The Principle of Relativity (Original Paper by A. Einstein and H. Minkowski)

In Michelson and Morley's experiment the distance travelled by the beam along OA (that is in a direction parallel to the motion of the platform) is 243 2 while the distance travelled by the beam along OB perpendicular to the direction of motion of the platform. [...] Suppose"4 PRINCIPLE OF RELATIVITY an observer — who is stationed at the origin of coordinates with the clock — associates a ray of light which comes to him through space and gives testimony to the event of which the time is to be estimated — with the corresponding position of the hands of the clock. [...] On account of its adaptability to the stationary system we call the time defined in this way as time of the stationary system." § 2. ON THE RELATIVITY OF LENGTH AND TIME The following reflections are based on the Principle of Relativity and on the Principle of Constancy of the velocity of light both of which we define in the following way :— 1. The laws according to which the nature of physical [...] Let the initial point of one of the systems (k) have a constant velocity in the direction of the X-axis of the other which is stationary system K the motion being also communicated to the rods and clocks in the system (k). [...] It is required to find out the motion of the point relative to the system K. If we now introduce the system of equations in § 3 in the equation of motion of the point we obtain X = )' 11 2 V )1/2 vt' t I + Vttl'4. z = 0 The law of parallelogram of velocities hold up to the first order of approximation.