© Blayne Heckel.
Originally devised as part of the fruitless 19th century effort to identify the "ether" that supposedly suffused space, the Michelson interferometer now finds application in a 21st century experiment: the search for gravitational waves. The diagram shows the original version of the instrument. A beam splitter divides laser light entering the input port into a transmitted beam and a reflected beam, perpendicular to each other. At the end of each beam's path, a mirror reflects the light back toward the beam splitter. If the two beams' paths have exactly the same length, the beams' electric fields oscillate in phase when the light returns to the beam splitter. The beams recombine to produce a beam that exits the beam splitter along the output port. If the two paths differ in length by half a wavelength, they are out of phase. In that case, they interfere destructively at the beam splitter and no light exits from the output port. The intensity of light leaving the output port changes from a maximum to zero as the relative distance to the end mirrors changes by a quarter of a wavelength—about 2.5 x 10-7 meters for typical laser light. Precisely measuring this light intensity allows experimenters to detect even smaller relative displacements of the mirrors. Any passing gravitational wave should compress spacetime in one direction and stretch it out in the perpendicular direction. Physicists believe that a modern version of the Michelson interferometer has the precise measuring ability to detect the difference between the two. (Unit: 3)