Visions 10 - Gravity waves

New devices exploiting the spin of the electron are poised to revolutionise the electronics industry

Astronomers will soon have a new tool to explore the frontiers of the Universe. A worldwide system of instruments is being built to detect gravitational waves from space. Gravitational waves are a prediction of Einstein's general relativity theory which describes gravity as distortions, caused by mass, of the very fabric of the Universe - spacetime. They are ripples in the spacetime fabric that travel outwards at the speed of light. The strongest gravity waves would emanate from cataclysmic cosmic events such as a supernova explosion or a pair of orbiting black holes crashing into one another. Gravitational waves have never been detected, although there is good evidence that they exist. In 1974, two Americans, Joseph Taylor and Russell Hulse discovered a pair of orbiting pulsars (super-dense stars composed of neutrons that emit regular radio pulses); calculations showed that their behaviour could be explained only by the emission of gravity waves.

Aerial view of GEO 600 with
a view down one of the tubes

The instruments
It is only recently that technology has become advanced enough to let us carry out a realistic search for gravitational waves. The waves would manifest themselves by distorting space and any objects in their path, causing them to stretch in one direction and shrink in the other direction at right angles. The problem is that the changes in dimension are minute, less than one part in a thousand billion billion! Nevertheless, scientists have thought up an ingenious way of observing them.

The principle is to measure the change in distance between far-apart masses using a laser and some clever optics. A half-silvered mirror splits the laser light into two identical beams which are sent down evacuated pipes (called arms), placed at right angles, to equidistant masses. The beams are then reflected back to a central detector where they meet to create an interference pattern. Any relative alterations in the lengths of the arms will result in minute changes in the pattern. This set-up is called an interferometer and, unbelievably, the detectors currently being built in this configuration will be able to observe changes as small as one-ten thousandth the width of an atomic nucleus! Not surprisingly, the test masses must be completely separated from the exterior environment, and they are suspended on wires to isolate them from terrestrial vibrations.

The US is building two such interferometers with arms 4 kilometres long, one in Hanford, Washington State and one in Livingston, Louisiana. Called LIGO (Laser Interferometer Gravitational Wave Observatory), they will operate in parallel to distinguish real effects from background noise. Detecting gravitational waves unequivocally, and determining their source, will require a network of detectors to work together. A German-British project GEO 600 being constructed near Hanover has 600-metre arms but comparable sensitivity due to pioneering technology developed at Glasgow University and collaborating institutions in Germany. This detector uses fused silica instead of steel wires to suspend the masses and more advanced optics to increase the signal output. Both projects hope to start taking data in early 2002. VIRGO, a French-Italian 3-kilometre detector at Cascina near Pisa, will start up in 2003, while the Japanese TAMA 300 detector with 300-metre arms is already operating but at a lower level of sensitivity.

Scientific aims
Once gravitational waves are shown to exist, researchers will turn their attention to a whole range of observations that will lead to a new era in astrophysics. Astronomers hope to detect gravitational signals from the most physically extreme events in the Universe: the birth of a neutron star resulting from the gravitational implosion that follows a supernova event; two neutron stars in a death spiral, perhaps coalescing into a black hole; a neutron star being eaten up by a black hole; and the ultimate - black holes colliding. For the first time, theorists will be able to check the finer points of general relativity: whether gravitational waves do indeed travel at the speed of light, stretching and squeezing matter. They will be able to confirm finally that black holes exist and analyse the violent changes in spacetime curvature predicted when black holes collide. Certain characteristics of signals from coalescing binaries could also lead to measurements of absolute distance, thus providing another handle on the Hubble constant which defines the rate of expansion of the Universe.

Part of the VIRGO
multi-stage pendulum

A detector in space
Yet more significant data will be collected by a space-borne project LISA (Laser Inter-ferometric Space Antenna) recently approved by the European Space Agency for launch before 2015. LISA will be a collaboration between American and European teams and will consist of three drag-free spacecraft to form an inter-ferometer with arms 5 million kilometres long. Two test masses in each spacecraft will make up three separate interferometers which are synchronously 'locked' together. The laser beams will have to be projected from one spacecraft to another via telescopes. The array will be placed in a special orbit following the Earth as it goes around the Sun but 20 degrees behind. Although the environmental noise in space will be much lower than with ground-based detectors, LISA will have to be shielded from the solar wind, the high energy particles streaming out from the Sun.

Because of its size, LISA will detect long-lasting signals of lower frequency emanating from the massive black holes at the centres of colliding galaxies. Observations with LISA of gravitational waves from black holes orbiting massive black holes will provide precise tests of gravitational theory at very high fields. LISA will also be able to map the distribution of signals from thousands of binary systems of different types.

Gravitational waves were
predicted as long ago
as 1916 by Einstein's
General Relativity theory

Looking further ahead, observations from space might be able to detect the faint gravitational waves left over from the formation of the Universe. These may eventually throw light on the inflationary period when the Universe is thought to have suddenly expanded just after the Big Bang. However, a flotilla of 'LISAs' would probably be needed to probe this otherwise inaccessible epoch, and astronomers are already developing concept studies along this line. In the meantime, there are plans to upgrade LIGO exploiting the GEO-invented technology. The Japanese and Australians are also considering further terrestrial detectors. In the next 10 years, we can therefore expect some exciting results, giving us a new view of our dynamic Universe.

Further information on gravitational wave experiments can be found at the following websites:

www.ligo.caltech.edu/
www.geo600.uni-hannover.de/
www.virgo.infn.it/
http://tamago.mtk.nao.ac.jp/
http://sci.esaint/home/lisa/

TOP OF PAGE

RETURN TO CONTENTS

Thanks go to Professor Jim Hough, Professor Norna Robertson and Dr Harry Ward of Glasgow University for their help with this paper