Today was a big day for physicists and space science, with the announcement of the first confirmed detection of gravitational waves, 100 years after they had been predicted by Albert Einstein as a major aspect of his general theory of relativity. The gravitational waves detected were produced the final fraction of a second of the merger of two black holes, creating a single, more massive spinning black hole. The news marks nothing less than a revolution in physics.
“Ladies and gentlemen, we have detected gravitational waves,” said David Reitze, the executive director of the LIGO Laboratory, at the press conference at the National Press Club in Washington, D.C. “We did it!”
The gravitational waves were first detected by astronomers on Sept. 14, 2015, at 5:51 a.m. Eastern Daylight Time (09:51 UTC), using the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, which are located in Livingston, La., and Hanford, Wash. The discovery 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.
“Our observation of gravitational waves accomplishes an ambitious goal set out over 5 decades ago to directly detect this elusive phenomenon and better understand the universe, and, fittingly, fulfills Einstein’s legacy on the 100th anniversary of his general theory of relativity,” said Reitze.
The discovery has been accepted for publication in the journal Physical Review Letters, and is a major milestone in physics.
“This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality,” said Gabriela González, LSC spokesperson and professor of physics and astronomy at Louisiana State University.
“The Advanced LIGO detectors are a tour de force of science and technology, made possible by a truly exceptional international team of technicians, engineers, and scientists,” said David Shoemaker of MIT, the project leader for Advanced LIGO. “We are very proud that we finished this NSF-funded project on time and on budget.”
According to the scientists, the two black holes were about 29 and 36 times the mass of the Sun. The event itself actually occurred about 1.3 billion years ago. It only took a fraction of a second for about three times the mass of the Sun to be converted into gravitational waves. Even in that short space of time, the maximum power output was about 50 times that of the entire visible Universe. The collision of these two black holes was a major cataclysmic event. The two black holes had been orbiting each other before the collision. General relativity states that they will gradually lose energy, through the emission of gravitational waves. As a result, they slowly move toward each other over time; during the last few minutes they speed up, before finally colliding in the last fraction of a second at nearly half the speed of light. The former two black holes now become one larger black hole. During the collision, a portion of the black holes’ mass is converted to energy, which is emitted as a final major burst of gravitational waves. The detector in Livingston recorded the burst 7 milliseconds before the detector in Hanford and scientists determined that the source was located in the Southern Hemisphere.
“It’s a runaway process,” said Frans Pretorius, of Princeton University in New Jersey. “The closer they get, the faster they spin.”
“The description of this observation is beautifully described in the Einstein theory of general relativity formulated 100 years ago and comprises the first test of the theory in strong gravitation. It would have been wonderful to watch Einstein’s face had we been able to tell him,” said Rainer Weiss, professor of physics, emeritus, from MIT.
“With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe – objects and phenomena that are made from warped spacetime. Colliding black holes and gravitational waves are our first beautiful examples,” said Kip Thorne of Caltech. “The total power output of gravitational waves during the brief collision was 50 times greater than all of the power put out by all the of the stars in the universe put together. It’s unbelievable.”
As famed physicist Stephen Hawking noted in Nature, “These amazing observations are the confirmation of a lot of theoretical work, including Einstein’s general theory of relativity, which predicts gravitational waves.”
The signals can also be converted to sound, which results in a distinct “chirp.”
So just what are gravitational waves? They are essentially ripples in the fabric of spacetime. They were the final prediction of Einstein’s general theory of relativity. The force of gravity is basically the result of a curvature in spacetime, with gravitational waves being ripples in it, caused by things such as they collision of two massive black holes. Massive objects will warp the spacetime around them. By the times these ripples reach Earth, however, they are very minute and difficult to detect.
Also, gravitational waves should not be confused with gravity waves. Gravity waves are related specifically to planetary atmospheres and bodies of water, physical perturbations driven by the restoring force of gravity.
Scientists have now “turned up the volume” on the sky, meaning that they will now be able to search for additional signs of gravitational waves. The discovery also has the potential to win a Nobel Prize.
“I used to say, as they are building the instrument, they can be thought of as a physics experiment,” said Avi Loeb of Harvard University in Massachusetts. “But as soon they detect a single source, they will be thought of as an astronomical observatory.”
“To make this fantastic milestone possible took a global collaboration of scientists – laser and suspension technology developed for our GEO600 detector was used to help make Advanced LIGO the most sophisticated gravitational wave detector ever created,” said Sheila Rowan, professor of physics and astronomy at the University of Glasgow.
There were hints of gravitational waves before as well; In 1974, physicists Joseph Taylor and Russell Hulse at the University of Massachusetts Amherst had indirectly confirmed the existence of gravitational waves. They watched radio flashes emitted by a pair of neutron stars whirling around one another; the shifts in the timing of the flashes matched Einstein’s predictions of how gravitational waves could transfer energy away from the stars. They won them the 1993 Nobel Prize in Physics for their work.
A lot of work had to be done to confirm the discovery before making it public. In 2014, researchers using the BICEP2 telescope thought they had detected primordial gravitational waves produced by the original Big Bang, but those turned out to be the effects of interstellar dust.
The discovery opens up a new field of study: gravitational-wave astronomy. Scientists will “listen to the waves” to learn more about the massive and bizarre objects which produce them, such as black holes, neutron stars, and supernovae.
“It’s been a very long road, but this is just the beginning,” González said. “This is the first of many to come. Now that we have detectors able to detect these systems, now that we know that binary black holes are out there, we begin listening to the universe.”
As MIT astrophysicist Scott Hughes told Gizmodo, “Whenever first detection happens, there’s gonna be a party, no question,” he continued. “But after that, when detection becomes routine, is when things start getting really interesting.”