In February 2016, astronomers announced the first-ever direct detection of gravitational waves, a phenomenon originally predicted by Albert Einstein a century ago in his theory of general relativity and sought after by physicists and astronomers for nearly 50 years. The discovery not only confirmed a prediction of general relativity but also opened an entirely new window to understanding the universe: the ability to explore the cosmos relying not just on light, but on gravity itself. Since the first announcement in 2016, five more gravitational-wave detections have been confirmed, including one detection in which both gravitational waves and electromagnetic radiation were observed from the same event, collectively generating new insights into nuclear physics, the cosmic origin of heavy elements, and some of the most extreme objects in the universe.
Gravity as a way of seeing
Virtually everything we know about the universe—its origin, its structure, its evolving dynamics—scientists have learned by studying light. Not just visible light, but electromagnetic radiation more generally, from low-frequency radio waves to high-frequency X-rays and gamma rays.
But light tells only part of the story. Just as hearing adds enormously to our experience of the everyday world compared to vision alone, scientists have for decades suspected they could understand the universe in more detail if only they could observe it through a “messenger” separate from light. Gravitational waves, in particular, could effectively serve as the sound waves of the universe—if scientists could develop technologies sensitive enough to detect them.
Gravitational waves in theory
Einstein’s theory of general relativity led to the prediction that certain accelerating objects—such as an astronomical body in orbit around another—would create gravitational waves: ripples in spacetime similar to those created by a boulder tossed into water. (Note: Definitions of technical terms such as “spacetime” are at the end of this fact sheet). Energetic or violent cosmological events, such as colliding black holes, merging neutron stars, and even the birth of the universe itself, were predicted to create gravitational waves.
According to theory, gravitational waves squeeze and stretch spacetime as they pass through it. Even when originating in cataclysmic events across the universe, the effect of a gravitational wave would typically be very small when it reaches Earth, distorting the length of a kilometer by an amount thousands of times smaller than an atomic nucleus.
Scientists also predicted that both light and gravitational waves travel at the same speed in a vacuum, but because gravitational waves are a fundamentally different physical process from light, no telescope designed to detect light—whether on a mountaintop or in orbit—can detect them.
Using pulsars to detect gravitational waves
According to general relativity, two bodies in orbit around each other won’t remain in exactly the same orbit for all time. Instead, the bodies emit gravitational waves and, in doing so, lose energy. Over millions to billions of years, that energy loss causes the bodies to spiral in toward each other before eventually colliding.
In 1974, scientists discovered the first pair of orbiting neutron stars, one of which was a pulsar. Binary stars are common, but not with a pulsar as one of the pair. The exceptionally regular pulses of light emitted by pulsars enabled precision measurements of the system over time.
Over several years, those measurements showed that the two neutron stars were spiraling toward each other at exactly the rate predicted by Einstein’s theory, providing the first indirect proof of the existence of gravitational waves.
Two American astrophysicists, Russell Hulse and Joseph Taylor, were awarded the Nobel Prize in Physics in 1993 in recognition of the original discovery of this binary pulsar, which ultimately opened up new avenues for studies of gravity.
Repeated observations of the precise pulses of light from a pulsar can also be used to detect background gravitational waves from across the universe, such as those emitted from pairs of merging supermassive black holes. Gravitational waves that pass by a pulsar will cause the timing of the pulses’ arrivals at Earth to vary; repeated observations of a pulsar could detect these small variations. (In this method, the pulsars are being used as celestial clocks to detect gravitational waves from much more massive objects and are not the emitters of the gravitational waves themselves.)
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration takes advantage of this pulsar timing technique, using telescopes in the United States (and soon in Canada) to observe an array of pulsars over time to search for evidence of background gravitational waves as well as those from individual supermassive-binary-black-hole systems. Similar collaborations capitalizing on this technique are the European Pulsar Timing Array and the Parkes Pulsar Timing Array (based in Australia). This method is expected to produce a gravitational-wave detection within the next five years.
Interferometric observations of gravitational waves
The direct detection of gravitational waves requires extremely sensitive instruments located far apart, which can confirm each other’s candidate detection events and identify the source’s location in the sky.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a collaborative project with over 1,000 researchers from 18 countries and is primarily funded by the National Science Foundation. The observatory includes two detectors separated by nearly 1,900 miles: one in Louisiana and the other in Washington state. A similar third detector, run by the Virgo Collaboration, is in Italy.
Nearly 100 years after Einstein’s prediction, and four days prior to officially beginning operations, the Advanced LIGO detector in Livingston, Louisiana, received a signal; less than 10 milliseconds later that signal arrived at the second detector in Hanford, Washington. After months of intensive examination and re-examination by the LIGO Scientific Collaboration and Virgo Collaboration teams to rule out alternative explanations for the observation—from instrument calibration issues to distant seismic events—the scientists announced in February 2016 that the signal was the first detection of gravitational waves, arising from the collision of two black holes 1.3 billion light-years away.
The 2017 Nobel Prize in Physics was awarded to three American physicists (Barry Barish, Kip Thorne, and Rainer Weiss) for their contributions to the first observation of gravitational waves and the development of the exquisitely precise LIGO detectors that enabled this discovery.
In October 2017, the LIGO and Virgo Collaborations announced another detection of gravitational waves, this time coincident with a gamma-ray burst detected by an orbiting satellite, leading scientists to conclude they had witnessed the merger of two neutron stars 130 million light-years away.
Electromagnetic signals are expected to accompany the gravitational-wave emission from merging neutron stars. With both LIGO and Virgo operating, it was possible to pinpoint the location of the gravitational-wave signal to a small portion of the sky. In the minutes, days, and weeks following the initial detection, telescopes around the world and in orbit sprang to action to capture light from the debris of the merger in radio, infrared, optical, ultraviolet, and x-ray frequencies, enabling astronomers for the first time to observe a neutron star collision and its aftermath in both light and gravitational waves.
These “multi-messenger” observations using both light and gravitational waves confirmed the link between neutron-star collisions and mysterious gamma-ray bursts—a connection that had been hypothesized over 30 years ago.
They also revealed the previously uncertain cosmic origin of heavy elements such as gold, platinum, and uranium. Nuclear reactions capable of creating elements heavier than iron (such as these) had been hypothesized and modeled, but they require extremely energetic, neutron-rich environments to take place; the merger of two neutron stars was just such an environment. Multiple telescopes around the world captured the glow from these newly synthesized atoms in real time as the halo of heavy elements dispersed in the days following the neutron-star merger.
The European Space Agency (ESA), in collaboration with the National Aeronautics and Space Administration (NASA), is planning a space-based gravitational-wave mission called the Laser Interferometer Space Antenna (LISA) that is scheduled to launch in 2034. LISA would include three spacecraft flying in formation millions of miles apart; by bouncing laser beams back and forth between them, LISA would be able to detect minute changes in the separation between the spacecraft that result from passing gravitational waves. LISA would be sensitive to the gravitational waves produced by different cosmic phenomena than LIGO and Virgo, including the violent mergers of supermassive black holes in the distant universe, and it is anticipated to bring new insights into how galaxies form and evolve.
In mid-2017, ESA and NASA concluded a successful 18-month proof-of-concept mission called LISA Pathfinder to test and validate the precision control and measurement technologies needed to make LISA a reality.
Because we knew you would ask
A neutron star is the remnant of the core of a massive star after it runs out of fuel. Among the most dense objects known in the universe, neutron stars may have a mass a few times that of the Sun packed into a sphere only about 12 miles across, or the size of a city.
Black holes are objects so dense that not even light can escape their gravity. Like neutron stars, black holes with masses up to twice that of the Sun are formed in the violent death of large stars. Supermassive black holes—with masses hundreds of thousands to billions of times the mass of the Sun—are found at the centers of massive galaxies; their origin is not yet well understood.
Pulsars are rotating neutron stars whose intense magnetic fields create beams of electromagnetic radiation. As the neutron star rotates, those beams of light sweep across the sky like the beacon of a lighthouse and appear to observers on Earth as regularly spaced pulses of light.
Spacetime is a mathematical description of the universe that combines time and the three dimensions of space into a four-dimensional continuum. Einstein’s theory of general relativity describes how a mass (such as the Sun) causes spacetime to bend, creating the effect we call gravity.
LAST UPDATED FEB. 7, 2018
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Key references for those who want to dig deeper
1. In The Foundation of the General Theory of Relativity, 1916, Albert Einstein consolidates his work in four previous papers to present his general theory of relativity, which states that the mere presence of mass - such as a galaxy, a star, or even a pebble - causes space and time to curve. That curvature produces the effect we call gravity.
2. Approximative Integration of the Field Equations of Gravitation, 1916 (Proceedings of the Prussian Academy of Science) presents Einstein’s first mathematical derivation of gravitational waves, although Einstein later found that he had made a mathematical error in this work. On Gravitational Waves, 1918 (Proceedings of the Prussian Academy of Science) corrects that error and in doing so, Einstein showed that two orbiting bodies will slowly lose energy by emitting gravitational waves.
3. Pulsar timing measurements and the search for gravitational waves, 1979 (The Astrophysical Journal) showed that precise measurements of pulsar timing could be used to detect the gravitational waves that originate across the universe from such phenomena as the interaction of supermassive black holes (that is, black holes with a mass of hundreds of thousands to billions of times the mass of the Sun).
4. A new test of general relativity - Gravitational radiation and the binary pulsar PSR 1913+16, 1982 (Astrophysical Journal) presents the first indirect observational evidence of gravitational waves based on data about the orbit of two neutron stars.
5. The Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration and Virgo Collaboration published Observation of Gravitational Waves from a Binary Black Hole Merger in 2016 (Physical Review Letters) detailing the first detection of gravitational waves, originating from the merger of two black holes, using the two LIGO detectors.
6. In 2017, GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral (Physical Review Letters) reports the LIGO Scientific Collaboration’s and Virgo Collaboration’s observation of gravitational waves from the merger of a pair of neutron stars. At the same time, a collection of nearly 4,500 authors also published Multi-messenger Observations of a Binary Neutron Star Merger (The Astrophysical Journal Letters) describing supplementary observations of the same neutron-star merger and aftermath in radio, infrared, optical, ultraviolet, x-ray, and gamma-ray light.
7. In Sub-Femto-g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results the European Space Agency-led Laser Interferometer Space Antenna (LISA) Pathfinder team reports in Physical Review Letters the positive results from a space-based proof-of-concept mission designed to test precision measurement technologies that may one day detect gravitational waves from supermassive black holes.
8. The Nobel Prize in Physics 1993 was awarded to Russell Hulse and Joseph Taylor for their 1975 discovery of an orbiting pair of neutron stars, which opened the door for the 1982 measurement of their orbits and the first indirect detection of gravitational waves. In 2017, Barry Barish, Kip Thorne, and Rainer Weiss shared the Nobel Prize in Physics for their work on the Laser Interferometer Gravitational-Wave Observatory (LIGO) leading to the first direct observation of gravitational waves in 2015.