gravitational lensing

History

Einstein predicted the bending of light in 1915, and in 1937 Fritz Zwicky suggested that galaxies could act as gravitational lenses. The first real lens was seen in 1979 — the double quasar Q0957+561.

How it works

A massive body curves spacetime around it, like a heavy ball denting a mattress. Light, traveling through curved space, bends around the obstacle and focuses at different points, creating unusual images.

History

Einstein predicted the bending of light in 1915, and in 1937 Fritz Zwicky suggested that galaxies could act as gravitational lenses. The first real lens was seen in 1979 — the double quasar Q0957+561.

How it works

A massive body curves spacetime around it, like a heavy ball denting a mattress. Light, traveling through curved space, bends around the obstacle and focuses at different points, creating unusual images.

History

The idea of light bending by a massive body had been discussed since the 18th century, but within Newtonian theory. In 1915 Einstein derived the correct deflection angle (twice the Newtonian value). In 1919 Eddington's expedition confirmed the deflection during a solar eclipse. In the 1930s Fritz Zwicky proposed using lensing by galaxy clusters as 'telescopes'. The first multiple system (double quasar) was found in 1979 by Dennis Walsh and colleagues. Today it is a key tool for cosmology, e.g., surveys by the Vera Rubin Observatory.

How it works

According to general relativity, Einstein's equations describe spacetime curvature. Light follows null geodesics, and its trajectory is described by an effective refractive index n = 1 - 2Φ/c², where Φ is the gravitational potential. The deflection angle for a point mass is α = 4GM/(c²ξ), where ξ is the impact parameter. When source, lens, and observer align, an Einstein ring forms with angular radius θ_E = sqrt((4GM)/(c²) * (D_ls)/(D_l D_s)), where D are distances. For distributed mass, lensing yields distortions from which a density map of matter, including dark matter, is constructed.

Open problems & fun facts

Can warm and cold dark matter be distinguished through small-scale lenses?; What is the nature of lensing on very large scales, affecting the cosmic microwave background?; Do primordial black holes exist that can be found via microlensing?; How to properly model complex lens potentials with substructure?

💡 Gravitational lensing helped discover the distant star Earendel, whose light traveled nearly 13 billion years – magnified thousands of times by a galaxy cluster!