Scientists across the world are giddy at the prospect that the final piece in the puzzle emerging from Einstein's theory of general relativity will fall into place on Friday.
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Gravitational waves explained
Professor Geraint Lewis from Sydney University explains what the discovery of gravitational waves will mean for science.
Every single prediction from Einstein's 1915 theory has been proven by direct experimental evidence - except one: the existence of gravitational waves.
We could be just hours away from their confirmation.
Their discovery could open up a new branch of astronomy and help us in the search for a grand unified theory of matter.
A joint press conference in Europe and the US at 2.30am AEST on Friday by scientists from the LIGO experiment in the US will issue "an update on these tiny ripples in the fabric of space-time".
If it is direct confirmation of their existence, it's kind of a big deal.
Every Star Trek or Doctor Who enthusiast will have heard of space-time - the very fabric of our universe.
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 were all mediated through space-time, with interactions limited to no faster than the speed of light.
Gravitational waves are an inevitable conclusion drawn from this theory.
Gravitational waves are tiny ripples in the fabric of space-time. Gravity is such a weak force that these ripples are so small as to be nearly impossible to detect.
It would take a massive event in the fabric of space-time for us to be able to detect them, an event such as the collision of black holes.
If a gravitational wave passed through your head, you wouldn't notice it - but what it would do is to 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.
Well 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.
LIGO stands for Laser Interferometer Gravitational-Wave Observatory. It has very, very accurate measuring devices.
The LIGO interferometer in Livingston, Louisiana. Photo: LIGO
Some scientists are cautious. In 2014, a team of physicists running the BICEPs experiment announced that they had discovered gravitational waves resounding in the echo of the Big Bang. They hadn't. The journal Nature famously scotched this in January 2015 with the headline "Gravitational waves discovery now officially dead".
It looks a little more promising this time.
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.
One Australian physicist involved with LIGO is Professor David McClelland at the Australian National University. He has refused to discuss any aspect of LIGO until after the announcement on Friday morning, suggesting it really will be a big deal.
And probably the world's best-known particle physicist, Laurence Krauss, blabbed about it on Twitter in September.
Rumor of a gravitational wave detection at LIGO detector. Amazing if true. Will post details if it survives.— Lawrence M. Krauss (@LKrauss1) September 25, 2015
Which he backed up further last month.
My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!! Exciting.— Lawrence M. Krauss (@LKrauss1) January 11, 2016
He has gone excitedly coy about it in the past few days.
News from LIGO sounds exciting. Not tweeting details before Thurs. Two days left before a new era in astronomy and physics could begin.— Lawrence M. Krauss (@LKrauss1) February 10, 2016
In an email to The Huffington Post, Professor 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."
Whereas the failed BICEPs announcement claimed to have seen gravitational waves in the echo of the Big Bang, the rumour is that this time LIGO has witnessed the echo of a more recent event - the collision of black holes.
Because gravitational waves are so weak, it takes a cataclysmic event to produce waves that we have even half a chance of measuring.
Any asymmetrical gravitational event should produce these waves, such as the orbit of the moon around the Earth. But we would never detect them.
Two colliding black holes is something different altogether.
Either way it goes, Friday's announcement will open up a new path to deepening our understanding of the universe.