in all inertial frames for events connected by light signals. The quantity on the left is called the spacetime interval between events a 1 = (t 1, x 1, y 1, z 1) and a 2 = (t 2, x 2, y 2, z 2). The interval between any two events, not necessarily separated by light signals, is in fact invariant, i.e., independent of the state of relative motion of observers in different inertial frames, as is shown using homogeneity and isotropy of space. The transformation sought after thus must possess the property that: