The rumours have been spreading like ripples – and so, it seems, have space and time.
By all indications, scientists are about to announce they have detected gravitational waves – a century-old prediction of Einstein’s theory of general relativity and one of the most anticipated signals in the history of physics. In the process, they will have opened up an entirely new way of seeing the cosmos – a Nobel Prize-worthy achievement.
Gravitational waves are literally waves in the fabric of spacetime – passing distortions generated by distant cataclysms, such as when two black holes that are locked in a death spiral collide and merge.
The details of Thursday’s upcoming announcement have not yet been confirmed by the 1,000-member international team of researchers working with Advanced LIGO, the first experiment to come along with a realistic chance of detecting gravitational waves as they sweep past our planet.
But it’s no secret what the experiment is looking for: signs that the universe is awash with such vibrations, along with the hope of using them to make new discoveries about the cosmos and to provide the strongest test yet of Einstein’s pre-eminent theory.
Calculations suggest that even now, with the experiment not yet operating at its maximum design sensitivity, it could spot one or more gravitational events a year.
Such events could be collisions involving black holes or neutron stars happening somewhere among the millions of galaxies that surround us.
Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) has now been running for six months and it’s become an open secret that it’s seen something. Team members are doing their best to keep mum, but with anticipation growing and experiment leaders and other senior scientists flying in to appear at a special news briefing in Washington, no one is expecting a negative result.
“It has been a great time the last few months analyzing the data. I am looking forward to telling the world what our results are so far,” said Harald Pfeiffer, a researcher at the Canadian Institute for Theoretical Astrophysics in Toronto and a member of the Advanced LIGO team.
But while the media buzz is growing and the early indications are more than promising, many experts say they will remain cautious until they have seen the data for themselves. Recent history is full of major scientific announcements that later proved to be an overreach or just flat wrong once others found a crucial error or incorrect assumption.
Conscious of this, the Advanced LIGO team even went so far as to direct some of its members to insert false data that might mimic a gravitational wave signal to test the system’s ability to spot a red herring.
And little wonder. The measurement of gravitational waves – something never before achieved – would be a remarkably difficult feat to pull off. Even a stray air current would be enough to throw off the experiment, which measures changes in a four-kilometre-long laser beam as small as 1/1000th the width of a proton.
“The idea of actually detecting gravitational waves and using them to learn things about distant galaxies … it’s just an extraordinarily audacious undertaking,” said Lee Smolin, a theorist who specializes in problems related to gravity and general relativity at the Perimeter Institute for Theoretical Physics in Waterloo, Ont.
The LIGO project, founded in 1992 by researchers at Caltech and MIT, is supported by the U.S. National Science Foundation and now includes many other institutional members.
The experiment consists of two facilities, one in Washington State and one in Louisiana, each with a pair of long arms that send laser light bouncing back and forth along identical four-km-long tunnels.
The challenge of measuring a distortion in space is that everything else distorts with it, including any sort of ruler or measuring tape, no matter how accurate. But laser light can only move at one constant speed, so if space is momentarily stretched out in the direction the light is travelling, its journey will take a tiny bit longer. Because the two arms are at right angles to each other, the difference between how long it takes light to travel down each one can reveal a gravitational wave signal.
And because there are two facilities widely separated, local effects due to vibrations from wind, seismic activity and even passing traffic – all of which might be mistaken for a gravitational wave if only one facility were built – can be ruled out. A real gravitational wave should be felt by both.
The advanced version of LIGO, with much higher sensitivity than before, began taking data last September. As to whether it has finally achieved the long-awaited detection, Dr. Pfeiffer would only say, “That would be worth calling a press conference for.”Report Typo/Error