An artist's impression of gravitational waves generated by binary neutron stars. Photo / R. Hurt/Caltech-JPL supplied via NASA
An artist's impression of gravitational waves generated by binary neutron stars. Photo / R. Hurt/Caltech-JPL supplied via NASA
Scientists announced Thursday that, after decades of effort, they have succeeded in detecting gravitational waves from the violent merging of two black holes in deep space.
The detection was hailed as a triumph for a controversial, exquisitely crafted, billion-dollar physics experiment and as confirmation of a key prediction of Albert Einstein's General Theory of Relativity.
It may inaugurate a new era of astronomy in which gravitational waves are tools for studying the most mysterious and exotic objects in the universe.
"Ladies and gentlemen, we have detected gravitational waves. We did it!" declared David Reitze, the executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO), drawing applause from a packed audience at the National Press Club that included many of the luminaries of the physics world.
New Zealand physicists have joined many others from around the world in praising the breakthrough.
Auckland University cosmologist Dr Richard Easther called it one of the most important discoveries of the past century - comparable to that of "God particle" Higgs Boson in 2013.
Professor David Wiltshire, of Canterbury University's Department of Physics and Astonomy, said the discovery meant that from now on, we would be able to "listen" to the Universe with "ears" that were not limited by the electromagnetic spectrum, completely changing our understanding.
"It is a moment in history every bit as important as when Galileo first pointed his telescope at the stars and planets, or when the first radio, x-ray, infrared or gamma ray telescopes were first turned on by 20th century astronomers."
Canterbury University alumnus Dr Ra Inta, who worked on the LIGO project, compared the euphoria among his colleagues to that of the 2011 Rugby World Cup victory.
Some of the scientists gathered for the announcement had spent decades conceiving and constructing LIGO.
"For me, this was really my dream. It's the golden signal for me," said Alessandra Buonanno, who started working on the problem of gravitational waves as a postdoctoral student in 2000 and is now a theoretical physicist at Germany's Max Planck Institute for Gravitational Physics.
The observatory, described as "the most precise measuring device ever built," is actually two facilities in Livingston, Louisiana, and Hanford, Washington. They were built and operated with funding from the National Science Foundation, which has spent $1.1 billion on LIGO over the course of several decades. The project is led by scientists from the California Institute of Technology and the Massachusetts Institute of Technology, and is supported by an international consortium of scientists and institutions.
Hebrew University's Roni Gross holds the original historical documents related to Albert Einstein's prediction of the existence of gravitational waves at the Hebrew university in Jerusalem. Photo / AP
LIGO survived years of management and funding turmoil, and then finally began operations in 2002. Throughout the first observational run, lasting until 2010, the universe declined to cooperate. LIGO detected nothing.
Then came a major upgrade of the detectors. LIGO became more sensitive. On Sept. 14, the signal arrived.
It was a clear, compelling signal of two black holes coalescing, LIGO scientists said in interviews before the news conference. The signal lasted only half a second, but it captured, for the very first time, the endgame of two black holes spiraling together.
"This was truly a scientific moonshot," Reitze said. "I really believe that. And we did it. We landed on the moon."
These black holes were each about the diameter of a major metropolis. They orbited one another at a furious pace at the very end, speeding up to about 75 orbits per second - warping the space around them like a blender cranked to infinity - until finally the two black holes became one.
The pattern of the resulting gravitational waves contained information about the nature of the black holes. Most significantly, the signal closely matched what scientists expected based on Einstein's relativity equations. The physicists knew, in advance, what gravitational waves from merging black holes ought to look like - with a rising frequency, culminating in what they call a chirp, followed by a "ring-down" as the waves settle.
The original historical documents related to Albert Einstein's prediction of the existence of gravitational waves. Photo / AP
And that's what they saw. They saw it in both Louisiana and Washington state. It was such a strong signal, Reitze said, that everyone knew it was either a real detection of a black hole merger or "somebody had injected a signal into the interferometers and not properly flagged it into the data set. It tuned out that fortunately that wasn't the case."
He said the team, knowing the checkered history of gravitational wave detections that were later discredited, took special care to have the results verified and peer-reviewed prior to the big announcement. The scientists even looked for the possible handiwork of a computer hacker, Reitze said. All reviews held up.
The LIGO success has been a poorly kept secret in the physics world, but the scientists kept their historic paper detailing the exact results secret until Thursday morning.
There is no obvious, immediate consequence of this physics experiment, but the scientists are ecstatic and say this opens a new window on the universe. Until now, astronomy has been almost exclusively a visual enterprise: Scientists have relied on light, visible and otherwise, to observe the cosmos. But now gravitational waves can be used as well.
Gravitational waves are the ripples in the pond of spacetime. The gravity of large objects warps space and time, or "spacetime" as physicists call it, the way a bowling ball changes the shape of a trampoline as it rolls around on it. Smaller objects will move differently as a result - like marbles spiraling toward a bowling-ball-sized dent in a trampoline instead of sitting on a flat surface.
The original historical documents related to Albert Einstein's prediction of the existence of gravitational waves. Photo / AP
These waves will be particularly useful for studying black holes (the existence of which was first implied by Einstein's theory) and other dark objects, because they'll give scientists a bright beacon to search for even when objects don't emit actual light. Mapping the abundance of black holes and frequency of their mergers could get a lot easier.
Since they pass through matter without interacting with it, gravitational waves would come to Earth carrying undistorted information about their origin. They could also improve methods for estimating the distances to other galaxies.
LIGO scientists, speaking to The Washington Post in advance of Thursday's news conference, say they saw a weaker signal from a black-hole merger about a week after the first detection.
"The geometry of spacetime gives a burp at the end of [the merger]," said Rainer Weiss, an MIT professor of physics emeritus who has labored on LIGO since the 1970s.
No one had ever seen direct evidence of "binary" black holes - two black holes paired together and then merging. The Sept. 14 signal came from about 1.3 billion light years away, though that's a very approximate estimate. That places the black hole merger in very deep space; the signal that arrived in September came from an event that happened before there were any multicellular organisms on Earth.
A picture shows a 3km-long arm part of the Virgo detector for gravitional waves located within the site of the European Gravitational Observatory. Photo / Getty Images
The reason that gravitational waves have been so difficult to detect is that their effects are tinier than tiny. In fact, the signals they produce are so small that scientists struggle to remove enough background noise to confirm them.
LIGO detects gravitational waves by looking for tiny changes in the path of a long laser beam. In each of the lab's two facilities, a laser beam is split in two and sent down two perpendicular tubes 2.5 miles long. Each arm of the beam bounces off a mirror and heads back to the starting point. If nothing interferes, these two arms recombine at the starting point and cancel each other out.
But a photodetector is waiting in case something goes wrong. If the vibration of a gravitational wave warps the path of one of the lasers, making the two beams almost infinitesimally misaligned, the laser will hit the photodetector and alert the scientists.
To catch movement that small, scientists have to filter out ambient vibrations all the time. And sometimes even seemingly perfect results can end in disappointment: To prevent false positives, LIGO has an elaborate system in place to occasionally inject ersatz signals. Only three scientists on the team know the truth in such cases, and in at least one instance their colleagues were prepared to publish the results when they finally revealed the ruse.
This fail-safe gave pause to many scientists when rumors about the LIGO detection began to circulate in recent months. But the team confidently confirmed that its readings were not falsely injected - it really spotted a pair of black holes.
One of the two black holes had a mass about 36 times greater than our sun. The other registered at 29 solar masses. Both were rather massive as black holes go -- 10 solar masses is more typical.
"For the first time we have a signature of the heavy black hole forming. That was a surprise," said Vicky Kalogera, a Northwestern University astrophysicist who has been with LIGO for 15 years. "It wasn't a vanilla-type of black hole that we had expected."
When the two black holes came together - spiraling in gradually rather than colliding suddenly in a linear crash - the resulting black hole was not the 65 solar masses you'd expect from basic arithmetic, but only 62. The rest was converted to energy that radiated across space in a grand gravitational burp.
That burp first reached the LIGO facility in Louisiana, then the one in Washington state just 7 milliseconds later. The sequence is important, as it allowed physicists to chart the black-hole collision back to somewhere in the southern sky. And the incredibly brief time delay supports something that theorists have long believed about gravitational waves: They move at the speed of light.
Einstein's theory led to the prediction of gravitational waves. Photo / Getty Images
"This is the most direct test of our concepts of black holes," said David Spergel, an astrophysicist at Princeton who was not part of the LIGO team.
The scientists are scrutinizing their data for signs of other violent cosmic events. LIGO's sensitivity continues to improve, and meanwhile other labs will work to catch up to their findings.
"This is such a fantastic new window into the universe - all the rules are different," said Michael Turner, a University of Chicago cosmologist who also was not involved with the new discovery. "This is the Galileo moment of gravity waves."
A black-hole collision sounds like a dramatic event, but it's not really the big news for the physicists. The headline is that LIGO finally worked. Success in detecting gravitational waves is a win for Big Science and for the institutions that backed the project.
"It had a very rough beginning," Weiss said. "The [National Science Foundation] had a tough time explaining to other people why they would back such a crazy thing."
Einstein's theory led to the prediction of gravitational waves, but, as Weiss noted, "Even Einstein wasn't very sure about this."
New Zealand scientists react
Dr Karl Wette, an alumnus of the University of Auckland and post-doctoral researcher at the Max Planck Institute for Gravitational Physics, said the announcement was a "huge deal" for physicists.
"This discovery is the icing on the cake for Einstein's theory of gravity," said Dr Wette, who worked on the LIGO project.
"Discoveries from fundamental research are not only fascinating in themselves - for what they tell us about our world - but often have all sorts of future consequences."
For example, he said, modern electronics would not exist without our understanding of atomic structure that began with New Zealander Ernest Rutherford's pioneering work.
"This discovery has come from the hard work of thousands of LIGO scientists, engineers, and students from many countries over many decades - a true big science effort - and it's very humbling to be able to say I was part of that effort.
"The feeling in the LIGO community reminds me of post-Rugby World Cup 2011 - euphoric, but also relieved, that we've finally achieved this milestone.
"We are optimistic that, as our detectors improve in the years ahead, we will be able to announce many new discoveries - and perhaps find something completely unexpected."
Dr Ra Inta, an alumnus of the University of Canterbury who also worked on the LIGO project, said described the detection as a historical event, kicking open a completely new window to astronomy.
"What makes this detection even more exciting is that it came from an unexpected astronomical event: two very large black holes - each about thirty times the mass of our Sun - eating each other up in a few seconds.
"This not only provides evidence for one of the most mysterious objects in the universe, a black hole, but, prior to this observation, we didn't know of any stellar-mass black holes anywhere near this massive."
The event was so energetic, that the total mass of three suns was dumped into pure space-time geometry, which is what the LIGO detectors measured, Dr Inta said. "In this sense, observing gravitational waves is more like listening to the universe than looking at it.
"However, this detection is so wild, it's like a deaf person suddenly being thrust into the most mind-blowing concert ever.
"This event was detected pretty much as soon as the LIGO detectors had been upgraded, so watch this space."
Discovery validates Kiwi's decades-old work
The breakthrough will be all the more special for Kiwi scientist Professor Roy Kerr, who found the solution of Einstein's equations which describes rotating black holes.
Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole.
This collision of two black holes had been predicted by Professor Kerr but never observed.
Professor Kerr, Professor Emeritus of Canterbury University, had to struggle to be listened to by astronomers on announcing his result in a 10-minute conference talk in 1963.
Reacting to this morning's announcement, he said: "This observation is the product of one of the most outstanding collaborations of science and technology ever.
"It not only required unbelievable technological advances to be able to measure the incredibly small gravitational vibrations but also several decades of theoretical work needed to calculate the theoretical signals that have now been observed.
"From the frequency of the signal it is clear that this is not two neutron stars colliding but is a pair of fairly heavy black holes. Spinning black holes exist."
