Originally published in ANU Reporter.
The discovery of gravitational waves is the culmination of a search by a generation of ANU physicists, reports DR PHIL DOOLEY, BSc (Hons) ’90, PhD ’99.
An excited hush fell over the briefing room at Parliament House as Professor David McClelland stepped up to the microphone.
“I’m pretty sure you all know by now but I want to say it. We’ve done it,” he said as his voice quavered.
Spontaneous applause broke out, as McClelland allowed himself a smile. Camera flashes popped and TV cameras zoomed in.
“We detected a gravitational wave that was generated 1.3 billion years ago when two black holes crashed into each another… the most violent event ever witnessed.”
The announcement was sweet reward for McClelland, an ANU laser physicist who has spent his career working towards this moment.
Albert Einstein predicted the existence of gravitational waves but thought they were too small for humans to ever detect.
To prove Einstein wrong and right in a single stroke is rare treat for a scientist.
“This is a moment that will be remembered for a thousand years,” McClelland said.
Gravitational waves are vibrations of space and time themselves, one of the most outlandish predictions of Einstein’s 1916 General Theory of Relativity. Yet, they appeared exactly as predicted and join the long list of successes of Einstein’s theory over the last century.
The first success of Relativity came three years after Einstein’s publication, when a solar eclipse allowed astronomers to pick out the tiny deflection of distant starlight by the sun’s gravity.
In 1990, at the ANU Physics Department, McClelland and Emeritus Professor Hans Bachor AM gathered people from across the world to assess the challenges of that dream.
The microscopic effect was exactly what Einstein’s revolutionary concept of space and time that was warped by huge masses predicted. The charismatic genius became a superstar overnight.
Einstein’s vision of stretchy space was not just static, it also predicted large masses moving or colliding would make waves through space, effectively a spacequake.
To make a gravitational wave of any significance needs a cataclysmic event happening to a huge mass, a star exploding or black holes colliding.
But such events are rare and unlikely to happen close to Earth, so scientists need to extend their search beyond our galaxy.
25 years of chasing Einstein’s theory
It would be 75 years before physicists would dare dream of looking for gravitational waves.
In 1990, at the ANU Physics Department, McClelland and Emeritus Professor Hans Bachor AM gathered people from across the world to assess the challenges of that dream.
One-by-one, a who’s who of gravitational wave specialists laid out the seemingly impossible tasks.
The basic concept was clear: a stable laser beam bouncing between mirrors in a vacuum.
When a gravitational wave came past and warped the space between the mirrors, it would momentarily disturb the stability of the laser.
Surpassing scientific challenges
But the practicalities were not so simple.
“Everything needed to be roughly a factor of 1000 more powerful or sensitive than we had at the time,” Bachor recalls.
Laser technology was weak. Nobody knew how to build a good enough mirror, it had to be massive and perfectly smooth, and with losses less than one part in a billion.
The experiment would need to be enormous, preferably 10 or more kilometres long.
Damping out vibrations from the environment was another huge challenge. The smallest earth tremor, or a passing truck, threatened to obliterate the tiny ripples from distant spacequakes.
But they didn’t know the size and shape of the ripples they were searching for, says Professor Susan Scott, from the Research School of Physics and Engineering (RSPE).
“We really had no idea of the enormity of the task which lay ahead of us. We were searching for a needle in a cosmic haystack, extremely faint signals in very noisy data.
“We had early meetings with submarine intelligence experts but we quickly realised we were facing a much more daunting task than theirs,” she said.
But, as in 1916, the scientists were not discouraged.
“The amazing thing was that the mood at the end of the conference was ‘Let’s go for it,’ ” Bachor says.
“We were eternal optimists. These people were pioneers who had seen the progress over the previous 20 years. We trusted that the technology would evolve.”
The start of a technological revolution
But political realities posed a big obstacle to progress.
The Russians quickly realised they could not afford to build an experiment.
The UK could not find the land and teamed up with Germany to build a smaller experiment, GEO 600, only 600 metres long. Japan set up small experiments in disused mines.
Australia had the land but lost out on funding against stiff competition from more developed astronomy projects, such as the Square Kilometre Array, which could guarantee data.
The gravitational astronomy world’s hopes lay with the two four-kilometre long experiments at the Laser Interferometer Gravitational-Wave Observatory (LIGO) at either end of the US, and the French-Italian Virgo project.
The nations apportioned the tasks, and set to work.
Over the next 20 years technological revolution after technological revolution achieved the impossible.
Joining the hunt as an honours student in 1996, Daniel Shaddock, was dismayed by the challenge.
“When I first heard about the sensitivity needed for gravitational waves, I thought that’s crazy, we’re never going to see them,” said Shaddock, now a professor in the RSPE.
“But you start working through the challenges and before you know it, you’re in a position where you have the measurement technology that you need.”
Also drawn to the quest was Dutch student Bram Slagmolen, who travelled around the globe in 1999 to work with McClelland.
“We’ve done things I wouldn’t have dreamed of back then,” says Slagmolen, now a research fellow at ANU and the leader of the Australian Consortium for Interferometric Gravitational Astronomy.
“Although the final experiment would be on a large scale and hideously complex, we ran little experiments that gave us understanding and allowed us to go the next step.”
In Italy, the Virgo experiment was dogged by technical hitches, but by mid-2015, LIGO scientists – including many Australians led by McClelland – believed their twin US experiments could start, and flicked on the many systems required to isolate Einstein’s elusive echoes of intergalactic impacts.
The Universe delivers an extraordinary result
Then the Universe delivered something truly extraordinary.
The team had estimated they might have to spend a couple of years sifting through their data to find matched sets of tiny wobbles from outer space.
But while they were still doing final tests with simulated data, still days away from the first official science run, the detector twanged with a signal so clear that the team could see it above the background without any filtering.
“The signal matched perfectly to the predicted pattern of two black holes colliding, but it was so strong we thought it must be a blind injection of a simulated signal, ” Scott says.
“We were aware very quickly that this was not the case and so this signal that had thundered into our detectors was probably real, and from surprisingly large black holes.”
A long time ago in a galaxy far, far away, there had been an invisible collision that had unleashed more energy than any other event humans had witnessed.
The peak power had been 50 times that of total output of every star in the visible universe.
There was no accompanying flash of an explosion.
After a trip of 1.3 billion years across the cosmic pond, the ripples had died down so much that they moved LIGO’s mirrors by about one thousandth of the diameter of a proton.
The signal was imperceptible to normal humans but clear as a bell in the detector in Louisiana and then, seven milliseconds later, in the Washington State detector.
Just the beginning of gravitational wave physics
McClelland’s search is not over now their tool is working.
“This is just the beginning,” he says.
“It’s like the moment when Galileo first turned a telescope towards the skies and started a new epoch of astronomy. Here we shall begin a whole new and fundamentally different way of observing the Universe.”
Bram Slagmolen was jubilant.
“I was always convinced we would see them, it was just a question of when,” he says.
“We’ll definitely see more now, by the end of the year we should have multiple detections, in the bag, it will be fabulous for the whole community.”
Aside from observing the antics of invisible black holes, gravitational waves help scientists see further back in time than conventional astronomy, says Professor Scott.
“This is a big deal. One of our holy grails is to understand the beginning of the Universe.”
And so another search begins.
