Viewed from above, the two arms form an L shape. Laser light enters the arms through a beam splitter located at the corner of the L, dividing the light between the two arms. The light is allowed to bounce between the mirrors repeatedly before it returns to the beam splitter. If the arms have identical lengths, the returning light beams are in phase and the light falling on the photo detector will be a bright spot. If there is any difference between the lengths, the beams will be out of phase and the light falling on the photo detector will be reduced.

The laser interferometer is like a microphone that converts gravitational waves into electrical signals. LlGO requires at least two detectors, operated in unison, to rule out spurious signals and confirm that a gravitational wave has passed through Earth. The observatory, operational since 2002, has yet to detect the elusive, still-theoretical waves. The Advanced LIGO Project is expected to provide improved sensitivity and the first hard evidence. When LIGO was first proposed, scientists foresaw that advances in laser technology and mirrors would allow for even greater sensitivity. The Advanced LIGO Project became a natural upgrade for the observatory. "The first several hours of observation with new instruments will equal almost the first year of observation with LIGO's current instruments," said Albert Lazzarini, deputy director of LIGO at Caltech. We can probe something like several hundred galaxies out to the Virgo cluster (59 million light-years away) with LIGO, but increase that by a factor of one thousand and you go to the cosmological regime of measuring many tens of thousands of galaxies."