4 ms·

For clarity, both people would need clocks though, correct? The top comment sort of implies you could measure this effect simply with one clock in the room, som

• lisper 6y ago
  That is correct, you need two clocks. If you think about it, this is true for any time measurement. If you only had one clock, how would you know if it was properly calibrated?
• grayfaced 6y ago
  Even if you had 2 clocks that accurate, would it even be possible to properly synchronize them? If you used an electric signal to tell the clocks to measure, wouldn't that wire length need to be measured to the precision of a hydrogen atom?
  • segfaultbuserr 6y ago
    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.