I've gone though a lot of google searches and couldn't find the reason why one way speed of light is undefined. All i know is that it is about synchronizing clocks. So i made my own thought experiment Imagine two clocks A & B. Both are not active I keep A with me and keep B at 1 light second away. First i send a signal to B to activate its clock When A sends a pulse, it's clock starts running from 0 seconds and when it reaches B, it reads 1 second on it. So that we have two clocks that runs at same rate. Now i send a pulse to B when A reads 10s. Since B is 1Ls away, the pulse will reach there in 11s of B time. So now i can subtract 11-10 and that is 1s. d/t=s ie c So what's the problem?