Particle masses may not be eternally fixed. The proton, for example, could be gaining or losing weight relative to the electron. However, a change in these masses would alter the absorption spectra of the molecule methanol. As reported in Physical Review Letters, researchers have combined observations of methanol in a distant galaxy to place a robust constraint on variations in the proton-to-electron mass ratio over a time period of half the age of the Universe.
The proton-to-electron mass ratio could be changing in response to time dependence in the coupling strengths of the strong and electromagnetic forces. Previous tests of mass variation have looked for—but not found—shifts in the absorption lines of hydrogen and ammonia molecules in cosmologically distant objects. However, hydrogen lines are not very sensitive to the ratio, and ammonia lines have to be compared to those from other molecules, which may introduce systematic errors.
Methanol is a better probe because its rotational excitations are sensitive to the proton-to-electron mass ratio to varying, and thus comparable, degrees. Following up on previous work, Julija Bagdonaite of VU University Amsterdam, Netherlands, and her colleagues have analyzed 17 absorption lines from methanol in a galaxy located in the foreground of the quasar PKS1830-211. This absorption is the most distant detection of methanol, corresponding, because of the finite speed of light, to a time of 7.5 billion years ago. The data, which are a compilation from three different radio telescopes, show no signs of wavelength shifts, allowing the team to conclude that the proton-to-electron mass ratio has changed less than one part in 10 million—a limit similar to ammonia constraints but without the uncertainty deriving from the comparison of separate molecules. – Michael Schirber