Black Hole Blues and Other Songs from Outer Space

by Janna Levin

Instead of bouncing light along one arm, an interferometer sends light down two arms arranged in an L. Laser light is split into two beams, so that one beam travels along one arm of the L and the other travels along the orthogonal arm of the L. Each beam bounces off a mirror at the far end and returns down the respective arm to interfere back at the original apex. The recombined light is then split into two outputs. If the light travels the same distance in each direction, then the light in one output will recombine perfectly so that the output is bright. Light in the other output will combine in perfect cancellation so that the output is dark. If the arms are not the same length the light will come together but imperfectly, out of sync in a sense. The light will interfere with itself.