LIGO — Laser Interferometer Gravitational-Wave Observatory

Design and principal components

Each LIGO site is a Michelson-type laser interferometer with two long perpendicular arms. Key elements include:

• High-power, frequency-stable laser systems that provide the coherent light source (laser subsystem).
• A beam splitter that sends light down two orthogonal arms and recombines it to sense differential length changes (interferometer optics).
• Arm cavities formed by suspended, highly reflective mirrors (test masses) to increase the effective path length and sensitivity (mirror and suspension).
• Ultra-high vacuum tubes that house the laser beams to eliminate air disturbances (vacuum system).
• Seismic isolation and active controls to reduce ground and environmental noise (isolation and controls).

How LIGO detects gravitational waves

LIGO measures minute changes in the relative lengths of its two arms caused by passing gravitational waves. When a wave stretches space in one direction and compresses it in the perpendicular direction, the travel time of the laser beams in the two arms shifts slightly. Those changes alter the interference pattern when the beams recombine, producing a tiny signal at the photodetector. To enhance sensitivity the instrument uses optical cavities, power and signal recycling techniques and carefully calibrated readout electronics. For technical background see interferometry basics.

History and milestones

Conceived in the late 20th century and constructed with major funding from the NSF, LIGO underwent an initial science program and later a substantial upgrade to Advanced LIGO. The upgraded detectors achieved the sensitivity necessary to make the first direct detection of gravitational waves from a binary black hole merger; that signal was produced in 2015 and announced publicly in 2016. The founding and early development of the project involved many scientists and institutions; historical context and biographies can be found at project history.

Scientific impact and examples

LIGO observations have opened a new observational window on the universe, enabling measurements of black hole and neutron star mergers, tests of general relativity in strong gravity, and new ways to probe the rate and properties of compact binaries. Notable detections and multi-messenger follow-ups illustrate the observatory's importance; selected results and data releases are available through data and results. LIGO's discoveries have spurred developments in astrophysics, cosmology and instrumentation.

Collaboration, network and future directions

LIGO is operated as part of an international network that includes other interferometers, enabling source localization and improved confidence in detections. The collaborative structure, upgrade roadmaps and outreach efforts are documented by the collaboration and partners (collaboration and outreach). Future work focuses on sensitivity improvements, longer observing runs, and integration with new detectors to expand the accessible volume of the universe.