Scientists in the LIGO Scientific Collaboration—including ones from MIT and Caltech—have directly observed gravitational waves 100 years after Einstein predicted such waves existed in his general theory of relativity. As Jennifer Chu from the MIT News Office writes, Einstein expected the waves would be “vanishingly small” and difficult to detect. The waves detected by the scientists are the product of a collision of two massive black holes 1.3 billion lightyears away—“…a remarkably extreme event,” as Chu puts it.
The researchers detected the waves on September 14, 2015, at 5:51 a.m. EDT and published their results today in the journal Physical Review Letters.
The direct observation of the waves confirms earlier evidence of their existence. In 1974 physicists Russell Hulse and Joseph Taylor observed a binary system composed of a pulsar orbiting a neutron star, 21,000 light years from Earth. By 1982, Taylor and Joel M. Weisberg found the orbit of the pulsar was shrinking, indicating the release of energy in the form of gravitational waves.
The LIGO (Laser Interferometer Gravitational-wave Observatory) is an impressive instrument, consisting of two identical detectors in Livingston, LA, and Hanford, WA. “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.”
At each observatory, a 4-km long L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down the instrument’s arms (four-foot diameter tubes kept under a near-perfect vacuum). The beams monitor the distance between mirrors precisely positioned at the ends of the arms. According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector. A change in the lengths of the arms smaller than 10-19 m can be detected.
Based on the observed signals, LIGO scientists estimate that the black holes for the event detected were about 29 and 36 times the mass of the sun. About three times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 ms before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.
“Our observation of gravitational waves accomplishes an ambitious goal set out over five 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 Caltech’s David H. Reitze, executive director of the LIGO Laboratory.
Chu at MIT has posted a Q&A with Rainer Weiss, now a professor emeritus of physics, on the origins of the LIGO project. He said the concept started at MIT in 1967, when general relativity had been relegated to mathematics departments, because physical experiments to prove the theory were so hard to do. “Einstein had looked at the numbers and dimensions that went into his equations for gravitational waves and said, essentially, ‘This is so tiny that it will never have any influence on anything, and nobody can measure it,’” Weiss said. “And when you think about the times and the technology in 1916, he was probably right.”
He added that as time progressed, “…you had lasers, masers, electronics, computers, and a whole bunch of stuff people didn’t have in 1916.”
So it turns out Einstein was both right and wrong. He was right that gravitational waves exist, but wrong to think they couldn’t be measured. As Bruce Allen, managing director of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), explained, “Einstein thought gravitational waves were too weak to detect, and didn’t believe in black holes. But I don’t think he’d have minded being wrong!”
