In a different inertial frame, say with coordinates ( t ′ , x ′ , y ′ , z ′ ) , the spacetime interval d s ′ can be written in a same form as above. Because of the constancy of speed of light, the light events in all inertial frames belong to zero interval, d s = d s ′ = 0 . For any other infinitesimal event where d s ≠ ≠ 0 , one can prove that d s 2 = d s ′ 2 which in turn upon integration leads to s = s ′ . The invariance of the spacetime interval between the same events for all inertial frames of reference is one of the fundamental results of special theory of relativity.