If I have two clocks in space at rest wrt each other and just a meter apart, I could synchronise them. If they were far enough away from any other mass so gravitational forces are nullified, then if I just let these clocks sit there for a few million years, expansion will separate them but without effecting synchronisation. Then at some time in the future we have pre-set clock A for example, to send a light signal to clock b. We compare the two times and we would have a measure for the one way speed of light. Everything I have read says the one way speed of light is impossible to measure. So where does the above thought experiment break down? Would gravity between the clocks be enough to stop them receding from each other with expansion? I can't think of any thing else.