Testing electron boost invariance with $2S\mathrm{\text{−}}1S$ hydrogen spectroscopy

There are few good direct laboratory tests of boost invariance for electrons, because the experiments required often involve repeated precision measurements performed at different times of the year. However, existing measurements and remeasurements of the $2S\mathrm{\text{−}}1S$ two-photon transition frequency in ${}^1\mathrm{H}$—which were done to search for a time variation in the fine structure constant—also constitute a measurement of the boost symmetry violation parameter $0.83c_{(TX)}+0.51c_{(TY)}+0.22c_{(TZ)}=(4\pm 8)\times {10}^{-11}$. This is an 8 order of magnitude improvement over preexisting laboratory bounds, and with only one additional measurement, this system could yield a second comparable constraint.