3.2 Building LIGO
A simple concept with a challenging implementation
- A simple way to measure gravitational waves would be to shine a laser at a mirror and time how long it takes to return. A passing gravitational wave will change the distance the light travels, which will be detected as a change in travel time. Unfortunately, there are no clocks precise enough to make the measurements needed to detect gravitational waves with this method.
- Instead, detectors like LIGO use interferometry to observe gravitational waves. A single laser is split and sent down two paths at right angles to each other. When they recombine, they produce a pattern of interference that changes depending on the difference in the lengths of the two paths.
- Due to its quadrupolar nature, when a gravitational wave passes by it will change the two path lengths by different amounts, producing a corresponding change in the interference pattern.
- This is much better than the timing method because it is a relative measurement–there is no need for an absolutely precise clock since the laser is referenced to itself. As long as the beams are traveling different distances, there will be a signal.
The birth of LIGO
- The Laser Interferometer Gravitational-Wave Observatory (LIGO) was conceived by Rai Weiss of MIT and was designed between 1968 and 1972.
- His design called for 4km long laser paths, in order to mitigate the effects of the extremely small strain. 4km is the longest straight path that can be built without having to tunnel through the Earth. At that scale spacetime will stretch by about 10-18m.
- Construction on LIGO began in 1994 and completed in 1999. Today LIGO is comprised of two detectors, one in Louisiana and one in Washington state.
- The two US-based LIGO detectors are part of a global network of detectors that includes GEO600 and VIRGO, as well as the planned KAGRA, LIGO-India, and space-based LISA.
Achieving functional sensitivity
- LIGO must make sure that only gravitational waves cause detections. That means isolating the detectors from any ambient vibrations or motion, which could alter the laser travel time and produce a signal.
- The mirrors are attached to both passive and active seismic isolation systems. The passive systems use springs and operate like shock absorbers on a car. The active systems consists of seismic detectors that measure ambient vibrations and use small motors to cancel them out before they influence the mirrors.
- The mirrors are suspended from a series of four pendulums, adding another layer of passive isolation. A pendulum is a naturally good isolation system that prevents vibrations above its (very low) natural frequency from reaching the mirror.
- These mirrors are housed inside enormous vacuum chambers so that atmospheric gases do not interfere with gravitational wave measurements.
- The original LIGO reached design sensitivity in 2007 and could detect strains on the order of 10-21 in the frequency range of 10Hz-10kHz. However, it did not detect anything during its operation.
- In 2008 the LIGO team began to design and build the next phase of the detector, called Advanced LIGO. This effort aims to achieve a significantly better sensitivity as well as a broader frequency range that it can search.
- To go from initial LIGO to Advanced LIGO requires three major upgrades: better seismic isolation to improve low-frequency sensitivity, better materials to eliminate thermal vibrations in the mirrors for improved intermediate frequency sensitivity, and increased laser power to improve high frequency sensitivity.
- Not all of the Advanced LIGO upgrades have been implemented yet, but the detectors began running at increased sensitivity in 2015. Full design sensitivity for Advanced LIGO will be achieved by 2021.