General relativity is physicist Albert Einstein's understanding of how gravity affects the fabric of space-time.
The theory, which Einstein published in 1915, expanded the theory of special relativity that he had published 10 years earlier. Special relativity argued that space and time are inextricably connected, but that theory didn't acknowledge the existence of gravity.
Einstein spent the decade between the two publications determining that particularly massive objects warp the fabric of space-time, a distortion that manifests as gravity, according to NASA.
How does general relativity work?
To understand general relativity, first, let's start with gravity, the force of attraction that two objects exert on one another. Sir Isaac Newton quantified gravity in the same text in which he formulated his three laws of motion, the "Principia."
The gravitational force tugging between two bodies depends on how massive each one is and how far apart the two lie. Even as the center of the Earth is pulling you toward it (keeping you firmly lodged on the ground), your center of mass is pulling back at the Earth. But the more massive body barely feels the tug from you, while with your much smaller mass you find yourself firmly rooted thanks to that same force. Yet Newton's laws assume that gravity is an innate force of an object that can act over a distance.
Albert Einstein, in his theory of special relativity, determined that the laws of physics are the same for all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no matter the speed at which an observer travels, according to Wired.
As a result, he found that space and time were interwoven into a single continuum known as space-time. And events that occur at the same time for one observer could occur at different times for another.
As he worked out the equations for his general theory of relativity, Einstein realized that massive objects caused a distortion in space-time. Imagine setting a large object in the center of a trampoline. The object would press down into the fabric, causing it to dimple. If you then attempt to roll a marble around the edge of the trampoline, the marble would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space.
Experimental evidence for general relativity
In the decades since Einstein published his theories, scientists have observed countless of phenomena matching the predictions of relativity.
Gravitational lensing
Light bends around a massive object, such as a black hole, causing it to act as a lens for the things that lie behind it. Astronomers routinely use this method to study stars and galaxies behind massive objects.
The Einstein Cross, a quasar in the Pegasus constellation, according to the European Space Agency (ESA), and is an excellent example of gravitational lensing. The quasar is seen as it was about 11 billion years ago; the galaxy that it sits behind is about 10 times closer to Earth. Because the two objects align so precisely, four images of the quasar appear around the galaxy because the intense gravity of the galaxy bends the light coming from the quasar.
In cases like Einstein's cross, the different images of the gravitationally lensed object appear simultaneously, but that isn't always the case. Scientists have also managed to observe lensing examples where, because the light traveling around the lens takes different paths of different lengths, different images arrive at different times, as in the case of one particularly interesting supernova.
Changes in Mercury's orbit
The orbit of Mercury is shifting very gradually over time due to the curvature of space-time around the massive sun, according to NASA. In a few billion years, this wobble could even cause the innermost planet to collide with the sun or a planet.
Frame-dragging of space-time around rotating bodies
The spin of a heavy object, such as Earth, should twist and distort the space-time around it. In 2004, NASA launched the Gravity Probe B (GP-B). The axes of the satellite's precisely calibrated gyroscopes drifted very slightly over time, according to NASA, a result that matched Einstein's theory.
"Imagine the Earth as if it were immersed in honey," Gravity Probe-B principal investigator Francis Everitt, of Stanford University, said in a NASA statement about the mission.
"As the planet rotates, the honey around it would swirl, and it's the same with space and time. GP-B confirmed two of the most profound predictions of Einstein's universe, having far-reaching implications across astrophysics research."
Gravitational redshift
The electromagnetic radiation of an object is stretched out slightly inside a gravitational field. Think of the sound waves that emanate from a siren on an emergency vehicle; as the vehicle moves toward an observer, sound waves are compressed, but as it moves away, they are stretched out, or redshifted. Known as the Doppler Effect, the same phenomena occurs with waves of light at all frequencies.
In the 1960s, according to the American Physical Society, physicists Robert Pound and Glen Rebka shot gamma-rays first down, then up the side of a tower at Harvard University. Pound and Rebka found that the gamma-rays slightly changed frequency due to distortions caused by gravity.
Gravitational waves
Einstein predicted that violent events, such as the collision of two black holes, create ripples in space-time known as gravitational waves. And in 2016, the Laser Interferometer Gravitational Wave Observatory (LIGO) announced that it had detected such a signal for the first time.
That detection came on Sept. 14, 2015. LIGO, made up of twin facilities in Louisiana and Washington, had recently been upgraded, and were in the process of being calibrated before they went online. The first detection was so large that, according to then-LIGO spokesperson Gabriela Gonzalez, it took the team several months of analysis to convince themselves that it was a real signal and not a glitch.
[See our full discovery story here and our complete coverage of the historic scientific discovery here]
"We were very lucky on the first detection that it was so obvious," she said during at the 228 American Astronomical Society meeting in June 2016.
Since then, scientists have begun quickly catching gravitational waves. All told, LIGO and its European counterpart Virgo have detected a total of 50 gravitational-wave events, according to program officials.
Those collisions have included unusual events like a collision with an object that scientists can't definitively identify as black hole or neutron star, merging neutron stars accompanied by a bright explosion, mismatched black holes colliding and more.
This article was updated on June 4, 2021 by Space.com senior writer Meghan Bartels.