Gravitational lens

A gravitational lens is a fascinating phenomenon where matter, such as a cluster of galaxies or a point particle, bends the light from a distant object as it travels toward us. This bending of light is explained by Albert Einstein's general theory of relativity, which shows how gravity can curve the path of light. Even if we think of light as tiny particles called corpuscles moving at the speed of light, Newtonian physics also tells us that light should bend, though it predicts only half the amount of bending that Einstein's theory does.

The idea of gravitational lensing was first discussed in print by scientists like Orest Khvolson in 1924 and Frantisek Link in 1936. However, it is most commonly linked to Einstein, who worked on the concept in 1912 and published his findings in 1936.

In 1937, another scientist named Fritz Zwicky suggested that groups of galaxies could act as natural gravitational lenses. This idea was proven true in 1979 when astronomers observed something called the Twin QSO SBS 0957+561, showing that light from distant objects could indeed be bent by the gravity of large galaxy clusters.

Description

Unlike regular lenses, a gravitational lens bends light in a special way. The light that passes closest to the center of the lens gets bent the most, while light that passes farther away gets bent less. Because of this, a gravitational lens doesn't have one focal point but instead creates a line where the light focuses.

This bending of light was first talked about in 1924 and later explained by Albert Einstein in 1936. When the light source, the lensing object, and the observer are in a straight line, the light forms a ring around the lens, called an Einstein ring. If they are not in a straight line, the light appears as arcs instead. There are three main types of gravitational lensing: strong lensing, which creates visible rings and arcs; weak lensing, which causes small changes that can be measured by studying many distant galaxies; and microlensing, where the amount of light from a distant star changes over time.

Main article: Strong lensing

Main articles: Weak lensing, Microlensing

History

Long ago, scientists like Henry Cavendish and Johann Georg von Soldner thought that gravity could bend light, just like Isaac Newton had guessed centuries earlier. Later, Albert Einstein used his theory of general relativity to show that light bends even more than Newton’s ideas suggested.

In 1919, during a total solar eclipse, scientists led by Arthur Eddington and Frank Watson Dyson watched the stars near the Sun. They saw that the stars’ positions were slightly shifted because their light had bent around the Sun. This amazing discovery made Einstein and his theory very famous. Years later, in 1979, the first gravitational lens was found. It looked like two identical bright objects, but it was actually one object whose light had been bent by a massive object between us and it.

Approximate Newtonian description

Isaac Newton wondered if light could be bent by gravity. Using his ideas about forces, scientists can estimate how much a beam of light might change direction when it passes near a massive object. This simple calculation shows that the light bends, but not as much as predicted by Einstein’s theory of relativity.

Even though this Newtonian approach gives only half the bending seen in Einstein’s theory, it still shows that gravity can affect light.

Explanation in terms of spacetime curvature

See also: Kepler problem in general relativity

In general relativity, light follows the shape of space itself. When light passes near a big object, like a star or a galaxy, the space around that object is curved. This makes the light bend, just like how a lens can bend light. So, if you look at something far away behind a big object, the light from that far-away thing will bend around the big object and might appear in a different place in the sky.

The amount of bending depends on how heavy the object is and how close the light passes to it. Scientists use special formulas to calculate exactly how much the light will bend. This bending effect was one of the first pieces of evidence that showed Einstein’s ideas about space and time were right.

Search for gravitational lenses

Most gravitational lenses have been found by chance. However, scientists have conducted special searches to find more of them. One such search in the northern sky used radio waves and discovered 22 new lensing systems, which helped scientists learn more about distant objects and the universe.

New telescopes and surveys, like the Euclid Space Telescope, Vera C. Rubin Observatory (LSST), and the upcoming Nancy Grace Roman Space Telescope, are expected to find thousands more gravitational lenses. These discoveries help scientists understand the universe better. In 2023, the JWST found a very distant galaxy acting as a lens, showing how powerful these tools are for exploring faraway objects.

Solar gravitational lens

Main article: Solar gravitational lens

Albert Einstein predicted in 1936 that light from far away could be focused by the Sun into a point about 542 AU away. A spacecraft placed there could use this natural lens to see very distant objects clearly. This idea is very far from what we can do today with our current technology, but scientists have kept thinking about it.

In 2020, a scientist from NASA suggested using this idea to get very detailed pictures of planets outside our solar system. This could help us look for signs that these planets might support life.

Measuring weak lensing

Scientists have developed ways to measure how gravity bends light from faraway galaxies. One common method, called KSB+, helps correct for blurry effects caused by Earth’s telescopes. This method looks at the shapes of galaxy images to figure out how much the light has been bent.

While this method is easy to use, it works best when certain conditions are met. Future telescope surveys might need even more precise tools to get accurate results.