Lasers shine new light on gravity waves
Friday, 5 May 2000
When Galileo trained his hand-made telescope on the night sky in 1604, he was the first to see lunar craters, Jupiter's moons and the phases of Venus, all with equipment more primitive than modern binoculars.
But Galileo, and countless astronomers who followed him, laboured under restrictions imposed by more than just primitive equipment. Their telescopes improved with time but, until quite recently, they could only observe visible light.
Visible light forms a tiny slice of the electromagnetic spectrum. Below it are infra red, and radio waves; beyond it, and equally invisible to our eyes, are ultraviolet, X rays, gamma rays and more.
In the past 50 years, scientists have added immensely to their understanding of the universe by exploring it through these forms of radiation, but limitations still remain. Much of the universe is dark matter, which emits virtually no electromagnetic radiation.
Gravitational waves seem to offer astrophysicists the best hope of studying objects composed of dark matter, but the problems are considerable. Static objects, even those that are large, will not emit gravitational waves. These, Einstein predicted, are produced when objects are accelerated, or when strong gravitational fields interact dynamically. Likely sources include supernovae and the merging or collision of entities like neutrino stars and black holes. These phenomena should produce extremely intense gravitational waves.
But even these gravitational waves produce only weak forces by common standards. Passing through the Earth, they could move objects, but only by 1/10,000 the width of a proton, and for less than ten milliseconds. How does one search for such forces?
There are two approaches. One involves cooling a metal bar of niobium or aluminium to very low temperatures and listening for the ringing set up in it by the distortion due to gravitational waves. It's an approach being explored in several places, with the University of Western Australia so far having the most sensitive detector of this type. The other approach involves using a laser-based detector.
A laser beam is split into two halves, and bounced back and forth between widely spaced mirrors many times to increase the sensitivity of the detector. The beams can then be brought together and compared. Minute movements caused by the stretching and squeezing of space by gravitational waves should affect them differently, and be detectable as an interference pattern created by the recombined laser beams,
The technique requires pioneering technology; 100 Watts in a laser beam that is almost perfect and very stable. Even here, the methodology divides. Two research teams are working on the problem, one at Stanford University, the other at Adelaide University in South Australia, funded by large ARC grants.
"The Stanford approach uses a relatively conventional strategy that is lower risk but compromises the quality of the output." Says Dr Peter Veitch, of the Department of Physics and Mathematical Physics. " The Adelaide approach, on the other hand, will produce a much better quality output but is somewhat more risky. It uses techniques that are commonly used with high power lasers but have never been tried for this type of neodymium laser for various reasons."
"Nobody else has thought of trying to do what we now consider to be an obvious way to produce such a laser." says Dr Veitch. "The competition between the two approaches is evenly poised at the moment but we are confident that our design will be chosen."
The assumed peculiarities of gravitational waves mean that a single detector can not tell where a signal comes from. A minimum of two detectors is needed, but even then they will struggle to filter background noise from the weak signals they seek.
At least four detectors, as widely spaced as possible, are required to get full directional information about the source of gravitational waves. All the northern hemisphere detectors are at similar latitudes, so an Australian instrument will eventually be necessary, but the costs involved mean that there is likely to be only one, and that international collaboration will be required to fund its construction.
The costs may be great, but the benefits could be immense, not only in commercial spinoffs from the technology being developed, but in a better understanding of some of the most elusive mysteries of the universe; how it works and, ultimately, even how it came to be.
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