 The Special Theory of Relativity (Einstein, 1905) was presented as a geometric theory of spacetime in Minkowski  (1908).[5]  For our purposes, the key change from Newtonian spacetime to Minkowski spacetime is that in the latter it is no longer the case that the temporal interval between any two points or events in the spacetime, \(p\) and \(q\), is a well-defined quantity. In fact, the temporal interval between two points in the spacetime (and hence the simultaneity of two points in the spacetime) is not defined at all until a coordinate system or frame of reference (with some arbitrarily chosen spacetime point as origin of the frame) is chosen. A peculiar feature of special relativity (as opposed Newtonian physics) is that each inertial frame defined by an “observer” passing through the chosen origin and moving with some constant non-zero velocity that is less than the speed of light (the only coordinate systems or frames of reference that will be considered in our discussion of the special theory) picks out a distinct set of points as simultaneous with the origin. This feature of special relativity is called the relativity of simultaneity.