Gravitational waves: First glimpse opens new measurement horizons

February 11, 2016 // By Paul Buckley
Gravitational waves, ripples in the fabric of spacetime, have been observed for the first time more than 100 years after Albert Einstein predicted their existence in his general theory of relativity.

The gravitational waves, which are thought to be the result of a cataclysmic event in the distant universe, look like opening a new window on the cosmos and will provide a new generation of measurement tools with which to study the universe in more detail.  The new LIGO discovery is the first observation of gravitational waves themselves, made by measuring the tiny disturbances the waves make to space and time as they pass through the earth.

Gravitational waves carry information about their origins and about the nature of gravity that cannot be obtained from elsewhere. 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. The 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 (09:51 UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington. The LIGO observatories are funded by the National Science Foundation (NSF), and were conceived, built and are operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (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.

Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. 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 milliseconds before the detector in Hanford - scientists can say that the source was located in the Southern Hemisphere.

At each observatory, the 2 1/2-mile long L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down the arms. The beams are used to 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 when a gravitational wave passed by the detector. Credit: LIGO Laboratory

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide at nearly half the speed of light and form a single more massive black hole, converting a portion of the combined black holes' mass to energy, according to Einstein's formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. These are the gravitational waves that LIGO observed.