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This animation shows the trajectory of light beams that move with a 2 mirror (Fabry-Perot) interferometer as the interferometer moves transversely relative to a fixed frame observer. The separation beween the mirrors is L so in the moving frame the time between mirror intercepts is t0=L/c. Since the speed of light is always c and the trajectory that he sees is slanted because of the motion it will take longer to complete a round trip between the mirrors than if the mirror were not moving. The velocity component along the y direction will be

`v_y=sqrt(c^2-v_x^2)`

so that the mirror intercept time increase factor is`t/t_0=c/sqrt(c^2-v_x^2) = 1/sqrt(1-v_x^2/c^2)`

which is usually called gamma in relativity convention. The term `v_x/c` is usually called `beta`. In the animation, the trace with the black dot at its leading end is the one that would be seen by an observer moving at the same velocity as the interferometer. The trace with the red dot at the leading end is what an observer watching from outside the moving interferometer would see. Since we base our time scale on the times between mirror intercepts, the outside observer time `t(v_x)` is considerably greater than the time t(0) as expected by the longer pathlengths. For a parallel mirror interferometer like this, it is usually the time for light to go between mirror intercepts that is important. It is clear that the black dot gets ahead of the red dot so these times appear different in this animation. However we also see that the`n_("interferometer")=[(B-A)/v_x](c/L)`

while the number in the laboratory frame is`n_("laboratory")=[(B-A)/v_x](c/L)sqrt(1-v_x^2/(c^2)`

The lab frame observer would conclude that time passes more slowly (less ticks between events) when the interferometer is moving. This agrees with results for time passage (time dilation) of special relativity.