Scientists have for the first time observed elusive gravitational waves – tiny ripples in the fabric of space-time caused by violent astronomical events.
The discovery, announced overnight in Washington DC, confirms the last outstanding prediction made by Albert Einstein's general theory of relativity.
This final piece in Einstein's puzzle could open up a whole new field of astronomy and help us in the search for a grand unified theory of matter, uniting relativity with the world of quantum physics.
Before today, every single prediction from Einstein's 1915 theory has been proven by direct experimental evidence – except one: the existence of gravitational waves.
The discovery was made at the advanced Laser Interferometer Gravitational-wave Observatory (LIGO) in the US. Australian scientists from the Australian National University, University of Western Australia, University of Adelaide, University of Melbourne, Monash University, Charles Sturt University and the CSIRO are celebrating their role in the discovery.
The LIGO interferometer in Livingston, Louisiana. Photo: LIGO
A recent $200-million upgrade of LIGO helped scientists detect the gravitational waves. One of the components in the upgrade was the installation of high-performance optical mirrors, many of which were coated by researchers from CSIRO.
The mirrors are among the most uniform and highly precise ever made, which ensures that LIGO's laser remains clean and stable as it travels through the detectors.
"What LIGO does is it actually takes these vibrations in space time, these ripples in space time, and it records them on a photo detector and you can actually hear them," said LIGO Executive Director Dave Reitze during the announcement in DC. "So what LIGO has done, it is the first time the universe has spoken to us through gravitational waves.
"This is remarkable - up until now we've been deaf to gravitational waves but today we are able to hear them."
Details surrounding the announcement will be outlined in Canberra this morning with the chief scientist, Alan Finkel, and ANU vice-chancellor and Nobel laureate, Brian Schmidt.
Confirmation of gravitational waves is almost certain to lead to a Nobel Prize for physics.
A century ago Einstein's theory upended Newton's understanding of gravity by showing that matter and time were inextricably linked. Rather than gravity being an instantaneous attraction between objects across infinite space, Einstein showed that space-time was the four-dimensional structure of the universe whereby matter, energy and gravity are all mediated, with interactions limited to being no faster than the speed of light.
Gravitational waves are an inevitable conclusion drawn from this theory. Its confirmation today joins a long list of confirmed predictions: gravitational lensing, the orbit of Mercury, dragging of space-time; the Shapiro effect, gravitational redshift; time dilation; the equivalence principle and others.
Gravity is such a weak force that these gravitational wave ripples are so small as to be nearly impossible to detect. Einstein thought they would never be observed. Only cataclysmic events in the fabric of space-time, such as the collision of black holes, could produce measurable waves.
If a gravitational wave passed through your head, you wouldn't notice it – but what it would do is stretch your head ever so slightly one way while simultaneously squeezing it the other, then vice-versa.
Kind of like this gif on the Einstein Online website.
Or like this, in three dimensional space.
How slightly?
The experiment designed to directly measure these waves comprises two sets of four-kilometre long laser arms set perpendicular to each other in an L-shape. Scientists expect that the impact of a gravitational wave on this set-up will stretch the arms by a distance so small it is less than 10,000 times smaller the width of a proton.
Geraint Lewis is a professor of astrophysics at the University of Sydney.
"The rumours have been building for a long time," he said. "With good news emerging from LIGO after a recent upgrade in technology and with Friday's press conference, everyone is expecting that they will announce they've detected something," he said.
In an email to The Huffington Post, the world's best-known particle physicist Laurence Krauss said the discovery of gravitational waves would be a huge milestone.
"It opens a new window on the universe," he said. "Gravitational wave astronomy could be the astronomy of the 21st century. More than that, it may reveal important information on the nature of gravity, black holes and fundamental physics.
"Every time we have opened a new window in the past, we have been surprised," he said. "I would be surprised if we weren't surprised again."
