  Measuring the mass-radius relation of individual white dwarfs is an
empirically challenging task that has been performed for only a few dozen
stars. We measure the white dwarf mass-radius relation using gravitational
redshifts and radii of 137 white dwarfs in wide binaries with main sequence
companions. We obtain the space velocities to these systems using the main
sequence companion, and subtract these Doppler redshifts from the white dwarfs'
apparent motions, isolating their gravitational redshifts. We use Gaia data to
calculate the surface temperatures and radii of these white dwarfs, thereby
deriving an empirical gravitational redshift-radius relation. This work
demonstrates the utility of low-resolution Galactic surveys to measure the
white dwarf equation of state. Our results are consistent with theoretical
models, and represent the largest sample of individual white dwarf
gravitational redshift measurements to date.
