Consider two person one is travelling in a space ship and one is on the moon, now in the space ship there is a device where a light beam or photon is bouncing between two mirrors now when the space ship moves shouldnt the light beam or photon stay where it is relative to the observer on the moon meaning that space should should go ahead a the photon would not move along with the space ship
should'nt the light beam or photon stay where it is
Light, electromagnetic wave, is a classical manifestation of the superposition of zillions of photons. Photons are elementary particles. Both move with velocity c , and cannot "stay" anyplace. c is an enormous velocity with respect to motions of moons and spaceships.
At relativistic speeds the geometry of the reflecting devise will change relative to the observer on the moon due to length contraction.