"Einstein knew that distances... and lengths of time are not objective, and he wanted to know what is objective. The S, this is objectiveS^2 = x^2 - t^2The spacetime separation between here-now and there-then is objective. The spacetime separation between two events, here-now and there-then. That's objective. So this [x] is the distance between here and there, and this [t] is the time between now and then."