February 11, 2016 – For the first time, scientists have observed ripples in the fabric of space-time – called gravitational waves – arriving at 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 to the cosmos.
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 September 14, 2015 at 3:51 a.m. MDT (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 the 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.
The LIGO detections represent a much-awaited first step toward opening a whole new branch of astrophysics. Nearly everything we know about the universe comes from detecting and analyzing light in all its forms across the electromagnetic spectrum – radio, infrared, visible, ultraviolet, X-rays and gamma rays. The study of gravitational waves opens a new window on the universe, one that scientists expect will provide key information that will complement what we can learn through electromagnetic radiation.
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.
The existence of gravitational waves was first demonstrated in the 1970s and 1980s by Joseph Taylor, Jr., and colleagues. In 1974, Taylor and Russell Hulse discovered a binary system composed of a pulsar in orbit around a neutron star. Taylor and Joel M. Weisberg in 1982 found that the orbit of the pulsar was slowly shrinking over time because of the release of energy in the form of gravitational waves. For discovering the pulsar and showing that it would make possible this particular gravitational wave measurement, Hulse and Taylor were awarded the 1993 Nobel Prize in Physics.
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, As Einstein Predicted
Einstein’s theory of general relativity describes the inner workings of gravity, which is the most important force on the scale of stars, galaxies, and the universe as a whole. Although it is complex and detailed, there is beauty and elegance in the theory’s form.
This theory describes spacetime as a fabric permeating the universe. Einstein posited that spacetime is not a static and rigid stage on which celestial bodies act, but is instead flexible, able to be distorted and warped as large masses move through it.
Picture a trampoline and a bowling ball. If you place the ball in the center of the trampoline its mass will cause a dip in the fabric where the ball is placed.
If large, very dense masses move in a particular way, such as pair of black holes or neutron stars spiraling into one another, the disturbances in the gravitational field will travel outward as gravitational waves.
In the analogy, if a pair of bowling balls were set to roll around each other on the trampoline, the fabric of the trampoline would undulate and oscillate, like ripples on a pond, reacting to the movement. The bowling balls would lose energy as a result of causing these oscillations, and they would ultimately collide with each other at the center.
Gravitational Waves Detected by LIGO
These plots show the signals of gravitational waves detected by the twin LIGO observatories at Livingston, Louisiana, and Hanford, Washington. The signals came from two merging black holes, each about 30 times the mass of our sun, lying 1.3 billion light-years away.
The top two plots show data received at Livingston and Hanford, along with the predicted shapes for the waveform. These predicted waveforms show what two merging black holes should look like according to the equations of Albert Einstein’s general theory of relativity, along with the instrument’s ever-present noise. Time is plotted on the X-axis and strain on the Y-axis. Strain represents the fractional amount by which distances are distorted.
As the plots reveal, the LIGO data very closely match Einstein’s predictions.
The final plot compares data from both detectors. The Hanford data have been inverted for comparison, due to the differences in orientation of the detectors at the two sites. The data were also shifted to correct for the travel time of the gravitational-wave signals between Livingston and Hanford (the signal first reached Livingston, and then, traveling at the speed of light, reached Hanford seven thousandths of a second later). As the plot demonstrates, both detectors witnessed the same event, confirming the detection.
“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.
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.
NSF is the lead financial supporter of 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.
“Scientists have been looking for gravitational waves for decades, but we’ve only now been able to achieve the incredibly precise technologies needed to pick up these very, very faint echoes from across the Universe,” said Professor Karsten Danzmann, the Director of the Albert-Einstein-Institut in Hannover, and a member of the LIGO collaboration.
Several of the key technologies that made Advanced LIGO so much more sensitive were developed and tested by the German UK GEO collaboration. Significant computer resources were 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 of the City of New York and Louisiana State University.
“In 1992, when LIGO’s initial funding was approved, it represented the biggest investment NSF had ever made,” said France Cordova, NSF director. “It was a big risk. But NSF is the agency that takes these kinds of risks. We support fundamental science and engineering at a point in the road to discovery where that path is anything but clear. We fund trailblazers. It’s why the U.S. continues to be a global leader in advancing knowledge.”
LIGO research is carried out by the LIGO Scientific Collaboration (LSC) 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 Universitat Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom and the University of the Balearic Islands in Spain.
LIGO was originally proposed as a means of detecting gravitational waves in the 1980s by Rainer Weiss, professor of physics, emeritus, from MIT; Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics, emeritus; and Ronald Dreyer, professor of physics, emeritus, also from Caltech.
Independent and widely separated observatories are necessary to determine the direction of the event causing the gravitational waves, and also to verify that the signals come from space and are not from some other local phenomenon.
Over the next several years, the Advanced LIGO detectors will be ramped up to full power, increasing their sensitivity to gravitational waves, and in particular allowing more distant events to be measured. With the addition of further detectors, initially in Italy and later in other locations around the world. UK scientists continue to contribute to the design and development of future generations of gravitational wave detectors.
The LIGO Laboratory is also working closely with scientists in India at the Inter-University Centre for Astronomy and Astrophysics, the Raja Ramanna Centre for Advanced Technology, and the Institute for Plasma to establish an Advanced LIGO detector on the Indian subcontinent. Awaiting approval by the government of India, it could be operational early in the next decade. Additional detectors will greatly improve the ability of the global detector network to localize gravitational-wave sources.
“Hopefully this first observation will accelerate the construction of a global network of detectors to enable accurate source location in the era of multi-messenger astronomy,” said David McClelland, professor of physics and director of the Centre for Gravitational Physics at the Australian National University.