Peering Into the Fabric of Spacetime –“a Totally Different, Totally Unexpected Universe”




The physicist Neils Bohr once said to a colleague, “your theory is crazy, but it’s not crazy enough to be true.” For many scientists, the most exciting result of the LIGO discovery of gravitational waves is that astronomy can now enable them to probe the “dark” universe: objects and forces that don’t absorb, reflect, or emit light, yet make up 96 percent of the universe –a seismic shift in astronomy, which has been studying the light side of the universe for 5,000 years.

“When we get this new window on the universe, history and experience has shown us that we find something totally different, something totally unexpected,” said Perimeter Institute for Theorectical Physics faculty member, Avery Broderick. “This has happened over and over again in astronomy, where we’ve opened up windows in the X-ray and the radio, and we see a totally different universe,” Broderick said. “I would be shocked if we don’t see the same thing when we look with gravitational eyes, and see the gravitational wave universe as totally different. This is going to be absolutely critical to understanding how the dark universe and the light universe fit together.”

The British novelist Martin Amis once said we are about five Einsteins away from explaining the universe’s existence. We took a step closer this past January, when a century after they were proposed in Einstein’s theory of general relativity, scientists have finally verified that gravitational waves and black holes exist.

“This is not just the detection of gravitational waves,” said David Reitze, Executive Director of the LIGO Laboratory. “What’s really exciting is what comes next. Four hundred years ago, Galileo turned a telescope to the sky and opened the era of modern observational astronomy. I think we’re doing something equally important here today. I think we’re opening the window of gravitational astronomy.”

“This is just the beginning,” said Gabriela González of aLIGO. “Now that we know binary black holes are there, we’ll begin listening to the universe.” Evidence of “ripples in spacetime” detected by the LIGO Scientific Collaboration marks the dawn of gravitational wave astronomy, say astrophysicists. The gravitational waves were generated when two black holes merged 1.3 billion years ago.

“It tells us something about the power of the human mind to understand nature at its deepest level,” said cosmologist Neil Turok, Director of Perimeter Institute, one old the world’s leading experts on the cosmological constant and a cyclic model of the universe.

“This was truly a scientific moon shot, and we did it,” said Reitze.

Caltech astrophysicist Kip Thorne, who co-founded the LIGO experiment, describes a simulation of the black hole merger that created the gravitational wave: “The storm was brief – 20 milliseconds – but very powerful,” Thorne said. “The power output was 50 times greater than all power put out by all of the stars in the universe put together. Because it was so brief, the total energy was what you’d get by taking three suns, annihilating them, and putting that into gravitational waves.”

In the early hours of September 14, 2015, during an engineering test a few days before the official search was to begin, aLIGO’s two detectors recorded a very characteristic signal made by both facilities of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Washington and Louisiana simultaneously. After numerous consistency checks, the resulting 5-sigma discovery was published.

The measured gravitational waves match those expected from two large black holes merging after a death spiral in a distant galaxy, with the resulting new black hole momentarily vibrating in a rapid ringdown. A phenomenon predicted by Einstein, the historic discovery confirms a cornerstone of humanity’s understanding of gravity and basic physics. It is also the most direct detection of black holes ever.

The illustration above depicts the two merging black holes with the signal strength of the two detectors over 0.3 seconds superimposed across the bottom. Expected future detections by Advanced LIGO and other gravitational wave detectors may not only confirm the spectacular nature of this measurement but hold tremendous promise of giving humanity a new way to see and explore our universe.

“It was exactly what you would expect from Einstein’s general relativity from two black holes spiralling and merging together,” said Reitze. “It took months of careful checking and rechecking to make sure what we saw was something that was a gravitational wave. We’ve convinced ourselves that’s the case.”

Gravitational waves are ripples in the fabric of spacetime, created when two massive objects – such as black holes or neutron stars – hurtle around each other at extremely high speeds and collide. First put forward 100 years ago as a consequence of Albert Einstein’s theory of general relativity, they have challenged theorists and experimentalists alike as one of the few elements of the theory that had not been experimentally proven. Until now.

LIGO, a system of two identical interferometers constructed to detect the tiny vibrations of passing gravitational waves, was conceived and built by MIT and Caltech researchers, funded by the US National Science Foundation.

The original LIGO experiment ran from 2002 to 2010 as a proof of concept. After significant upgrades to the detectors in Louisiana and Washington, Advanced LIGO did its first observation run in September 2015.




The first detection, at the Louisiana observatory, had a peak value of 10-21 meters. “For four kilometers [the length of the LIGO detector], that’s a tiny, tiny fraction of a proton diameter. That’s incredibly tiny,” said González. “We know it’s real, because seven milliseconds later, we saw the same thing in the Hanford detector. This is it. This is how we know we have gravitational waves.”

The signals exactly match what Einsteinian gravitation predicts for the merger of two black holes. The signals also indicate the wave carried three solar masses of energy. The signal is so strong, the researchers reported in a paper published in Physical Review Letters, that it exceeds the “five sigma” standard of statistical significance physicists use to claim a discovery.

“The LIGO measurement is a spectacular confirmation of not just one, but two of the key predictions of Einstein’s theory of gravity: the existence of gravitational waves and black holes,” Turok said. “Einstein developed his theory based on clues from experiment and prior theories, but even more on a remarkable intuition that gravitation is due to the bending of spacetime. A full century later, we’re seeing his predictions verified with exquisite precision.”

Even more than verifying Einstein, LIGO’s detection of gravitational waves provides science with a new tool with which to potentially answer many more basic questions.

And it might lead researchers to the next great scientific theory, Perimeter researcher Luis Lehner said during the “Ripple Effects” panel hosted by the Perimeter Institute following the LIGO announcement. “When we can get more and more data, we might be able to see departures [from what is expected], and that may guide us in what replaces relativity,” he said.

As more gravitational wave detectors come online in the next few years, scientists will be able to glean increasingly rich information about the universe around us. “That will give us a very important network that will allow us to … reduce serendipity from astronomy, at least for some sources,” Lehner said.

Perimeter cosmologist Latham Boyle is also excited about the unknowns that gravitational waves could now reveal. As he explained during the Perimeter panel, there was a span of decades between the discovery of radio waves and their use in astronomy.

“For 40 or 50 years, nobody built radio telescopes,” Boyle said. “Finally, when they did, there was a flood of all kinds of crazy discoveries. [There was] the cosmic microwave background radiation, which is kind of a cosmic selfie, then they discovered these things called pulsars, and they discovered quasars.

“They discovered all this stuff that people would have called you crazy if you’d suggested it before. As soon as you turned it on, it was out there. It’s just a historical fact that often you see wilder stuff. That’s one of the most exciting things for me.”

The Daily Galaxy via The Perimeter Institute




"The Galaxy" in Your Inbox, Free, Daily