What phenomenon describes the bending of light around massive objects?

The phenomenon describing the bending of light around massive objects is gravitational lensing. In Einstein’s general theory of relativity, mass and energy curve spacetime, and light follows those curved paths as it travels. A galaxy, galaxy cluster, or black hole between Earth and a distant object can therefore act like a cosmic lens.

The effect can magnify, stretch, brighten, or multiply the image of the background source. With near-perfect alignment, the source may appear as an Einstein ring; with less alignment, it can form arcs or several separate images. The first widely celebrated observation of gravitational light deflection came during the 1919 solar eclipse, when measurements suggested that starlight passing near the Sun had been bent.

Gravitational lensing is not the same as frame dragging, which concerns the twisting of spacetime caused by a rotating mass. It is also distinct from gravitational redshift and gravitational time dilation, which affect light’s frequency and the rate at which clocks appear to run. Modern astronomers use lensing to study dark matter, measure galaxy masses, and observe extremely distant galaxies that would otherwise be too faint to see.