A phenomenon that allows us to see an object simultaneously at two different points in time.
In 1916, Albert Einstein published his groundbreaking paper on general relativity, which, among other things, showed that space and time were intertwined as space-time, and that every object with mass exerted a gravitational influence on its surrounding space.
This brought with it an interesting conclusion, namely that an object with enough mass would not only exert a gravitational influence on other celestial objects, but also on light itself. This theory was confirmed during a solar eclipse in 1919 when British astrophysicist Arthur Eddington observed that stars passing close to the Sun appeared out of position because their light was being bent.
It was postulated that if a distant object, an intervening one and the Earth were all aligned, then the image of that distant object would appear distorted or even multiplied as its light bent around the intervening object on its way to Earth. However, scientists including Einstein, only considered the gravitational lensing effects of a single star, which they deemed would be almost impossible to observe. It was not until 1937 that Swiss astronomer Fritz Zwicky suggested that larger objects like galaxies could produce the same effect, although sadly he would not live to see his theory proved.
In 1979, five years after Zwicky’s death, the first gravitational lens was discovered, albeit by accident.
A team using the Kitt Peak National Observatory in Arizona, USA was responsible, but their discovery was the cause of contention for some time. What they saw was the Twin Quasar, an occurrence of apparently two quasars located over 8 billion light years from Earth in the Ursa Major constellation. They were remarkably similar, with an almost identical redshift and visible light spectrum, suggesting that they were actually the same object appearing in two different images due to an intervening galaxy cluster located over 4 billion light years closer to Earth.
However, later in 1979 further studies revealed a relativist jet emerging from quasar A, but the same could not be seen at quasar B. This put the suggestion that the Twin Quasar was the first gravitationally lensed object to be observed in doubt, until similar jets were discovered at quasar in 1983.
It was also found, through three decades of observations, that the intervening galaxy cluster was off-centred. So, although the two quasars were the same object, the light from quasar A reaches Earth 14 months before quasar B. This is one amazing aspect of gravitational lensing; it allows us to see an object simultaneously at different points in time.
But the Twin Quasar was not finished with its surprises just yet. One other type of gravitational lensing, called microlensing, occurs when a smaller object like a planet in the lensing object further distorts the light of the distant object. In 1996, such a distortion was observed in the light curve of the Twin Quasar, which was controversially attributed to an extra galactic planet (one found outside the Milky Way) three times the mass of Earth. At a distance of 4 billion light years this would make it the most distant planet known to date, but the chance alignment will never occur again and therefore we will never know for sure.
However, microlensing such as this has proven very useful in the search for planets. A Polish astronomical project called the Optical Gravitational Lensing Experiment (OGLE), which began in 1992, has successfully discovered several extrasolar planets by observing the distortion in the lensing effect of distant objects (see the ‘Finding planets through microlensing boxout). This led to the discovery of OGLE-2005-BLG-0390Lb, a super-Earth 21,500 light years away that is one of the most distant confirmed planets we know of to date.
We now know of many gravitationally lensed objects in the universe, and by using this technique we are able not only to discern the properties of massive distant objects like quasars, but also to map the amount of expected dark matter in the universe. Gravitational lensing is a remarkable effect but it has taken us nearly a century to truly appreciate and understand its uses. Continued observations will no doubt uncover more of these fantastic phenomena and help us to comprehend the effect of massive objects on space-time even further.