The UK: Taking gravitational wave detection to infinity (and beyond)
It’s not just massive gravitational objects that have been making waves in the field of gravitational wave research, the UK has also been making waves for more than 40 years (now that’s how you strain a metaphor).
After a three-year shut down, the international LIGO-Virgo-KAGRA network of gravitational wave detectors went back online recently following a series of upgrades that make it 30 per cent more sensitive. The UK’s contribution to this network is funded by the Science and Facilities Technology Council (STFC) so we thought this would be a good time to explore the network and our contribution to it.
Need to bone up on gravitational wave science? Check out our Q&A here.
A true collaboration
The days of the lone scientist making a ground-breaking discovery are pretty much a thing of the past. Today, scientific discoveries are usually the result of the collaborative efforts of hundreds of scientists from dozens of counties. The historic direct detection of gravitational waves at LIGO was no exception and involved more than a thousand scientists, from 83 institutions, across 15 countries. Although the UK was just one contributor in this massive melting pot of international talent, British engineering and scientific ingenuity was crucial to making the discovery possible and will continue to be at the heart of the emerging science of gravitational wave astronomy.
The UK has been involved in gravitational wave research for more than four decades and, with the help of funding from STFC, our scientists have pioneered the technology behind gravitational wave detection; led the analysis of data gathered from around the world; and played a crucial role in developing the techniques that allow scientists to identify the source of the gravitational waves they detect.
Scientists from 12 different universities across the UK, including major teams from the universities of Cardiff, Birmingham and Glasgow, are involved in the LIGO scientific collaboration, with many taking leading roles from detector calibration to data analysis.
On the ground: LIGO and beyond
For the past 40 years, the search for gravitational waves has been conducted from the ground — with successive generations of detectors, each more sensitive than the last, hoping to be the first to measure the telltale ‘wobble’ that indicates a gravitational wave has passed through. Today, there is a global network of detectors working together to detect and analyse gravitational waves and scientists are already designing the next generation of detectors, which will take gravitational wave astronomy to next level.
LIGO
At the centre of the original groundbreaking gravitational wave detection was Advanced LIGO (Advanced Laser Interferometry Gravitational-wave Observatory), which consists of two identical four-kilometre-long, L-shaped detectors based in the US. But, before LIGO, there was GEO600.
Built in 1995, GEO600 is a 600-metre-long gravitational wave interferometer in Germany that is funded by the Max Planck Society and the STFC. Although GEO600 didn’t actually detect gravitational waves, it was an important proving ground for the technology and many the techniques and technologies developed, in part, by UK scientists were adopted by LIGO.
As well helping to lay the foundations for the original LIGO detectors, the UK played a key role in making the upgrades that allowed Advanced LIGO to become the ten-times-more-sensitive, gravitational-wave-detecting machine it is.
Gravitational wave detectors like LIGO are laser interferometers that rely on being able to measure the smallest changes in the lengths of their detector arms and essential to those measurements are the mirrors that guide the lasers back and forth along the arms and into the detectors. LIGO’s mirrors were designed and built by a Glasgow-led team of UK universities and the STFC.
Equally important are the optical components, sensors and electronics that make guiding, measuring and analysing the laser signals — much of which was also designed and supplied by the UK.
VIRGO and KAGRA
Elsewhere in the world, the Virgo detector — a three-kilometre-long interferometer based in Italy — has undergone a upgrade programme that has seen a ten-fold increase in sensitivity.
Meanwhile, in Japan, the Large-scale Cryogenic Gravitational Wave Telescope KAGRA is under construction deep in the Kamioka mine. KAGRA will boast an interferometer with a baseline length of three kilometres — making it one of the most precise gravitational wave detectors in the world.
Virgo and KAGRA complement LIGO, which will also be getting a third detector built in India in 2024. This will complete the so-called ‘second generation’ of gravitational wave observatories.
The third generation
Looking further into the future, scientists have already started to think about the third generation of observatories. The proposed design for the aptly-named Einstein Telescope (ET) will have three gravitational wave detectors housed in three ten-kilometre-long detector arms — LIGO and Virgo only have one detector in each facility. If built, the design, which was proposed by eight European research institutes (including the University of Birmingham, University of Glasgow and Cardiff University), will bring new levels of sensitivity to gravitational wave detections.
Taking the search into space
There is a general rule with laser interferometry that the longer the detector’s arms, the more sensitive it will be — a fact that will be exploited by the proposed Einstein Telescope’s ten-kilometre-long arms — but, on Earth, there is a limit to how long these arms can be.
Engineers can get past some of these constraints by cleverly using mirrors to increase the distance the laser travels — LIGO’s arms are four kilometres long, but its mirrors bounce the beams back and forth about 280 times, which gives the arms a ‘virtual length’ of 1,120km — but if you want a detector with arms tens of thousands, or millions of kilometres long, you have to take you detector into space.
LISA
Currently slated for launch in 2034, the European Space Agency’s (ESA) Evolved Laser Interferometer Space Antenna (eLISA) will consist of three identical spacecraft that will fly in formation to create a detector with arms one million kilometre long. The distance between the spacecraft will be precisely monitored to detect a passing gravitational wave. Because the arms are so long, eLISA will be able to detect gravitational waves emitted by some of the Universe’s earliest black holes, thousands of binary star systems and maybe even from the Big Bang itself. It is hoped that eLISA will also be able to answer some of the Universe’s biggest questions — such as the nature of the mysterious dark matter that makes up some 80 per cent of all the matter in the Universe.
The UK is helping to lay the foundations for eLISA with its work on LISA Pathfinder. Launched in 2015, LISA Pathfinder is currently testing the technology needed to build a space-based gravitational wave observatory. The LISA Technology Package (LTP) is a miniaturised version version of the technology that may one day help eLISA ‘listen’ to rumblings of the Big Bang itself.
Like its Earth-bound cousins, a space-based observatory’s detectors have to be isolated from external influences — even something as seemingly feeble as the pressure exerted by sunlight could interfere with the signal.
At the heart of LISA Pathfinder are two test masses (identical 46mm cubes) that are kept in a near-perfect free-fall under the influence of gravity alone, unaffected by external forces so that they remain almost motionless relative to one another. Only by ensuring that several masses can be kept perfectly motionless in respect to one another, will a future detector be able to measure any changes in position caused by a passing gravitational wave.
The UK has played a significant roll in developing and supplying the technology being used to demonstrate the concept — providing (among other things) the test-mass caging mechanism and the optical bench that is capable of detecting changes in the test mass positions as small as one hundredth of a millimetre — as well as helping to analyse data returned during the test run.
So far, LISA Pathfinder has exceeded even the most optimistic expectations by demonstrating that the two cubes are free falling through space under the influence of gravity alone five times better than required.
So after a century languishing in the doldrums of theoretical curiosity, it looks like gravitational wave astronomy has a bright future — and you can be sure that the UK will be at the centre of it.
