A gravity lens is an astrophysical phenomenon whereby the gravitational field of a massive object bends the light from a more distant object. This effect can be used to magnify or distort the image of the more distant object.
The most famous example of a gravity lens is the so-called “Einstein Cross”, which is formed by the gravitational lensing of quasars by a foreground galaxy. However, there are many other examples of this phenomenon, both in our own Galaxy and beyond.
Gravity lenses are not just restricted to light; they can also affect other forms of electromagnetic radiation, such as radio waves and X-rays. In fact, the first evidence for gravitational lensing was actually discovered in the radio regime!
One of the key predictions of Einstein’s theory of general relativity is that massive objects will bend the space around them. This means that light travelling through this curved space will also be bent. The amount of bending depends on two things: the mass of the object doing the bending, and how close to it the light passes.
If we imagine a very massive object (such as a galaxy) between us and a more distant object (such as another galaxy), then its gravity will cause any light from that background object to bend around it. This means that we will see multiple images of the background object, each offset from one another by some amount. The closer we are to alignment with one particular image,the brighter it will appear; this is known as “gravitational magnification”. Conversely, if we are further away from alignment, then one or more images may appear distorted or even disappear altogether; this is known as “gravitational microlensing”.
Gravity lenses can therefore be used to study extremely faint objects that would otherwise be undetectable – for example, planets orbiting other stars in our Galaxy – or to probe very small scales in astrophysical phenomena (such as quasar accretion disks).