Ground-based Gravitational Wave Detectors

Gravitational waves are ripples in the fabric of space-time caused by energetic and violent processes in the Universe. These ripples travel at the speed of light through the Universe and hold clues to the nature of gravity itself.

Existence of gravitational waves came to light in 1974. Astronomers working at the Arecibo Observatory in Puerto Rico discovered two extremely dense, heavy stars rotating around each other. Astronomers began to measure how the period of the star’s orbits changed over time.

After 8 years, it was determined that the stars were getting closer to each other at precisely the rate predicted by general relativity, providing evidence that the rotating stars were emitting gravitational waves. Since this discovery, scientists have designed and built a network of gravitational wave detectors, which include; LIGO and its upgrade Advanced LIGO; GEO600/HF; Virgo/Advanced Virgo; and KAGRA, to directly detect and analyse gravitational waves. Scientists have also begun designing the next generation of detectors, such as the Einstein Telescope to probe gravitational waves further.

The UK has a large involvement in the detection of gravitational waves in which it plays a leading role. Furthermore, the technological advancements and effort in designing gravitational wave detectors will have a positive impact on the UK.


LIGO and Advanced LIGO


LIGO Hanford Observatory (LHO) in aerial view. The 4-km interferometer arms are shown with the 5 main buildings along the orthogonal arm layout
(Credit: Advanced LIGO)

In 1916, Albert Einstein predicted that gravity travelled outward from a source as ripples in the curvature of spacetime that propagate as waves. Nearly 100 years later, the initial LIGO gravitational wave detectors set out to prove Einstein’s prediction. Built in 1999, and beginning their first search in 2002, the LIGO detectors completed observations in 2007 and were unsuccessful in detecting gravitational waves. It was therefore essential to improve the sensitivity of the instruments to enable detection of gravitational waves.

Advanced LIGO is ten times more sensitive, and over a much broader frequency band, than the initial LIGO detectors, and on the 14th September 2015, Advanced LIGO directly detected gravitational waves for the very first time!

The detection of gravitational waves has opened up a whole new window of Particle Astrophysics and offers an opportunity to see the Universe from a whole new perspective. Advanced LIGO is a National Science Foundation project (NSF) and the facility is operated by Caltech and MIT. The LIGO upgrade was funded by the NSF with financial and technical contributions from the UK’s Science and Technology Facilities Council (STFC), the Max Planck Society of Germany, and the Australian Research Council (ARC). STFC currently supports the operation of the Advanced LIGO detectors through computational support from UK institutions.

Advanced LIGO consists of three 4km laser interferometric gravitational wave detectors, one of which is being transferred to India. The remaining two detectors; The LIGO Hanford Observatory (LHO) based on the U.S. Department of Energy Hanford site in eastern Washington; and LIGO Livingston Observatory (LLO) located between Baton Rouge and New Orleans, Louisiana, were used in the first detection of gravitational waves.

Each detector contains two long 4km arms arranged in an “L” shape. These instruments act as “antennae” to detect gravitational waves. When a gravitational wave passes through the Universe, it stretches and contracts objects in space. The gravitational wave that was detected on the 14th September 2015 stretched and contracted the Earth, thus stretching and contracting the “antennae” like arms, resulting in the detection of gravitational waves.

The UK played a key role in the Advanced LIGO upgrade which was fundamental to the detection of gravitational waves. Glasgow led a UK team, consisting of the Universities of Birmingham, Strathclyde, West of Scotland and Cardiff, and STFC Rutherford Appleton Laboratory that created and delivered the ultra-low noise suspended mirrors which were critical to making the detection. The UK also designed and supplied associated optical components, all sensors, actuators, electronics and the expertise required to install and interface the suspensions to the rest of the detector.

The Advanced LIGO UK project was a complete success and with subsequent funding from STFC, the UK team continues to have responsibilities in operating and commissioning the detectors. Further information on Advanced LIGO, gravitational waves and how they are detected can be found here.




Two-dimensional illustration of how mass in the Universe distorts space-time
(Credit: NASA)

GEO600 is a 600m long gravitational wave interferometer situated between Hannover and Hildesheim in Germany. GEO600 has a history of demonstrating innovative technology advances, such as the first use of squeezed light to reduce background noise. GEO600 is funded by the Max Planck Society and STFC; and was jointly constructed by Germany and the UK. GEO600 is operated as part of the worldwide network of gravitational wave detectors; LIGO (USA); VIRGO (Italy); and KAGRA (Japan), to acquire data of scientific interest.

In 2009, upgrades for GEO600 began under the GEO-HF upgrade programme. This upgrade programme implemented a squeezed light technique, the inclusion of an output mode cleaner, an increase of employed laser power and a change in signal-recycling. All of these upgrades helped to improve the sensitivity of GEO600. GEO600 played a major role in the first detection of gravitational waves where the increased sensitivity of the LIGO detectors were based on many technologies developed and tested at GEO600. GEO600 provides the dual benefit of maintaining network coverage in “astrowatch” mode, covering the gravitational wave sky whilst the larger detectors are being commissioned between science runs and also acting as a sophisticated test bed for novel techniques.


Virgo and Advanced Virgo


Aerial view of Virgo, looking north
(Credit: The Virgo Collaboration)

Virgo is a 3km detector located in Cascina near Pisa, Italy, and commenced science runs in 2007. Virgo was designed and built by a collaboration between the French Centre National de la Recherche Scientific (CNRS) and the Italian Istituto Nazionale di Fisica Nucleare (INFN). The frequency range and high sensitivity of Virgo should allow detection of gravitational radiation produced by supernovae and coalescence of binary systems in the Milky Way and outer galaxies. The initial Virgo detector observed the sky between 2007 and 2011 together with LIGO. Virgo is currently undergoing a major upgrade, Advanced Virgo, and will be back online in 2016. Further information on Virgo can be found here.

The Advanced LIGO and Virgo detectors are currently being commissioned. The second observing run, O2, should start in the summer of 2016 and take data for six months; Advanced Virgo is expected to be online at this time, albeit with a lower range. This will be followed by a commissioning period to further improve the range. The third science run O3 will begin during 2017 and last for nine months. Final commissioning will take place in 2019.


Einstein Telescope – Future Generation

Einstein Telescope

Design of the Einstein Telescope
(Credit: Max-Planck-Gesellschaft, Munich)

The Einstein Telescope (ET) is a design for a third generation gravitational wave detector. As per Advanced LIGO, Virgo and GEO600/HF, ET is based on the measurements of tiny changes to laser beams that are being passed down connected arms several kilometres long. The ET project plans to build an observatory that hosts three gravitational wave detectors, overcoming limitations of current infrastructures composed by a single detector. The configuration will be designed to allow the observatory to evolve by accommodating successive upgrades or replacement components that can take advantage of future developments in interferometry and also respond to science objectives.

Although the current detectors will evolve over the coming years, the expected final sensitivity is not enough for high precision astrophysical studies. For this reason, a new generation of gravitational wave observatories, such as ET, were defined. The ET aims to be far more sensitive than current instruments and is currently in its Research and Development phase, with construction possibly starting in 2018. Further information on ET can be found here.


UK involvement

Gravitational wave research has existed in the UK for more than 40 years and has been funded by STFC or its predecessor councils. Gravitational wave research covers a programme that maximises the discovery potential and subsequent exploitation of a worldwide network of gravitational wave detectors. This includes pioneering novel technologies for making gravitational detections, leading the analysis of data from the global gravitational wave network and astrophysical modelling of gravitational wave sources. Over 100 UK researchers are involved in gravitational wave research from the Universities of Glasgow, Cardiff, Birmingham, Southampton, Cambridge, Sheffield, West Scotland and Strathclyde, the Royal Observatory and Rutherford Appleton Laboratories. Investing in gravitational waves allows UK researchers to push the boundaries at GEO600, Virgo, LIGO and ET.



There have been various technological advancements resulting from the ambition to detect gravitational waves. Some of these technologies, although originally designed specifically for gravitational wave detection, have branched out and are being used in other industries. These include the use of lasers in producing light emitting diodes, computer chips and smart phone circuit boards. Algorithms used to detect the characteristic chirp of gravitational waves are now being used to detect chirps in sonar and radar. There is also scope to use technology from gravitational research to detect hydrocarbons and the prediction of natural disasters, such as earthquakes.

In 2012 the development of oxide-bonding techniques for Advanced LIGO suspensions led to an STFC technology transfer award to SpanOptic Ltd, and a UK patent for bonding silicon carbide.

The Triana open-source software was developed for analysing data from GEO600 and is now on many industrial and interdisciplinary collaborative projects, including; grid infrastructure; medical computing; data mining; and modelling in engineering.

The procurement of Advanced LIGO lasers, suspension and optical components has returned more than £2.5M to UK industry through contracts with UK companies such as SpanOptic Ltd, LaserLines Ltd and Crystan Ltd. Several start-up companies have also been established from breakthroughs in precision optical coatings and sensors.

At the core of gravitational wave research are PhD students and Postdoctoral Research Assistants who are developing their scientific research and project management skills. These individuals are provided with training in media relations, creativity, business skills and public outreach. Investing in the next generation of scientists is key to the future development of state-of-the-art technology. Gravitational wave research is also involved with outreach events for the public, inspiring students and helping the general public to understand what research is taking place and why.

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