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 The Sagnac Effect:

An Experimentum Crucis in Favor of the Aether?

by.

(Received January 19, 1914.)

1. The common fundamental idea of the optical experiments aimed at bringing a decision to the question of the existence or non-existence of the aether, and which were actually executed or imagined as executed, is as follows:

If there is an aether, then the optical processes in all systems moving relative to the aether, must proceed in a deviating way; especially, a different value of the speed of light than the standard value $$c$$ must be measured in all such systems.

The amount of change to be expected is determined (besides the direction of motion) by the magnitude of the relative velocity $$v$$ of the system relative to the aether (or when viewed oppositely, by the velocity $$-v$$ of the relative "aether stream" or "aether wind").

The largest relative velocities, which had been accounted for by now, are of order of magnitude 30 km/sec, thus the expected result is given by the ratio $$\tfrac{v}{c}$$ = ca. $$10^{-4}$$.

Yet, until now we were of the conviction, that an effect of this order of magnitude, i.e., of the first order, cannot be measured mainly due to the fact, that it would require observations at two different points of the system.

Thus the decision was relegated to effects of second order (thus in the best case $$\tfrac{v^{2}}{c^{2}}$$ = ca. $$10^{-8}$$). Already showed, that terms of second order should be demonstrated by measurement at one and the same point; the experiment of  became the experimentum crucis.

It was mainly the negative result of this experiment, which finally led many physicists, especially the proponents of the recent