A pair of widely-separated ground-based gravitational wave detectors, LIGO is a masterpiece of complex and sophisticated engineering. Super-stabilized lasers, enormous vacuum systems, the purest optics, unprecedented vibration isolation, and servo controls all work symbiotically for one singular purpose: To sense the ephemeral passage of a gravitational wave. The whole of LIGO is so much greater than the sum of its individual parts. Nevertheless, LIGO’s parts are remarkable in and of themselves. The pages in this section describe some of LIGO's most important mechanical systems in detail. If you're already relatively familiar with how LIGO works, this is the place for you to delve ever deeper into LIGO's remarkable engineering.
Why Gravitational Waves?
In 1993, astrophysicists Russell Hulse and Joseph Taylor received the Nobel Prize in Physics for their 1974 discovery of a binary pair of neutron stars 21,000 light years from Earth. Seven years later, after tracking the radio emissions of one star in the pair over a period of years, Taylor and two other colleagues (Joel Weisberg and Lee Fowler) noted that the time it takes for the two stars to orbit each other was decreasing exactly in a way that general relativity predicted if the two stars were radiating gravitational waves. Analyses of other binary neutron star systems confirmed this effect firmly concluding that gravitational waves were not just theoretical.
More than 40 years later, on September 14, 2015, gravitational waves were directly detected by LIGO's interferometers. This accomplishment was achieved because LIGO is designed to sense the infinitesimally faint whispers of gravitational waves through the imprint of that radiation on laser light. How faint are these whispers? LIGO must detect a change in arm length of about 10-19 m (10,000 times smaller than a proton). Achieving this degree of sensitivity requires a remarkable combination of technological innovations in precision lasers, vacuum technology, and advanced optical and mechanical systems.
While LIGO’s search for gravitational waves relies heavily on its most prominent systems (lasers, mirrors, and photodetectors), it is the extraordinary supporting engineering and infrastructure that makes LIGO’s task even remotely possible. Each of LIGO’s complex engineering systems, though intrinsically different, work together for one singular goal: eliminating noise. Noise, such as physical vibrations from the environment (from cars driving on nearby roads to waves crashing on distant ocean shores), quantum fluctuations within the laser itself, nanometer-scale changes in the shapes of optics, even molecules crossing the path of the laser could hamper LIGO's efforts to make its sensitive detections. Working together, LIGO’s engineering systems and subsystems keep enough of this noise at bay for LIGO to hear the faintest whispers of gravitational waves from the depths of space.
The links below contain greater detail of some of LIGO's most impressive engineering and technology components.