...The two “measuring sticks” are actually two perpendicular tubes with light beams shooting through them and bouncing off mirrors at each end. The beams then meet at the center and combine in such away as to cancel each other out (180 degrees out of phase). Any change at all in the lengths of EITHER tube (measuring stick) will result in the beams NOT perfectly combining at the center and thereby no longer canceling each other out.
The Michelson-Morley experiments.