A Technician Works on 1 of LIGO's Optics (IMAGE) National Science Foundation Caption A technician works on one of LIGO's optics. At each observatory, the two-and-a-half-mile long L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down the arms. The beams are used to monitor the distance between mirrors precisely positioned at the ends of the arms. According to Einstein's theory, the distance between the mirrors will change when a gravitational wave passed by the detector. Credit LIGO Laboratory Usage Restrictions Use with credit line License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.