For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.
Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. 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 but never observed.
The gravitational waves were detected on Sept. 14, 2015 at 5:51 a.m. EDT by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, La., and Hanford, Wash. The LIGO Observatories are funded by the National Science Foundation and were conceived, built and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.
‘Dreaming about this for a long time’
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. Three UChicago physicists are among the many co-authors of the study detailing the discovery.
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 at UChicago. “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.”
More about the LIGO collaboration
LIGO research is carried out by the LIGO Scientific Collaboration, a group of more than 1,000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, universities in the United Kingdom and the University of the Balearic Islands in Spain.
LIGO was originally proposed as a means of detecting these gravitational waves in the 1980s by Rainer Weiss, professor emeritus of physics from MIT; Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics Emeritus; and Ronald Drever, professor emeritus of physics, also from Caltech.
Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in The Netherlands with Nikhef; the Wigner RCP in Hungary; the POLGRAW group in Poland and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy.
The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first-generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run. The U.S. National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University and the University of Wisconsin-Milwaukee. Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University and Louisiana State University.