The → phase difference between two light waves
moving in opposite directions along a closed circular loop when the loop
is rotating. More specifically, consider
a beam of light split into two beams which are then
allowed to propagate in two opposite directions along the rim of a rotating
disk. When they are recombined, a phase difference
occurs between them. The position of the
→ interference fringes is dependent on the
→ angular velocity
of the setup. This → relativistic effect
illustrates the impossibility of synchronizing clocks situated in a
rotating → reference frame, as described by
Einstein in 1905. The Sagnac effect is used, for example, in optical
gyroscopes installed in airplanes or in devices used for measuring the
Earth rotation.
The Sagnac effect is very important for the correct working of the
→ Global Positioning System.
See also: Named after Georges Sagnac (1869-1928), French physicist,
who discovered the phenomenon in 1913;
→ effect.
