The „experimentum Crucis”

1− v/c, where c is the speed of light in vacuo. In this case, the frequency of an atomic clock can vary depending on its velocity. The frequency of the clock can be influenced by other external factors as well. The Pound–Rebka experiments [2] demonstrated the gravitation potential effect of this variable. Therefore the conditions of the present experiment were chosen in such a way that except for the velocity, all other factors affecting the frequency of the clocks remained constant. Inertial atomic clocks moving with different velocities have constant frequencies as long as external factors remain constant. While the direction and velocity of a moving atomic clock („B”) in the inertial system K corresponds to the direction and velocity of the clock („A”) examined in the Ives–Stilwell experiment, the frequency of the former clock is definable without taking any additional measurements, assuming that the clocks are of the same type (Fig. 1).