Philosophy of Physics: A Very Short Introduction (Very Short Introductions)

by David Wallace

In Newtonian spacetime (i.e., spacetime before the lesson of the relativity principle), for any two events we could say separately how far apart they are in space and in time. In Galilean spacetime, we can still say how far apart two events are in time, but their space separation is undefined (except relative to some arbitrary inertial frame) unless they are simultaneous. In Minkowski spacetime, neither spatial nor temporal distance between events really makes sense unless we fix an inertial frame. They are replaced by a unified concept: spacetime distance or, as it is usually called, the interval.

replacing inertial frame with spacetime distance / interval Hermann Minkowski