The special theory of relativity was originally concerned with the measurement of space and time by means of rigid rods and spring-driven clocks; and with the relationship of such measurements to observations made with the aid of electromagnetic waves. Later on, other propositions were developed in connection with this theory, such as modifications of the laws of conservation of momentum and energy in impact processes. It is the part of the theory concerned with impact that we intend to emphasize here; but we first must give a short account of the problem of space and time measurement.

Before one can measure distances in space one must adopt a “unit distance”. To this end one should select two definite points and adopt the distance between them as the unit. Next one should employ a rigid rod whose length is just the unit distance, as verified by placing it between the two selected points. The distance between any two points may then be established by laying off the unit rod on the straight line connecting these points. To measure the time elapsed between two events one should proceed similarly with the aid of a spring that oscillates without external interference; such an oscillating spring will be called a “steady clock”. In making these measurements it is taken for granted that it does not matter at what time the distance between the points is measured and at what place the time elapsed between the two events is observed.