Gravitational Lensing

Gravitational lensing is a direct consequence of spacetime curvature. A massive object warps the spacetime around it, and light follows the straightest possible path—a geodesic—through this curved spacetime. From our perspective, this path appears bent. The degree of bending depends on the mass of the lensing object and the light’s proximity to it. There are three main classes of lensing. Strong lensing occurs with a massive object and precise alignment, producing multiple images, arcs, or a complete ‘Einstein ring’. Weak lensing involves subtle distortions of background galaxies’ shapes, which can be statistically analyzed to map mass distribution, including dark matter. Microlensing is a temporary brightening of a background star when a smaller object, like a planet, passes in front, briefly focusing its light. This is effective for finding objects that emit little or no light.

The first confirmation came in 1919 during a solar eclipse. Expeditions led by Sir Arthur Eddington and Sir Frank Dyson observed starlight passing near the Sun and confirmed its position was shifted by the amount predicted by Einstein’s theory. This result catapulted Einstein to international fame. Today, gravitational lensing is a fundamental tool in astronomy and cosmology, allowing scientists to weigh galaxy clusters, probe the distant universe by magnifying faint objects, and discover celestial bodies that would otherwise be undetectable.