Indications of a change in the proton-to-electron mass ratio have shown up in comparisons of the spectra of hydrogen gas as recorded in a lab with spectra of light coming from hydrogen clouds at the distance of quasars. This is another of those tests of so-called physical constants that might not be absolutely constant. For example, the steadiness of the fine structure constant (denoted by the letter alpha), defined as the square of the electron’s charge divided by the speed of light times Planck’s constant, has been in dispute (see PNU 410). Some tests say alpha is changing, others say it isn’t.
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