Outline how the constancy of the speed of light across all
inertial reference frames contributes to the stated
consequence:
If a very short pulse of light originates at space-time
coordinates (x1,y1,z1,t1), the future spacetime coordinates of the
pulse (x2,y2,z2,t2) obey the following relationship
(x2-x1)^2+(y2-y1)^2+(z2-z1)^2-c^2(t2-t1)^2=0 and the same
relationship holds for the pulse of light measured in any other
inertial reference frame.