Page:Zur Elektrodynamik bewegter Systeme II.djvu/5

 the electrodynamics of the moving system appears (with respect to a co-moving observer) to be influenced by motion only in so far, as the observer is able to distinguish local time $$t'$$ from general time $$t$$. The difference of both quantities is, according to (6), a fraction of the light propagation-time corresponding to vector $$r$$, which in the worst case ($$r$$ parallel to $$p$$) is equal to the ratio of the translational velocity to the speed of light.

Let us apply this to the motion of Earth: Everywhere, where the propagation of radiation is not the object of measurement, we define identical moments of time at different points of Earth's surface, by treating the propagation of light as timeless. In optics, however, we define these identical moments of time by assuming, that the propagation takes place in spherical waves for every relatively resting and isotropic medium. This means: the "time" which actually serves us for the representation of terrestrial process, is the "local time" $$t'$$, for which the equations I'b to IVb hold, – not the "general time" $$t$$.

What is required to experimentally distinguish $$t'$$ from $$t$$, can be shown by a proposal which recently was made by "for the decision of the question, as to whether the luminiferous aether is moving with Earth or not." Through the gaps of two gears whose common axis has the direction of Earth's motion, light of the same intensity shall be sent through in both directions. Then both gears shall be set in rotation with equal angular velocity. concludes: If the aether is at rest, then the propagation of light is different for the two paths; – the arriving light hits the gear at the end of the path in different locations upon both stations; – the intensities must have become different.

Now it is clear that for this experiment, not the same angular velocity is required as thinks, but equal collective rotation from the moment of observation at rest until the moment of observation at rotation. If the two collective rotations are equal for equal "general times" t of both stations, then one obtains a difference in brightness in the case of "dragged aether"