Four Causal Classes of Newtonian Frames

Abstract

The causal characters (spacelike, lightlike, timelike) of the coordinate lines, coordinate surfaces and coordinate hypersurfaces of a coordinate system in Relativity define what is called its causal class. It is known that, in any relativistic space-time, there exist one hundred and ninety nine such causal classes. But in Newtonian physics (where only spacelike and timelike characters exist) the corresponding causal classes have not been discussed until recently. Here it is shown that, in sharp contrast with the relativistic case, in Newtonian space-time the different causal classes of coordinate systems are drastically reduced to four. These causal classes admit simple geometric descriptions and physical interpretations. For example, it is shown that one can generate coordinate systems of the four causal classes by means of the sole linear synchronization group, i.e. by coordinate transformations that only change the origin of time linearly. The relativistic analogs of these examples are also considered.