Light-bending in full glory
WE’RE all familiar with the obvious effects of gravity. It keeps the Moon in orbit around the Earth and the Earth orbiting the Sun, and prevents us floating off into space. It also causes red wine from a tipped-over glass to pour downwards onto the carpet, which in my case is usually white.
However, there is an effect of gravity that is not seen in our everyday experience: the bending of light in a gravitational field, called gravitational lensing.
Recently, the Hubble Space Telescope has been used to take an image one of the most amazing examples of this phenomenon producing what is known as an Einstein ring, and it instantly became one of my favourite ‘Hubble’ pictures. Around the ring are four separate images of the same distant quasar, and astronomers have even detected a fifth image of it inside the ring, near the images of a much brighter pair of galaxies.
To understand what is happening here, it’s worth mentioning some of the history of this topic, and how Albert Einstein came to be connected with it.
Hundreds of years ago, Isaac Newton suggested that light should be deflected in a strong gravitational field, on the assumption that light was composed of tiny particles. For his deduction to be correct, it was not even necessary for light to be based on particles, because Newton simply applied the value of the acceleration, caused by gravity, of anything, which is basically independent of its mass anyway.
Einstein took a different approach. His famous General Theory of Relativity was essentially a theory of gravity, in which matter moves according to the distortion in spacetime produced by matter itself, rather like the path of a ball being deflected by passing close to a depression caused by a massive object in the middle of an elastic surface.
The effect of light from a distant object passing by a massive object, or set of objects, can produce quite amazing effects.
The particular picture by the Hubble Space Telescope is of an object whose discovery was announced in 2018 by a team of researchers led by John Lucey of the Centre for Extragalactic Astronomy at Durham University in the UK.
It has the catalogue number 2M1310–1714 and was one of two such discoveries announced in their research paper.
The astronomers made the discoveries from images they were examining while working on the Taipan Galaxy Survey, which is a catalogue of objects in the southern part of the sky.
The object 2M1310–1714 is in the direction of the far southern part of the constellation of Virgo, but I’m afraid you have no hope of seeing it just by gazing up to the sky.
The very distant quasar, at a distance of about 10 billion light years when its light began its journey to us, is almost directly in the same line of sight (as seen from Earth) as a pair of foreground galaxies that are considerably closer to us, whose light left about 3 billion years ago. Beams of light from the quasar are deflected as they pass by the galaxies, forming not just a ring of light but also the several separate images of the quasar. This quadrupling of the images is an effect called an ‘Einstein Cross’, the first example of which was discovered in 1985.
Gravitational lensing is an important phenomenon for astronomers, and has been used in much research. It has led to an increased understanding of the masses of clusters of galaxies and what lies beyond them, and offers clues as to the overall makeup of the universe.
One aspect of the bending of light by gravity has even led to the discoveries of some planets. As light from a distant star passes near a foreground one on its way to us, the presence of a planet in the nearer system can be revealed by having its own effect on the way the light rises and falls. This study is called gravitational microlensing, and has been the subject of important work by the University of Tasmania.