they were at the beginning of the universe?
While the masses of elementary particles like the electrons, quarks, and neutrinos seem unlikely to change with time, it is not so obvious for composite particles like the proton. The masses of these composites are largely made up of the energies of interactions of their constituent quarks and gluons and if the strengths of those forces change with time, then couldn’t the masses also change? This is a question with considerable import since the models of nucleosynthesis following he Big Bang depend on the masses of protons and neutrons.
As with many empirical questions in science, ingenious experimentalists have found a way to get at the answer. A team of physicists have analyzed 17 data points from ten different absorption spectral lines of the methanol molecule CH3OH from a distant galaxy PKS1830–211 and used that to deduce the ratio of the proton mass to the electron mass of those molecules. The galaxy has a redshift of z=0.89 which means that the light was emitted about 7.5 billion years ago. What the researchers find is that mass ratio has changed by less than one part in 10 million compared to the present time.
For those who have access to the journal Physics Review Letters, the paper appeared in the December 6, 2013 issue PRL 111, 231101 (2013) and can be seen online here.