Though amateur stargazers often assume objects in the universe are static, astronomers have discovered that some things aren’t what they appear to be. Gravitational lensing is a phenomena where the image of one heavenly body that gives off light—a nebula, quasar, or galaxy, among others—is altered as its light passes by or through another object. It’s as if the light has passed through a real lens, altering the image before it reaches the observer.
Gravitational lensing occurs because spacetime is curved around all objects in the universe. That’s what Einstein taught us: that mass indents spacetime, creating depressions. What we think of as gravity is actually an object following the curves of these spacetime highways.
That’s true for light as well. When light passes by a celestial object, it follows the curves of spacetime surrounding the object—like a toy car following the set path of a racetrack. As a result, the light gets bent.
Though it may sound like an alien concept, gravitational lensing is actually happening all around us, all the time. “I’m a gravitational lens,” says Paul Schechter, professor of astrophysics at MIT, illustrating its ubiquity.
In fact, while you are reading this article, you are a gravitational lens, because you exert a gravitational force and are bending the light source you are using to look at this page. But since objects on earth have very little mass and therefore exert little gravitational force, the amount of light that is distorted is imperceptible to the human eye. It is only when scientists look at the massive objects in the sky that gravitational lensing has any measurable effect.
Gravitational lenses are similar to optical lenses because they both bend light, though optical lenses utilize refraction instead of gravitational fields to get the job done. And like optical lenses, the light can be bent in different ways, creating different images. Similarly the size of the lens and the distance of the lens from the light source determines the image the viewer sees. But unlike an optical lens, a gravitational lens also has the ability to distort other forms of electromagnetic radiation, such as radio waves and X rays.
Gravitational lenses also come in different strengths. Strong gravitational lensing can be compared to a pair of thick prescription glasses that correct vision by bending light more drastically than glasses with a weaker prescription. If a person with 20/20 vision tries on a pair of strong corrective lenses, they will find that the images they see are more distorted than if they were wearing a weaker pair of glasses.
In strong lensing, the appearance of the background light source is distorted into arcs, double images or rings, depending on the situation, and is often a result of a very massive lens, such as an entire galaxy. If the background object is aligned with the lens and the observer in a straight line, it will appear as a ring, a halo of light surrounding the lens called an Einstein ring.
If the background image, the lens, and the observer are not perfectly aligned, the image is distorted into an arc instead of a ring.
A twin image of the background object can also occur, if the lens is extremely large and close to the object. This causes the light from the object to be cleaved in half and follow two separate paths to the observer, appearing as if there were two objects in the sky.
In weak lensing, the image of the background object is not as visibly altered, since the “prescription” is not as strong. Instead of forming circles or arcs, the object can appear merely stretched or magnified. This is often a result of a less massive lens, such as the mysterious component of the universe known as dark matter. Since dark matter cannot emit, absorb, or reflect light, but can bend it, scientists use the appearance of weak gravitational lenses to map out the distribution of dark matter in any given area.
Microlenses are the veritable reading glasses of gravitational lensing, and can occur if a very small object, such as a star, is the lens. The image of the background object may not appear distorted, but its brightness may vary over time. That’s because they are seeing multiple images and not realizing it. Because the duplicated images are so small and close together, the object just appears brighter, since it is giving off combined light from its multiple copies. Due to this additional brightness, microlensing sometimes allows scientists to view objects that would be too far away or dim to see.
Gravitational lensing was first hypothesized by the Russian physicist Orest Chwolson in 1924 and then more famously by Albert Einstein in 1936, as part of his general theory of relativity. But during their lifetimes, gravitational lensing was just a theory, only discovered by accident in 1979 in the form of a double image quasar.
Now more than thirty years later, Schechter, co-chair of the Science Definition Team, hopes to build a Wide-Field Infrared Survey Telescope, which will be used to measure weak gravitational lensing in order to learn more about dark matter and dark energy. With gravitational lenses now transformed from a theory into a necessary tool for understanding the universe, the average person may soon be learning about these lenses, instead of just being one.