Theories · 1912 – 1936
Gravitational Lensing
A massive object bends the light passing it and can act as a lens. Einstein worked it out, published it reluctantly, and thought nobody would ever see it.
The Idea
If gravity bends light, a massive object sitting between us and a distant source will bend the source's light around itself on all sides. Like a glass lens, it can magnify the source, distort it, or produce several images of it. If the alignment is perfect, the source appears as a circle of light around the foreground object: an Einstein ring. The equation above gives its angular size.
A Reluctant Paper
Einstein had done the calculation in a notebook as early as 1912, before general relativity was finished, and set it aside. In April 1936 a Czech amateur scientist named Rudi Mandl visited him in Princeton and pressed him to publish it. Einstein obliged with a short note in the journal Science, "Lens-Like Action of a Star by the Deviation of Light in the Gravitational Field."
He was not enthusiastic. For one star lensing another, the required alignment is so exact and the ring so small that he wrote there was no great chance of observing the phenomenon. In a private letter to the journal's editor he said the result had little value, but that it made the poor fellow happy.
He Was Too Pessimistic
The following year the astronomer Fritz Zwicky pointed out that entire galaxies, vastly more massive than stars, would make far better lenses, and that the effect should be observable.
It took until 1979. Astronomers found two quasars side by side with identical properties and realized they were looking at one quasar, imaged twice by a galaxy in front of it. The first Einstein ring was found with a radio telescope in 1987 and reported in 1988.
Where It Stands
Lensing is now one of the working tools of astronomy. It is used to weigh galaxies, to map matter that gives off no light, to find planets around other stars, and as a natural telescope to see galaxies too faint and distant to detect otherwise. In 2025 the European Space Agency announced that its Euclid telescope had found a nearly perfect Einstein ring around NGC 6505, a galaxy known since 1884. See Seeing With Gravity.
Sources
- NASA Jet Propulsion Laboratory — Euclid discovers an Einstein ring
- Einstein Online — the history of gravitational lensing
- European Space Agency — Euclid discovers a stunning Einstein ring
- Renn and Sauer, Max Planck Institute for the History of Science — Eclipses of the Stars: Mandl, Einstein, and the early history of gravitational lensing
- Science News — The amateur who helped Einstein see the light
- Physical Review — Zwicky, Nebulae as gravitational lenses (1937)
- National Radio Astronomy Observatory — discovery of the first Einstein ring
- NASA — Hubble's gravitational lenses