Surfing gravity's waves still tough after decades of trying

As a very young man I can recall reading a popular science magazine article about the search for gravity waves. Just as electromagnetic waves are the result of rapid changes in electric and magnetic fields and we can listen to radios, gravity waves occur when massive bodies with strong gravity fields undergo major changes.

Two black holes spiralling into one another fits the bill.

So gravity waves are supposed to be out there, having been forecast by Albert Einstein in his General Theory of Relativity, and it was just a matter of detecting them, with the apparatus outlined in that magazine article of so long ago seemingly very advanced to my youthful mind. These waves will not long escape observation by scientists, I thought.

However, fast forward more decades than I care to admit to and scientists have not yet detected gravity waves but they are still trying and believe the 100-year search (General Relativity was formulated in 1916) will come to an end late this year when two detection facilities become operational in the US.

Australia has an involvement in the effort with the Australian National University and the University of Adelaide contributing parts that help eliminate the noise that will affect these super detectors, infinitely more advanced than the equipment I read about in my youth. But will these new installations finally detect these elusive waves?

Daniel Shaddock, a professor in the ANU research school of physics and engineering said that gravitational waves had been proved to exist indirectly through the decay of the orbit of two neutron stars rotating around each other. But when it came to detecting the waves directly the problem was that space was "stiff" – even very large events caused waves that would have tiny effects on a detector on earth, many light years away.

In fact, the US detectors known as LIGO (Laser Interferometer Gravitational-wave Observatories) – two facilities in opposite corners of the US each containing two four-kilometre long vacuum pipes set at right angles to each other – are looking for a change that is smaller than the radius of a subatomic particle, in amongst a lot of other noise. Precisely-positioned mirrors bounce laser light up and down each of these arms, with measurements from the two facilities continuously compared to detect that miniscule change.

As Professor Shaddock points out the detectors are so sensitive that a person walking in a nearby field, or even the miniscule gravity of clouds passing overhead will affect them. All that means the equipment devised by the ANU and the University of Adelaide has to eliminate a lot of noise.

But once they are switched on and properly tested, the researchers involved hope that the two facilities will pick up a number of gravitational waves within just a few months – waves that will give them clues about the major events that caused them, such as black holes colliding, neutron stars merging or white dwarf stars falling into supermassive black holes. They will also look for the gravitational cosmic background created by the big bang itself.

Just as the advent of radio astronomy revolutionised our understanding of the universe, it is hoped that gravitational wave astronomy will also result in major discoveries. Instead of looking at light or radio waves – the electromagnetic part of the universe – the detectors will be looking at ripples in space itself. That is, provided that they do detect these waves.

An earlier version of LIGO did not detect anything despite the $US362 million put up by the US National Science Foundation to build it, but the new version will be ten times more sensitive, with the upgraded version, in theory, able to detect the last few minutes of two massive black holes spiralling into one another, to coalesce and then vibrate, just as two soap bubbles vibrate when they merge.

The next step up is the LISA (Laser Interferometer Space Antenna) space mission which will involve two masses in near perfect gravitational free fall (in a gravitational sweet spot dictated by the orbits of the earth and sun). A planned pathfinder mission for LISA, to test its feasibility, has been postponed due to lack of funds and, of course, there is still the earth-based gravitational wave observatory. After all that effort and considerable ingenuity, not to mention funds a blank or null result would be disappointing.

Professor Shaddock said that researchers are confident that they will detect gravitational waves with the earth based observatories but if they don't, well they would have to look at their models again, and that means they will have learnt something.