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

Unlike regular lenses, a gravitational lens bends light in a special way. The light that passes closest to the center of the lens bends the most, while light that passes farther away bends less. Because of this, there isn’t one single point where the light comes together, but instead a line. When the light source, the lens, and the observer are in a straight line, the light can form a ring shape around the lens. If they are not perfectly lined up, the light will look like arcs instead.

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, easy-to-see shapes like rings and arcs. Weak lensing causes small changes that can only be found 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

Long ago, scientists noticed that gravity could bend light. Even before Albert Einstein, some thinkers guessed this might happen. Einstein later used his ideas about how space and time work to calculate exactly how much light would bend.

In 1919, during a total solar eclipse, scientists 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 ideas very famous. Since then, scientists have found many more examples of light bending around big objects in space, showing how powerful gravity can be.

Approximate Newtonian description

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

They found that the bend would be about half of what Albert Einstein’s theory of relativity predicts. This shows how even simple ideas can give us clues 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 bending makes it look like the light is coming from a different place, similar to 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 itself is curved by heavy objects. This idea was a key part of Albert Einstein's work, showing how gravity affects not just things that are moving, but even light.

Search for gravitational lenses

Most gravitational lenses have been found by chance. Scientists used special tools like the Very Large Array in New Mexico to look for these lenses and found 22 new ones. This helped them 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. These tools will help scientists discover thousands of lenses over the next few years. This research helps us understand how the universe works and where things are located far away from us.

Solar gravitational lens

Main article: Solar gravitational lens

Albert Einstein predicted that light from far away can be focused by the Sun ’s gravity. If a spacecraft were placed about 542 times the distance from the Earth to the Sun, it could use this natural lens to see very distant objects more clearly. This idea was suggested for searching for signals from space, but sending a spacecraft that far is still a big challenge.

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

Measuring weak lensing

Scientists have developed special ways to study how gravity bends light from faraway galaxies. One common method, called KSB+, helps them measure how much the light is bent without getting confused by the telescope’s own blurring effects.

This method works well because it’s simple and easy to use. However, it assumes the blurring from telescopes looks a certain way. As we build even better telescopes, like the LSST, we might need even more precise ways to measure this bending of light.