In a rocket, time is measured by light bars consisting of two mirrors located at a distance l from each other. The number of vibrations of the light beam between these mirrors counts down the time in this rocket. How will the course of these clocks change based on observations at the station relative to which the rocket is moving at a speed of βc? Show that the distance between the mirrors, if the axis of the walkers is directed along the velocity βc, will decrease by a factor of γ = 1−β21.