Gravitational waves

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, gravitational-wave detections have become more commonplace as the sensitivity of the detectors improved. A total of 50 detections were announced by October 2020. Exceptional events include astronomical objects of unusual size, an early detection in which both gravitational waves and electromagnetic radiation (that is, light) were observed from the same event, and the June 2021 announcement of the final moments of a neutron star-black hole system spiraling inward before merging—a system that had been predicted to exist but had not until then been definitively seen.

• 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.

•  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.

• 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 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 definitive gravitational-wave detection in the coming years.

• 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, and Japan’s Kamioka Gravitational-Wave Detector began observations in 2020.

• 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 a detection of gravitational waves coincident with a gamma-ray burst detected by an orbiting satellite, leading scientists to conclude they had witnessed for the first time 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.

• In June 2020, the LIGO and Virgo teams announced the discovery of an astronomical object heavier that any known neutron star, but lighter than the lightest known black hole—an object in the “mass gap.” The object merged with a black hole, a cosmic event that released gravitational waves eventually detected on Earth, and left astronomers with a new puzzle to solve about the rare object.

• A new confirmed source of gravitational waves was announced in June 2021: a neutron star and black hole spiraling toward each other before the black hole consumes the neutron star. This type of binary pair had been theorized to exist, but with two separate gravitational-wave detections of the phenomenon there is now concrete observational evidence.

• 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 and 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.

• 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.