The discovery of gravitational waves in 2016 – predicted by Albert Einstein – was one of the biggest sciences stories of the decade. While the discovery didn’t take the Nobel Prize today, many in the scientific community think it is just a matter of time before the Nobel committee honours the Laser Interferometer Gravitational-Wave Observatory (LIGO) team for one of the most outstanding contributions in the field of physics. Science writer Patchen Barss explains the discovery and the key contributions of University of Toronto’s Harald Pfeiffer and his team.
A billion years ago and a billion light years away, a star 36 times more massive than our Sun expended its remaining fuel in a final blast of nuclear fusion. With nothing left to burn, the star began to collapse under its own gravity. The atoms in its massive core collapsed like crushed soda cans. Protons and electrons ground together to form new neutrons.
The star’s density kept increasing. Its gravity became so concentrated and intense that not even light could escape any longer. Spacetime warped and ruptured. The star became a black hole.
But that wasn’t the end of the story.
A second black hole, the product of an only slightly smaller stellar cataclysm passed by. The two became trapped in each other’s mighty gravitational fields. They circled one another, slowly at first, but then more and more quickly. Their collision course became a high-speed death spiral that sent waves of gravitational energy rippling out across their galaxy and into the cosmos at the speed of light, stretching and squeezing space itself.
Back on present-day Earth, came a different kind of merger: a collision of ideas between observational cosmologists and numerical relativity experts.
In the 1970s, observational scientists had begun working on “laser interferometry” instruments that might detect gravitational waves. Decades of effort culminated in the construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which comprises two massive detectors, one in Washington and the other in Louisiana.
Concurrently, the University of Toronto was leading an international effort to simulate black hole collisions and predict what the emerging gravitational wave patterns might look like.
The simulations belong to a field of study with the unglamorous name of “numerical relativity.” These supercomputer simulations nab few headlines, but without them, gravitational-wave research wouldn’t have gone far, even with LIGO’s whiz-bang technology. Scientists at U of T identified the need for powerful simulations early in LIGO’s planning stages, and drove the push to mature the theoretical science in time to make the most of LIGO’s observations.
“The development of these simulations was precisely designed to make us able to analyze the data collected by LIGO experiments,” says J. Richard Bond, a University Professor at the Canadian Institute for Theoretical Astrophysics (CITA) in the Faculty of Arts & Science. Bond drove the effort to recruit an expert devoted to numerical relativity.
“Detecting gravitational waves is a huge revolution. It will be front and centre in what’s going to happen over the next few decades,” he says. “You’re either on that bus or off it. Somebody here at the University had to be on the gravitational-wave bus.”
In fact, U of T attracted a whole busload of graduate students and postdoctoral fellows to work on numerical relativity. From the start, though, the person driving that bus has been Harald Pfeiffer.
Before Pfeiffer became an associate professor at CITA, he had already established his reputation in numerical relativity at Cornell University and Caltech.
“I have always been interested in black holes and Einstein and gravity and computers,” he says. “At Cornell, I worked with one of the world’s experts on solving Einstein’s equations on supercomputers. The relevance to LIGO was there all along.”
In the early 20th century, Albert Einstein proposed his Theory of Relativity a model of gravity and the universe that scientists have been testing and exploring ever since. Many non-scientists can recite Einstein’s most famous equation: E=mc2. But the so-called mass-energy equivalence equation is just one tiny part of the math behind relativity. Researchers are still finding new predictions based on Einstein’s equations, and using them to understand and simulate cosmic events that would otherwise defy imagination and intuition.
“The first time people tried to simulate black-hole collisions on computers was in 1964,” says Pfeiffer. “But even when I started my PhD, nobody had yet figured out how to do it. We made steady progress but only on arcane technical sub-problems. The big problem eluded everybody until 2005 when finally all the pieces came together.”
LIGO faced hurdles of its own. Through the 1980s and 1990s, the project faced technological and budgetary delays. Between 2002 and 2010, the first major version of LIGO worked exactly as planned. But, during that time the cosmos failed to cooperate, sending no detectable waves our way. An international team of scientists continued to make refinements and improvements to increase LIGO’s sensitivity.
LIGO’s L-shaped detectors work by splitting a laser beam into two waves radiating at right angles to one another. Each beam travels precisely the same distance – four kilometres – through a vacuum, bounces off a fine-tuned mirror and returns along the same path to the split point. In the absence of gravitational waves, the returning beams cancel each other out. The detector stays quiet.
But passing gravitational waves would lengthen space in one direction and squeeze it in the other. Each beam would travel a slightly different distance, get out of sync with the other, and create a distinct, detectable interference pattern.
Researchers built two such detectors thousands of kilometres apart, which allowed them not just to detect waves, but also to triangulate them to determine the location of their source.
They still needed to know what to look for, though.
“In the first 10 years, my research and LIGO were not directly touching each other,” says Pfeiffer. “However, on both sides of the fence there was momentum building and building rapidly.”
Both sides were working toward a goal that nobody was sure would be achievable. Still, they were spiraling in on one another, circling toward an explosive discovery.
In September 2015 a new, vastly more sensitive iteration of LIGO came online. By then, Pfeiffer and his team had simulated thousands of collisions, creating a bank of “pattern templates” that gave observers clues about what to look for, and how to interpret what they found.
Not long after, gravitational waves from that distant, ancient black-hole collision finally reached the Earth.
Space compressed in one direction, stretched in another. The laser beams fell out of sync.
That chirp, revealed to the world at an international press conference in February 2016, was an audio interpretation of a laser interference pattern created by billion-year-old gravitational waves.
Using Pfeiffer’s simulations, researchers conclusively identified the pattern as the first-ever direct detection of gravitational waves.
“U of T’s key contribution was this waveform modeling,” says Pfeiffer. “If you know the shape of the signal you’re looking for, it’s like knowing the colour of a needle in a haystack. It’s easier to find.”
In June 2016, LIGO scientists announced that the detectors had chirped again: a second detection. In this case, though, the black holes involved had about one third the combined mass of the first collision. It was a “quieter” crash with a weaker signal, which meant the simulations played an even more important role in its interpretation.
“The second detection would have been an extremely marginal discovery without the simulations,” says Pfeiffer. “It would have been flagged as an interesting detection, possibly between two black holes, but nothing more precise.”
The pattern templates also save time – rather than deciphering data for days on end, observers can say right away, “You’ve got waves!”
“Real-time is important, because there’s a whole band of astronomers across the world who are not part of LIGO,” says Peter Martin, a CITA professor. “They want to turn optical or radio telescopes to the point of detection quickly to see whether any electromagnetic flash comes with gravitational radiation.”
Researchers continue to improve LIGO, with plans to double its sensitivity. That puts pressure on Pfeiffer to keep building simulations based on Einstein’s relativity equations.
“As boring as it sounds, there’s still a lot of work to be done in improving the waveforms that LIGO is looking for,” says Pfeiffer. “It’s really cool having this big breakthrough, but 99 percent of science is the tedious day-to-day work.”
LIGO plans to continue observations in 2016, and it will join forces with a French-Italian gravitational-wave detector in 2017. Plans include studying more colliding black holes, scoping out their properties in unprecedented detail, and checking whether Einstein’s theory continues to work flawlessly in light of ever more precise data.
Astronomers will also search for gravitational waves from sources other than black holes, including from less-massive-but-still-whoppingly-massive bodies like pulsars and other neutron stars that spin at high speed.
Bond, though, has his eye on another target.
“In Toronto, I and many others are heavily invested in discovering gravitational waves formed during the first moments of the universe,” Bond says.
“The sheer challenge of figuring out how to solve Einstein’s equations would have been enticing enough of a problem,” Pfeiffer says. But he found it doubly exciting when those equations allowed scientists to precisely reconstruct the story of that distant, cataclysmic collision from a billion years earlier.
“It is amazingly satisfying, to see the effort of thousands of people come together,” he says. “Building the LIGO instruments, developing the software to analyse the data, and also our own contribution toward detecting and deciphering the signals. It was only through this huge joint effort that we could discover black holes colliding.”