of equation (23) assumes the rôle of this invariant. With respect to an arbitrary inertial system,
may be determined by measurements; with a given unit of measure it is a completely determinate quantity, associated with an arbitrary pair of events.
The invariant
differs, disregarding the number of dimensions, from the corresponding invariant of the Euclidean geometry in the following points. In the Euclidean geometry
is necessarily positive; it vanishes only when the two points concerned come together. On the other hand, from the vanishing of
it cannot be concluded that the two space-time points fall together; the vanishing of this quantity