Abstract
We report the detection of redshifted neutral hydrogen in absorption against the BL LAC type quasar AO 0235+164 is reported. The line occurs at 932.116 +- .005 MHz corresponding to a redshift z=0.52385 +- 0.00001, a value identical, to within the errors, to the optically derived redshift. This is the first instance in which both radio and optical high-redshift absorption lines are measured. The coincidence of these redshifts yields stringent limits to the constancy of the wavelength invariance of the Doppler expression, supports the optical line identifications, and yields limits on the variation of certain atomic quantities over cosmic time scales. The Mg/H abundance is derived as is a lower limit to the location of the absorbing cloud with respect to the quasar. (AIP)
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