Gravitational lens

A gravitational lens is when matter, like a cluster of galaxies or a tiny point particle, bends the light from something far away as that light travels toward us. This bending of light was described by Albert Einstein in his general theory of relativity. Even if we think of light as tiny particles moving at the speed of light, Newtonian physics also tells us that light should bend, though not as much as Einstein’s theory predicts.

Scientists like Orest Khvolson in 1924 and Frantisek Link in 1936 were the first to write about this effect, but it is often linked to Einstein, who did his own calculations in 1912 and wrote about it in 1936. Later, in 1937, Fritz Zwicky suggested that groups of galaxies could act like these lenses. This idea was proven true in 1979 when astronomers observed something called the Twin QSO SBS 0957+561, showing that galaxy clusters really can bend light from distant objects.

Description

A gravitational lens bends light in a special way. Light that passes close to the center of the lens bends the most, while light farther away bends less. This creates a line instead of one point where the light comes together. When the light source, the lens, and the observer are in a straight line, the light can form a ring around the lens. If they are not perfectly lined up, the light will look like arcs.

Scientists first talked about this in 1924, and Albert Einstein explained it more in 1936. There are three main types of gravitational lensing. Strong lensing makes big shapes like rings and arcs. Weak lensing causes small changes that we find by looking at many distant stars together. Microlensing changes how bright a star looks over time without changing its shape. Gravitational lenses work on all kinds of light, not just what we can see, and they can even affect waves from space.

History

A long time ago, scientists discovered that gravity can bend light. Even before Albert Einstein, some people thought this might be true. Later, Einstein used his ideas about space and time to figure out just how much light would bend.

In 1919, during a total solar eclipse, scientists watched the stars close to the Sun. They noticed that the stars looked a little out of place because their light had bent around the Sun. This amazing finding made Einstein and his ideas very well-known. After that, scientists found many more examples of light bending around large objects in space, showing just how strong gravity can be.

Approximate Newtonian description

Isaac Newton wondered if light could be bent by gravity. Using simple physics, scientists can guess how much a beam of light might bend when it passes near a big object.

They found that the bend would be about half of what Albert Einstein’s theory of relativity predicts. This shows that even easy ideas can help us learn about the universe, even if they aren’t the whole story.

Explanation in terms of spacetime curvature

See also: Kepler problem in general relativity

In the theory of general relativity, light follows the curves of space. When light passes near a big object, like a star or a galaxy, its path bends. This makes it look like the light is coming from a different place, like how a normal lens works. The amount of bending depends on how heavy the object is and how close the light passes to it.

The bending happens because space is curved by heavy objects. This idea was important in Albert Einstein's work, showing how gravity affects even light.

Search for gravitational lenses

Most gravitational lenses are found by chance. Scientists use special tools, such as the Very Large Array in New Mexico, to look for these lenses. These tools help scientists learn more about faraway objects and the universe.

New projects and telescopes, like the Euclid Space Telescope and the Vera C. Rubin Observatory, will find many more lenses. This research helps us understand how the universe works.

Solar gravitational lens

Main article: Solar gravitational lens

Albert Einstein said that the Sun’s gravity can bend light from far away places. If we put a spacecraft very far from Earth, about 542 times farther than Earth is from the Sun, we could use this natural lens to see very distant objects more clearly.

People thought this could help us search for signals from space, but it is still very hard to send a spacecraft that far.

In 2020, a scientist suggested using this idea to take very detailed pictures of planets outside our solar system. With this method, we might see these distant worlds well enough to look for signs that they could support life.

Measuring weak lensing

Scientists have special ways to study how gravity bends light from faraway galaxies. One method, called KSB+, helps them measure how much the light is bent. It works well because it is simple and easy to use. But it needs the telescope’s blurring to look a certain way. With better telescopes, like the LSST, we may need even better ways to measure this bending of light.