By Jeremy Manier and Steve Koppes
Photo courtesy of SXS Collaboration

We’ve been dreaming about this for a long time. It’s our first time ever seeing something like this, and it truly opens up a new chapter in physics.”
—Daniel Holz
Associate professor in physics and LIGO collaborator

Early on the morning of Sept. 14, 2015, two detectors separated by about 1,800 miles made the first observation of ripples in the fabric of spacetime, ushering in an entirely new way of studying the universe.

The discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory, announced in Washington, D.C. on Feb. 11 by leaders of the international collaboration, fulfills a century-old scientific quest. Albert Einstein’s 1915 theory of general relativity predicted that cataclysmic cosmic events would produce gravitational waves that could be detected from Earth.

University of Chicago physicists played an important role in determining that the LIGO detectors had detected gravitational waves from the merger of two black holes, which collided to form a more massive, spinning black hole. Three UChicago physicists are among the many co-authors of the study detailing the discovery, which has been accepted for publication in the journal Physical Review Letters.

For scientists at UChicago, the finding speaks powerfully to their field’s past and its exciting future.

“We’ve been dreaming about this for a long time,” said LIGO collaborator Daniel Holz, associate professor in physics. “It’s our first time ever seeing something like this, and it truly opens up a new chapter in physics. You don’t get to do that very often.”

In addition to providing the first observation of ripples in spacetime, the discovery is a dramatic confirmation that black holes are real, Holz said. “The physics community was convinced, but we’ve never seen one up close,” he said. “Now we’re going right to the heart of these objects, from a billion light years away. These measurements leave little doubt that black holes exist.”

Legendary UChicago physicist Subrahmanyan Chandrasekhar was the first to propose in 1930 that massive stars might collapse into objects like black holes—an idea that prominent physicists initially ridiculed. As important as it is to validate the theories of Einstein and Chandrasekhar, the LIGO findings do much more than that, said Edward “Rocky” Kolb, dean of UChicago’s Physical Sciences Division.

“This opens up a new window into the universe, to understand the most violent events that happen,” Kolb said. “We’re in a great position at the University of Chicago to exploit this new opportunity. Using instruments like the Magellan Telescopes in Chile and the future Giant Magellan Telescope, in which UChicago is a founding partner, we will try to see the fireworks that should accompany what we’ve just heard through gravitational waves.”

‘Mind-blowingly extreme’ cosmic events

Holz’s UChicago collaborators on the LIGO project are Ben Farr, a McCormick Fellow in the Enrico Fermi Institute, and graduate students Hsin-Yu Chen and Zoheyr Doctor. Together, they played a significant role in analyzing the signals to help characterize the source of the gravitational waves, which cause ripples in the fabric of spacetime. LIGO detects this warping of space using laser interferometers, which are sensitive to minute changes in the length of the cavities that the lasers travel through.

“The detector tells you when it sees wiggles–the two detectors, although separated by thousands of miles, wiggle in a predictable way at almost the same time,” Holz said. “That tells you there must have been a gravitational wave event. Then you try to understand what produced the wiggles.”

The Sept. 14 event was so intense that in the moment before the colliding black holes swallowed each other, they emitted more energy than the entire rest of the universe combined. By studying the LIGO data over a period of months, Holz’s team contributed to the international effort to calculate the properties of the black hole collision, such as the mass of the black holes, how far away they are and where they happened in the sky,

Holz previously had written papers suggesting that LIGO analysts should be on the lookout for collisions of two black holes, since they should produce waves strong enough and frequently enough to be observed on Earth. The scale of the cosmic smash-up that LIGO observed is almost unimaginable, Holz said.

“Most black holes have masses in the range of our sun, but these two are significantly more massive,” Holz said. “Each black hole compresses 30 suns into an object that’s about one hundred miles across, and they crash into each other at almost the speed of light. It’s just mind-blowingly extreme.”

The team also has played a key role in testing how well the colliding black holes match what relativity theory predicts.

“Does this agree with the predictions of Einstein or are there some little differences? We’re trying to help address that question,” Holz said. “The short answer is that our observations agree perfectly with Einstein’s theory, which is quite remarkable.”

For the UChicago team, the feeling post-discovery is almost bittersweet, Holz said, because “there’s an awareness that it’s such a unique moment. It’s so thrilling, so intense, so revolutionary.”

Yet collaborators are excited about the next phase of discovery. With continuing upgrades to the detectors’ sensitivity, the detection of gravitational waves should become commonplace.

“This is a completely new way of doing astronomy,” Holz said. “Traditional telescopes enhance our sight, but gravitational waves are a lot like sound—a sound that actually ripples through spacetime. Up until now, we’ve been deaf to the universe. Now we’re hearing it for the first time.”

Originally published on February 11, 2016.