Gravitational lens

A gravitational lens is when matter, like a cluster of galaxies or a small point particle, bends the light from something far away as that light comes toward us. This happens because of Albert Einstein's general theory of relativity. This theory explains how gravity can change the path that light travels. Even if we think of light as tiny bits called corpuscles moving at the speed of light, Newtonian physics also says that light should bend a little. But it says the bend should be only half of what Einstein’s theory predicts.

The idea of gravitational lensing was first talked about in books by scientists such as Orest Khvolson in 1924 and Frantisek Link in 1936. Most people connect it to Einstein, who studied it in 1912 and shared his work in 1936.

In 1937, a scientist named Fritz Zwicky said that groups of galaxies could work like natural gravitational lenses. This became true in 1979 when astronomers saw something called the Twin QSO SBS 0957+561. They saw that light from far away could really be bent by the gravity of big groups of galaxies.

Description

A gravitational lens bends light in a special way. Light that passes close to the center of the lens gets bent the most. Light that passes farther away gets bent less. Because of this, a gravitational lens does not have one focal point. Instead, it 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

A long time ago, scientists like Henry Cavendish and Johann Georg von Soldner thought that gravity could bend light. This idea came from Isaac Newton.

Later, Albert Einstein used his ideas about space and time to show that light bends even more than Newton thought.

In 1919, during a total solar eclipse, scientists watched the stars near the Sun. They saw that the stars looked in slightly different places because their light had bent around the Sun. This helped make Einstein and his ideas very famous. Many years later, in 1979, the first gravitational lens was found. It looked like two bright objects, but it was really one object whose light had been bent by a big 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 guess how much a beam of light might change direction when it passes near a big object. This simple way of thinking shows that the light bends a little, but not as much as Einstein’s theory of relativity says.

Even with this simpler idea, it shows that gravity can change the path of light.

Explanation in terms of spacetime curvature

See also: Kepler problem in general relativity

In general relativity, light follows the shape of space. When light passes near a big object, like a star or a galaxy, space around that object curves. This makes the light bend, like a lens can bend light. If you look at something far away behind a big object, the light from that far-away thing will bend around the 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 how much the light will bend. This bending effect helped show that Einstein’s ideas about space and time were right.

Search for gravitational lenses

Most gravitational lenses are found by chance. But scientists look for them on purpose, too. One search in the northern sky used radio waves and found 22 new lensing systems. This helped scientists learn more about faraway 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. A spacecraft placed far away could use this natural lens to see very distant objects clearly. This idea is still far from what we can do today, but scientists keep thinking about it.

In 2020, a scientist from NASA suggested using this idea to get 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 found ways to measure how gravity bends light from galaxies far away. One way, called KSB+, helps fix blurry pictures from Earth’s telescopes. This method looks at the shapes of galaxy pictures to learn how much the light has bent.

This method is easy to use but works best when some conditions are just right. Future telescope surveys may need even better tools to get the most accurate results.