Page:The Meaning of Relativity - Albert Einstein (1922).djvu/53

Rh only when the two points concerned come together, the other hand, from the vanishing of



it cannot be concluded that the two space-time points fall together; the vanishing of this quantity $$s^2$$, is the invariant condition that the two space-time points can be connected by a light signal in vacuo. If $$P$$ is a point