We might think that we have pretty well understood gravity. If we drop a ball from a certain height, soon after, it will touch the ground. But something much deeper is happening, and the picture below illustrates it perfectly.
Albert Einstein published his theory of general relativity in 1915, which has since been widely accepted to describe gravity. One of the key principles is that space exists not in three dimensions, but as a four-dimensional space-time. Anything that has energy must literally change the shape of space-time, thus changing the effects of gravity. In truth, there are few things that illustrate this picture better than the Einstein ring. And precisely, a magnificent example of this phenomenon has recently been captured by the Hubble Space Telescope.
As Einstein had shown, the existence of a space-time bent by matter can manifest itself in the path of light rays. It can therefore be expected that a sufficiently large concentration of matter will act on light from a star or galaxy. In the middle of the image you can see two bright lights — it’s actually a cluster of hundreds of galaxies called SDSS J0146-0929. There is so much mass in this area that the cluster deforms the light of the objects behind it.
The red ring is not just a ring: It is also a galaxy, but its light is strongly distorted by the presence of the huge cluster in the foreground. When the two objects are almost exactly aligned, the light thus forms a circle, called an Einstein ring, around the cluster of galaxies in the foreground.
Gravitational lenses have become an important tool for astronomers today. They have recently proven that they can measure the mass of a star using the way it tilts the light of the stars behind it – something that Einstein himself believed impossible.
Gravitational lenses also revealed the oldest spiral galaxy ever recorded. More recently, a galaxy has magnified a star in the background by nearly 2,000 times — this star is now one of the farthest ever seen.