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On the Crest of a Gravity Wave

Credit:Henning Dalhoff / Science Photo Library

Credit:Henning Dalhoff / Science Photo Library

By Stephen Luntz

Gravitational wave detectors may soon provide a new way of viewing the universe, but Australia has passed up the chance to have one located here – for now at least.

The recent detection of the Higgs boson represented the final frontier for the Standard Model of Particle Physics, for once putting science on the front page of the world's newspapers.

The search for the Higgs boson parallels the quest to detect gravitational waves, a key feature of General Relativity. Both require enormous facilities to detect something both subtle and hugely significant, and in both cases it is hoped that their discovery will be simply the first step to far greater insight into the workings of the universe.

Within a few years facilities around the world are expected to detect gravitational waves for the first time. In the process they will launch a new era in studying the universe, including information about its birth, its most catastrophic events and the particles that make it up.

It is likely that Australian equipment will play a key role. However, none of the initial observations will be made locally, as a funding window for an Australian-based detector has closed.

“People talk about the Square Kilometre Array as enabling us to detect the radiation from the Big Bang, but that is not strictly accurate,” says Prof Jesper Munch of Adelaide University’s Department of Physics. “For the first 300 million years the universe was opaque to all electromagnetic radiation. However, gravitational waves could propagate through this early universe, and we can thus in principle detect signatures from the time of the Big Bang. It is probably the only way we can get signals from the origin of the universe.”

Gravitational waves were a key prediction of Einstein’s theory of General Relativity. Einstein proposed that massive objects can curve space-time, and that when mass is rapidly accelerated the motion could create a distortion that would travel outward as a gravitational wave.

Larger masses, and faster acceleration, would create larger gravitational waves, but Einstein doubted even the largest would ever be detected directly because the effect on ordinary instruments is extremely small. Now, however, with help from lasers and other modern technology the race is on to observe one within a century of the 1916 publication of General Relativity.

The facilities to detect them are very large laser interferometers, including the Laser Interferometer Gravitational Wave Observatories (LIGO) in the US. These are large L-shaped vacuum chambers measuring 1.2 metres in diameter with each arm 4 km long. Heavy mirrors are suspended at each end to form so-called test masses with which gravitational waves from distant sources interact.

A powerful laser beam is fired at a beam splitter located at the vertex of the L. The split beams travel down each arm of the L, bouncing off mirrors many times to increase the effective length of the arm before returning to the vertex to be recombined. Under normal circumstances the light beams from the two arms are perfectly out of phase, so that upon recombination they cancel out and produce darkness.

But alterations in space-time caused by gravitational waves slightly change the phase of the light. Depending on the source of the wave, one beam will become affected more than the other, so when the beams combine they will not cancel exactly but instead will produce a weak light signal.

Unfortunately, however, local effects such as the jiggling of the mirrors from earthquakes or even a truck crashing can interfere with the beams. Hence there is a need for several distant detectors to observe and confirm the same signals.

There is a certain irony to the use of such systems to detect gravitational waves – the design is based on the 1887 experiment by Albert Michelson and Edward Morely to detect the ether that was believed to fill the universe. The failure to find ether threw physics into confusion, leading to Einstein's theory of Special Relativity and his general theory a decade later.

Initial gravitational wave detectors have been constructed in the United States (LIGO) and Italy (VIRGO). While some signals have been detected, for a variety of reasons these could not be confirmed as gravitational waves rather than local disturbances.

While there is no confidence that gravitational waves have been observed directly, Prof Russell Hulse and Prof Joseph Taylor of Princeton University made observations that are only explicable if gravitational waves exist. The pair revealed that a pair of pulsars known as PSR B1913+16 were losing energy as they orbited each other at exactly the rate predicted by General Relativity. Under the theory, this energy is carried outwards in the form of a gravitational wave.

Hulse and Taylor were awarded the 1993 Nobel Prize for Physics for the observations, demonstrating the confidence of astrophysicists that gravitational waves had indeed been confirmed. Consequently, Munch says, “few people take seriously models of the universe without gravitational waves. General Relativity has survived every test thrown at it.” So while detection of the first gravitational wave will be exciting, it is expected to confirm a well-established theory.

Munch says the ability to detect gravitational waves will mark “the onset of gravitational wave astronomy”. He believes gravitational observatories will complement radio and visible light telescopes to enable us to study cataclysmic events better than either could on their own.

“Everything we know about the universe comes from the electromagnetic spectrum,” Munch says. “Light waves, radio waves, X-rays are all part of the spectrum.

“Gravitational waves are something different. It is a little like seeing an orchestra if you couldn’t detect sound waves – you can see all these interesting things going on, but once you can hear the music you can understand so much more.”

Munch says that studies of gravitational waves could increase our understanding of the quantum nature of gravity, observing the behaviour of nuclear particles like the Higgs boson inside neutron stars at much higher densities than those produced at CERN during its recent discovery.

While physicists hope to find gravitational waves that really do come from the dawn of time, more local events could also produce gravitational waves worthy of study. Future detectors will seek the waves that Hulse and Taylor inferred. For instance, collisions between black holes and other large objects are thought to release larger, more dramatic gravity waves with different signatures than those spun off by pulsars in orbit.

Supernovae come in several forms that arise from very different causes. The models suggest the gravitational waves from these forms would look very different, providing early insights into the type of supernova event being witnessed. “Computer simulations of supernovae suggest that the gravitational waves produced could tell us what is going on,” Munch says.

With no confirmed observation of a gravitational wave it might seem a big jump to be measuring with such subtlety that we can tell explosions and collisions apart, let alone learn more about them. However, Munch says: “Changes currently being made to the existing detectors will make them ten times more sensitive, so we can observe events out to ten times the distance. This means the volume of the universe – and therefore the rate of events we will be able to see – will be 1000 times larger.”

Several events have been observed that might be gravitational waves, but there weren’t sufficient detectors functioning at the same time to confirm them. “At the moment we would expect to have one or two events a year that we could detect, and they only last for a few seconds, so the chance of confirmed detection is small,” Munch says. “With the extra sensitivity, the rate of events should go to several a day.”

A gravitational wave at a single detector can be hard to distinguish from terrestrial events such as earthquakes and human activities. However, this problem can be removed by establishing several gravitational wave detectors at substantial distances. Moreover, if the detectors are appropriately spaced they can use minute differences in the timing of the wave to detect its source. Ideally, Munch says, sufficiently distant facilities on the scale of the upgraded LIGOs and a detector under construction in Japan would be able to narrow the source of a gravitational wave to a single degree of the sky, making it possible to match the wave to an event observed in other ways.

Four detectors, not located in a single plane, are required to accurately identify the source of a gravitational wave. It was for this reason that the LIGO Laboratory, backed by the US National Science Foundation (NSF), offered support for the establishment of LIGO-Australia.

Given existing facilities in the US and Europe, the perfect location for the next point would be in the Indian Ocean off Western Australia, with a location north of Perth the closest on land. The town of Gingin, already home to Australian International Gravitational Observatory (AIGO), seemed perfect. The AIGO is currently a small-scale research version, just 80 metres long, of the interferometers that hope to make the first reliable detections.

However, the NSF's offer was conditional on further funding, estimated by Munch at around $160 million over 10 years, being raised from other sources. “To raise that we tried to get money from the federal government, the state government and the universities that would be operating it – anyone who had money,” Munch says.

The cost is smaller than the Australian Synchrotron, let alone the SKA, but by Australian standards this is still very much big science. “Large-scale science is a privilege not a right,” says Munch, adding that one of the obstacles to gaining funding was that money needed to be sought before the outcome of the decision on the location of the SKA was known.

“Scientifically there is no direct benefit from locating the detector and the SKA together, but there would be advantages in sharing computer processing and infrastructure,” Munch says. Uncertainty over the SKA may have made potential funding sources more reluctant to commit.

The deadline for Australian funding having passed, the LIGO Laboratory is now in negotiation to build the facility in India. Although India is not quite as good as Australia for triangulation, Munch says the difference is small and the LIGO-India is likely to go ahead.

Australia continues to be involved through the Australian Consortium for Interferometric Gravitational Astronomy, a coordinated effort involving teams from the University of Western Australia, the Australian National University and Adelaide, Melbourne and Monash universities. “We’re supplying equipment and people to detectors in America, and perhaps to one that is being built in Japan. And Australian equipment will also be part of LIGO-India if it goes ahead,” Munch says.

The establishment of a facility in India will decrease the need for one in Australia, but Munch hopes our time will come again. “It’s hard to get money for things that haven’t been shown to work yet. When the detection of gravity waves proves successful, more facilities will eventually be built,” Munch says.

He doubts, however, that this will happen until the age of gravitational astronomy is well and truly underway.