Jim Hough of the University of Glasgow is confident that Gravitational Waves will finally be detected within the next five years. The Institute of Gravitational Research was set up four years ago and its current work is focused on the detection of gravitational waves. The work of the Institute is strongly supported by funding from PPARC, the Particle Physics and Astronomy Research Council, with £7.5m contributing directly to the Advanced LIGO project…
PPARC is the UK's strategic science investment agency for particle physics and astronomy. By directing, coordinating and funding research, education and training in particle physics and astronomy, PPARC delivers world-class science. Other funding for the Institute comes from SHEFC, the Scottish Higher Education Funding Council. Working in Scotland, the research team forms a natural bridge between work being carried out in Europe and in the USA.
Einstein’s General Theory of Relativity predicted the existence of gravitational waves as long ago as 1917 but these elusive ripples in the curvature of space-time have still to be directly detected. After 45 years when no-one showed any particular interest in proving their existence through experiment – and another 40 years of serious earth-based detective work with ever more sensitive equipment – the exploration is moving into space, with the intention of providing conclusive evidence that these tiny fluctuations caused by the acceleration of mass really do exist. However, Professor Jim Hough predicts the breakthrough will come in the next few years, even before the research goes space-side.
Professor Hough is Director of the Institute of Gravitational Research. This is a group based at Glasgow University that has been deeply involved in the development of earth based detection systems and is now playing an important role in the work on LISA, the Laser Interferometer Space Antenna. The research activities of this group, supported by PPARC funding, are currently focused on the development of detectors to search for gravitational waves from astrophysical sources.
Professor Hough explains, "Because the gravitational interaction is very weak, large masses and high accelerations are needed to produce gravitational waves that can be measured. Violent astrophysical events such as supernovae or coalescing binaries are the focus of our attention."
The existence of these wrinkles in relativity is no longer seriously questioned by scientists. The gradual changes in the orbit of a binary pulsar called PSR 1913 +16 (a pair of orbiting neutron stars, one of which is a pulsar emitting precisely timed radio pulses) can be explained only if angular momentum and energy is carried away from this system by gravitational waves.
"The reason why we have not directly detected gravitational waves over the years is because we have not had sufficiently sensitive equipment. With earth based detection there is also the challenge of interference from seismic activity and other environmental noise."
"In the early days, the search for gravitational waves was driven by an element of competition between scientists. Today however, the emphasis is on collaboration as researchers have recognised the scale of the project. Recent developments in precision measurement, lasers, optics and control systems have come together to bring the required sensitivity. The leading edge science that has resulted from our desire to detect gravitational waves also has wider applications in industry. The technological spin-off is high for our work on stable lasers, special reflective coatings and our extensions of a bonding technology-hydroxide-catalysis bonding – which exhibits very low mechanical loss."
As Principal Investigator for the UK on GEO 600, the Gravitational Wave Detector being developed by a German/British consortium, Prof Hough has played a significant role in developing technology that will now also be used at LIGO in the USA. GEO 600 and LIGO (Laser Interferometer Gravitational Wave Observatory) are long arm interferometers that offer the best chance of detecting gravitational waves from the ground. Hough’s own work in developing very delicate suspensions for gravity wave detectors and his research into ultra-stable lasers have both played a significant role in increasing the sensitivity of the detection equipment. Between them, the team at Glasgow have expertise in precision interferometry techniques, the development of ultra-low light loss mirrors, the electronic measurement of signals, data analysis and the bonding of silicon carbide.
These optical and mechanical technologies are being transferred to the next generation of the US laser interferometer system LIGO, and the work originally carried out for GEO is playing a crucial role in the planning and development of Advanced LIGO. The new technologies will result in a ten-fold increase in the sensitivity of the detector.
Current branches of astronomy are largely restricted to observing and measuring the surfaces of objects in space. Gravitational Wave Astronomy will allow us to see right into the heart of objects such as black holes, develop our understanding of quasars and explore the mass distribution of the universe. It will also be possible to look back at events that took place early in the formation of the universe and learn more about the significance of objects such as cosmic strings.
The significance of the space project LISA may therefore be in pioneering research in gravitational wave astronomy rather than proving the existence of gravitational waves themselves. LISA is now not due to launch until 2013, well after the time when Professor Hough predicts we will have conclusive evidence of gravitational waves. LISA is jointly sponsored by the European Space Agency and NASA and is expected to detect waves generated by binaries within our galaxy and by massive black holes in distant galaxies. LISA will use an advanced system of laser interferometry and the most delicate measuring instruments ever made to directly detect gravitational waves.
As a system of three spacecraft, LISA will effectively have arms that are 5 million km in length, in contrast with the 4km arms of LIGO. This means that signals will be about a million times stronger and are therefore much easier to measure and interpret. In space, LISA won't be affected by the environmental noise that is such an issue for detectors on the Earth's surface. Because of seismographic activity and other vibrations, ground detectors can only make observations at frequencies above 1Hz, but LISA will be able to observe at much lower frequencies.
However, other environmental factors will have an impact on LISA. Such factors include the drift of the spacecraft and buffeting by the solar wind. Making these small disturbances negligible is a major technological challenge of the mission. The Glasgow team are directly involved in the LISA Pathfinder project, with optical bench development taking place in Glasgow.
While there seems to be little doubt that scientists are on the cusp of a major breakthrough in directly detecting gravitational waves, the excitement for Professor Hough and his colleagues lies not only in proving the existence of these enigmatic waves but also in the development of a new branch of astronomy. Projects such as Advanced LIGO and LISA are already poised to look deep into black holes and bring us new insights into how our universe began.