The One-Way Speed of Light Conundrum - Technology Review: "One interesting question immediately arises: how do you measure the one way speed of light? It turns out there are various methods. One idea involves the emission and absorption of gamma rays by certain kinds of atoms in a solid. The process of absorption is very sensitive to the energy of the gamma rays. So if the speed of light (and therefore its energy) varies with direction, then the rate of absorption ought to change too.
In the 1960s and 70s, various physicists looked for a directional dependence by placing a gamma ray emitter at the edge of a rotating disc and an absorber at the centre. They then looked for any difference in the rate of absorption as the disc rotates but found none."