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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.

Our Work

Center for Astrophysics | Harvard & Smithsonian astrophysicists research the predictions of general relativity in many ways:

  • Capturing the first image of a supermassive black hole using the Event Horizon Telescope (EHT). This image of the black hole at the center of the nearby galaxy M87 reveals how gravitation affects the matter in orbit and the light that material emits, providing a novel test of general relativity in a regime where gravity is very strong.
    CfA Plays Central Role In Capturing Landmark Black Hole Image

  • Using gravitational lensing to search for the earliest galaxies in the universe. While they’re too faint to be seen directly, closer-by galaxies and clusters sometimes magnify their light, allowing us to learn about the ancestors of the Milky Way and other modern galaxies.
    Discovering Distant Radio Galaxies via Gravitational Lensing

  • Reconstructing the location of most of the mass in the universe using gravitational lensing. Next-generation observatories like the Large Synoptic Survey Telescope (LSST) will provide a census of millions of galaxies from their gravitational distortions.
    Mapping Dark Matter

  • Performing follow-up observations of gravitational wave events, to confirm the nature of the source. Collisions between neutron stars produce a lot of light in the form of short duration gamma ray bursts in addition to gravitational waves. Astronomers observed such a collision in 2017 using Dark Energy Camera on the Blanco Telescope in Chile, providing complementary data to the observation from LIGO.
    Astronomers See Light Show Associated With Gravitational Waves

  • Studying gravitational wave sources that aren’t visible to LIGO, but will be to future gravitational observatories. Using visible light telescopes, astronomers have observed that white dwarf binaries are relatively common in the galaxy, and some of them are in sufficiently tight orbits to be emitters of gravitational waves. One pair in particular orbits every 12.75 minutes, which will make it the strongest source for the future Laser Interferometer Space Antenna (LISA).
    Space-Warping White Dwarfs Produce Gravitational Waves

  • Determining whether black holes are actually what GR predicts. While GR is very clear that black holes exist, alternative theories propose different objects that behave in different ways. The challenge is that black holes appear very small in our telescopes, so it’s hard to observe their behavior. However, researchers have ruled out a number of alternative explanations, based on many observations of black holes.
    Do Stars Fall Quietly into Black Holes, or Crash into Something Utterly Unknown?

  • Testing general relativity’s prediction about the shape of a black hole. The Event Horizon Telescope is designed to take a picture of the “shadow” of the Milky Way’s supermassive black hole, which is the dark region through which no light passes. The shape of this shadow is predicted by GR, so the EHT will provide the first precision measurement of a fundamental property of a black hole.
    Event Horizon Telescope Reveals Magnetic Fields at Milky Way's Central Black Hole

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