Advancing Basic Science for Humanity
How LIGO Works
LIGO is the world's largest gravitational wave observatory. It consists of two detectors situated 1,865 miles (3,002 kilometers) apart in isolated regions in the states of Washington and Louisiana. Each L-shaped facility has two arms positioned at right angles to each other and running 2.5 miles (4 kilometers) from a central building. Lasers are beamed down each arm and bounced back by mirrors, essentially acting as a ruler for the arm. Sensitive detectors can tell if the length of the arms of a LIGO detector varies by as little as 1/10,000 the width of a proton, representing the incredibly small scale of the effects imparted by passing gravitational waves. LIGO has two observatories to act as a check on the other to rule out that a potential gravitational-wave signal detection is not due to a local, terrestrial disturbance; both facilities will detect a true gravitational wave moving at the speed of light nearly simultaneously. Although the twin LIGO facilities act as a single observatory, they are not designed for "observing" in the conventional sense. Instead of eyes, the facilities can be thought more of as "ears" listening for gravitational waves, or even as a skin trying to "feel" the slightest of movements.
A Giant Interferometer1
LIGO's observatories are technically known as interferometers. Used in many scientific fields, interferometers merge two or more sources of light in order to create an interference pattern. Such patterns result from overlapping waves of light. When the peaks of two waves of light overlap, they combine to form a larger peak (constructive interference). In contrast, when the valley of one light wave overlaps with the peak of another light wave, the two waves cancel each other out (destructive interference). Interference patterns provide scientists with clues about the properties of the sources that emitted the light.
Within LIGO, the lasers beamed down its arms bounce back and are set to cancel each other out completely. As a result, no light reaches another LIGO component called a photodetector. If, however, a gravitational wave were to pass through the LIGO facility, it would stretch one detector arm and compress the other, throwing off this perfect destructive interference. Some light would then reach the photodetector. The pattern of this light would provide information about the changes the arms underwent, and thus reveal properties about the incident gravitational waves and their source.
There are many things besides gravitational waves that can upset LIGO’s sensitive instrumentation, including vehicles on nearby roads, seismic waves from earthquakes, slight temperature differences between the detector arms and even the tidal tugging of the Sun and the Moon. Therefore, the experiment's designers went to extraordinary lengths to compensate for these myriad vibrations and forces.
One countermeasure is called seismic isolation. Devices detect and then immediately offset ground vibrations, keeping LIGO’s components, such as its mirrors, at rest. Those mirrors are suspended by a special four-stage pendulum to further cancel external noises that are not from astrophysical sources.
The tubes containing the lasers, mirrors and other LIGO components are also kept at an extreme vacuum. In fact, LIGO hosts one of the largest and purest "empty" spaces on Earth. Removing as many molecules as possible from the tubes cuts down on mirror deformations, laser distortions and temperature fluctuations manifesting as air currents.
LIGO's optics system of lasers, mirrors and photodetectors have also been engineered with incredible precision. For example, the mirrors are made of precisely polished, highly pure fused silica.
Advanced LIGO (aLIGO)3
The initial technology deployed for LIGO was sensitive to movement of 1/1000 the diameter of a proton, but with an upgrade begun in the 2010s, LIGO's sensitivity was boosted 10-fold. The many enhancements included increasing the power of the lasers from 10 watts to 200 watts and mirror seismic isolation technology improvements.
Overall, aLIGO will be able to detect possible gravitational wave-producing events three times farther away than the initial LIGO setup. Accordingly, a far larger volume of space will now be within "earshot" of the LIGO project, with the opportunity to catch far more potential sources of space-time ripples.