The discovery was made possible by recent upgrades to the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), which enabled researchers to probe a larger volume of the universe during its first observation run.
From a Black Hole Collision to LIGO
The 2½-mile-long, L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down arms, consisting of four-foot-diameter tubes kept under near-perfect vacuum. The beams are used to monitor the distance between the mirrors that are precisely positioned at the ends of these arms.
According to the theory of general relativity, the distance between the mirrors will change infinitesimally when a gravitational wave passes by the detector. The devices are sensitive enough to detect a change in the length of the arms less than 1/10,000 the diameter of a proton.
The detected waves were produced during the final fraction of a second in the collision of two black holes, resulting in a single, more massive black hole. Such collisions have been predicted, but never observed until now.
A computer simulation created by the Simulating eXtreme Spacetimes Project (SXS) showing space and time warping around two colliding black holes. (Video courtesy of SXS.)
Based on the signals, LIGO researchers estimate that the event occurred 1.3 billion years ago between black holes approximately 29 and 36 times the mass of the sun. The collision converted three times the sun’s mass into gravitational waves in a fraction of a second, with a peak power output of 50 times the baseline of the entire visible universe.
The 7-millisecond delay between the signals’ arrival at two separate LIGO detectors—first in Livingston, LA and then in Hanford, WA—indicates that the source was located in the Southern Hemisphere.
General Relativity and Gravitational Waves
Einstein’s theory predicts that a pair of black holes orbiting each other will lose energy though the emission of gravitational waves. This will cause them to gradually approach each other over billions of years – accelerating their approach once reaching a close enough proximity.
A computer simulation showing how the collision of two black holes would look to us; stars appear warped due to the incredibly strong gravity of the black holes bending light in a process called gravitational lensing. (Video courtesy of SXS.)
In the final fraction of a second before colliding, the two black holes reach velocities of nearly half the speed of light. The black holes form a single, massive black hole and convert a portion of the combined mass to energy. This energy, in the form of gravitational waves, is in accordance with Einstein’s famous formula: E=mc 2 .
The signals of gravitational waves detected by the LIGO observatories. The two top plots show data received at Livingston and Hanford, along with the predicted shapes for the waveform according to general relativity. The bottom plot compares data from both detectors—Hanford’s data have been inverted for comparison, due to the two detectors’ difference in orientation. (Image courtesy of LIGO Laboratory.)
However, the new LIGO discovery is the first observation of gravitational waves themselves.
Rainer Weiss, emeritus professor at MIT and one of the scientists who originally proposed LIGO, commented that, “This observation is beautifully described in Einstein’s 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.”
The discovery has been accepted for publication in Physical Review Letters .