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Einstein's Theory of Gravitation

Our modern understanding of gravity comes from Albert Einstein’s theory of general relativity, which stands as one of the best-tested theories in science. General relativity predicted many phenomena years before they were observed, including black holes, gravitational waves, gravitational lensing, the expansion of the universe, and the different rates clocks run in a gravitational field. Today, researchers continue to test the theory’s predictions for a better understanding of how gravity works.

A Century of Relativity

Albert Einstein published his full theory of general relativity in 1915, followed by a flurry of research papers by Einstein and others exploring the predictions of the theory. In general relativity (GR), concentrations of mass and energy curve the structure of spacetime, affecting the motion of anything passing near — including light. The theory explained the anomalous orbit of Mercury, but the first major triumph came in 1919 when Arthur Eddington and his colleagues measured the influence of the Sun’s gravity on light from stars during a total solar eclipse.

Physicists made many exotic predictions using general relativity. The bending of light around the Sun is small, but researchers realized the effect would be much larger for galaxies, to the point where gravity would form images of more distant objects — the phenomenon now called gravitational lensing. GR also predicted the existence of black holes: objects with gravity so intense that nothing getting too close can escape again, not even light.

General relativity showed that gravitation has a speed, which is the same as the speed of light. Catastrophic events like collisions between black holes or neutron stars produce gravitational waves. Researchers finally detected these waves in 2015 using the Laser Interferometer Gravitational Observatory (LIGO), a sensitive laboratory that took decades to develop.

For many aspects of astronomy — the motion of planets around stars, the structure of galaxies, etc. — researchers don’t need to use general relativity. However, in places where gravity is strong, and to describe the structure of the universe itself, GR is necessary. For that reason, researchers continue to use GR and probe its limits.

  • Black holes are extremely common in the universe. Stellar-mass black holes, the remnants of massive stars that exploded, are sometimes the source of powerful X-ray emissions when they are in binary systems with stars. In addition, nearly every galaxy harbors a supermassive black hole at its center, some of which produce powerful jets of matter visible from across the universe. GR is essential to understanding how these objects become so bright, as well as studying how black holes form and grow. The Event Horizon Telescope (EHT) is a world-spanning array of observatories that captured the first image of a supermassive black hole, providing a new arena for testing GR’s predictions.

  • Gravitational waves are a new branch of astronomy, providing a complementary way to study astrophysical systems to the standard light-based observations. Researchers use GR to provide “templates” of many possible gravitational wave signals, which is how they identify the source and its properties. Gravitational wave astronomy combines with light-based astronomy to characterize some of the most extreme events in the cosmos: collisions of black holes and neutron stars.

  • Astronomers use gravitational lensing to locate some of the earliest galaxies in the universe, which are too faint to be seen without the magnification provided by gravity. In addition, the distortion created by lensing allows researchers to study dark matter, and map the structure of the universe on the largest scales.

  • Not long after Einstein published GR, researchers realized the theory predicts that the universe changes in time. Observations in the 1920s found that prediction was true: the universe is expanding, with galaxies moving away from each other. Using GR, cosmologists found the cosmos had a beginning, and was once hotter and denser than it is today. GR provides the mathematical framework for describing the structure and evolution of the universe from its beginnings 13.8 billion years ago, and into the future.

This artist’s illustration depicts two merging neutron stars and the gravitational waves they emit. As the LIGO and Virgo gravitational wave observatories have confirmed, collisions of black holes and neutron stars emit enough gravitational waves to be seen billions of light-years away.

Credit: NSF/LIGO/Sonoma State University/A. Simonnet