where
= velocity of light in vacuo.
The new principle of relativity set up by Einstein is as follows: Systems that are moving uniformly and rectilinearly with respect to each other are completely equivalent for the description of physical events. The equations of transformation that allow us to pass from the co-ordinates of one such system to those of another possible system, however, are not then (for the case when both systems are moving parallel to their
-axes with the constant velocity
):—
