Experimental Search for Anisotropy in the Speed of Light

A triangular interferometer passes two beams around the interferometer in opposite directions. A portion of the optical path contains a piece of glass. If the phase velocity of light is anisotropic, the fringe pattern will shift as the interferometer is rotated relative to the fixed stars. The interferometer is sensitive to anisotropies that behave as the first or third Legendre polynomials, $P_1\left(cos\phi \right)$ and $P_3\left(cos\phi \right)$, where $\phi$ is the angle between a preferred direction in space and the direction of light propagation. The maximum change in the speed of light is less than 0.03 ± 2.5 cm/sec for the $P_1$ anistropy, and is less than 0.7 ± 0.45 cm/sec for the $P_1$ anisotropy. If an ether wind can be described by a Fresnel dragging coefficient when light passes through glass, the ether wind would be detectable by this experiment. This experiment sets an upper limit of 0.045 ± 3.8 cm/sec for the ether wind at the surface of the earth. This is about one-millionth of the earth's orbital velocity.