The temporal widths of a light pulse as measured in different inertial frames are shown to have a relation more complicated than that of a simple time dilatation. The result is compared with the dilatation in the twin paradoxGedanken experiment. The light pulse measurement requires two observers in two different frames. The measurements of the observers are compared. For the twin experiment a comparison is made between two clocks which have undergone different histories between the two points at which their (...) world lines intersect. (shrink)

Measurements in spacetime can be classified as spacelike or timelike, according to the positional and temporal characteristics of the measuring process. A well-known adjective for the spacelike measurement is “synchronous.” To describe the timelike measurement the term “syntopic” is introduced. The use of these terms is illustrated in discussing the measurement of time dilatation and the Lorentz contraction.