In electronics, there's a trick to measure the delay between two signals when the length of your cable is unknown. First, Connect Cable 1 to Input 1, Cable 2 to

In electronics, there's a trick to measure the delay between two signals when the length of your cable is unknown. First, Connect Cable 1 to Input 1, Cable 2 to Input 2, record the delay between Signal 1 and Signal 2. Next, swap Cable 1 and Cable 2, record the delay again. Then, add two delays together, delays in both cables are cancelled, finally divide it by two to get the true signal delay. You can calculate the delay of both cables if you want, and calibrate it out for your later measurements. It works because it assumes swapping the cable has no effect on the delay. But the same cannot be said for your hypothetical atomic clock - creating an electronic switch with matched delay is an obvious challenge, but then, even minor physical movement of the cable will change its delay.