Light begins to bend in the presence of any gravity, no matter how weak. Even a 15-pound bowling ball will technically deflect the beam of a passing flashlight.
According to Albert Einstein’s general theory of relativity, gravity is not a traditional invisible force reaching out to grab things. Instead, it is a curve in the fabric of space and time. Any object with mass creates a dent in this fabric, and light simply follows the straightest possible path—a geodesic—across that curved terrain.
While every object bends light, the effect of everyday mass is astronomically small—far beyond our ability to measure it. Even an object as large as the Earth only bends a beam of light passing along its surface by about a billionth of a degree.
To see light bend in a measurable way, you need a tremendous amount of mass. The first proof of this phenomenon came in 1919 during a total solar eclipse. Astronomer Arthur Eddington photographed the stars sitting just behind the Sun. Because the Sun’s mass warped the space around it, the light from those background stars traveled along a curved path to reach Earth, making the stars appear slightly shifted from their known positions.
When an object is massively heavy—like an entire galaxy—it can bend light so severely that it acts like a cosmic magnifying glass. If a foreground galaxy sits perfectly between Earth and an even more distant light source, the gravity forces the distant light to bend around all sides, forming a ring. Astronomers call this an "Einstein Ring." In the image captured by the Hubble Space Telescope below, a massive luminous red galaxy has warped the light of a distant blue galaxy into a giant cosmic horseshoe.
From a photon’s perspective, it is never actually turning or being pulled off course. Light always travels perfectly straight. It is the universe itself that is curved.