ABSTRACT
Chapter 27 introduced the spacetime distance and the relevant transformations (the Lorentz transformations) of coordinates that leave this distance unchanged: Given two events the spacetime distance between those events is the same for all observers. We also saw that, except for a factor of c, the spacetime distance is simply the proper time interval between the two events, where as the reader may recall, the proper time is the time measured by a clock that is present at both events. In this chapter, we use the proper time to define some important relativistic quantities.
