A 'Quasar' is a class of extremely bright object that existed in the early universe. They are observed today at very large distances, they are in fact the most distant objects known. Light from these Quasars sometimes passes close to other galaxies on its way to the Earth (see above). The light passing by the galaxy is actually deflected in its path giving rise to 'ghost' images of the Quasar. If the Quasar, Earth and massive galaxy lie in a perfect straight line, a very rare occurrence, then the Quasar image can actually appear as a perfect ring! These 'mirages' are called Einstein Rings since Einstein predicted their existence in the early 20^th Century. More generally the ghost images appear as points or arcs and are described as 'Gravitational Lenses'.

 The first Gravitational Lens arcs were photographed using photographic plates in the '70s and early '80s but their significance was not appreciated. In 1985 an arc was recorded on a more sensi tive electronic detector (a Charge Coupled Device or CCD), two years later another was discovered and it was realised that these objects were Gravitational Lenses.
 
 

Gravitational Lens Image Gallery

Abell 2218 taken by the HST

This image shows about 200 arc like structures, each one of which is a gravitationally lensed 'mirage' of a more distant object.

Abell 2218 taken with the William Herschel Telescope

This image is a combination of visible wavelength and Infra Red photos, taken with the WHT on La Palma. To record this image through the distorting medium of the Earth's atmosphere required an exposure time of greater than 24 hours.

Cluster AC 114 taken with the Hubble Space Telescope

At centre right is a very interesting pair of hook like galaxy images. Closer inspection reveals that they are mirror images, reflected about another intervening pair of galaxies! The two 'hooks' are in fact gravitationally lensed mirages of the same galaxy; the intervening pair are acting as a gravitational lens.

The Cluster CL 2244-02 taken with the Hubble Space Telescope

This cluster of elliptical galaxies lies at a distance of 3 Billion light years and has a mass 20,000 Billion times that of our sun. This has been deduced from the size of the gravitationally lensed arc on the right hand side of the image. Interestingly the Luminosity of this galaxy cluster is only 200 Billion times that of the sun, so 99% of the cluster's mass is composed of invisible material : so called 'Dark Matter'. The exact nature of this matter, whose presence is observed even in our own galaxy, is unknown. Dark Matter is surely one of the most intriguing puzzles in modern astronomy.

  A selection of Gravitational Lenses from Hubble
 
 

News : March 2001 The ING Wide Field Survey detected an unusual gravitationally perturbed galaxy. There was no visible nearby companion object that could cause this perturbation. Could a dark galaxy be responsible? Full article here.

Image of perturbed galaxy

A Black Hole warping space into a higher dimension

Mathematically it is quite straightforward to describe Space containing more than three dimensions. We can describe mathematically, four dimensional versions of common polyhedrons such as cubes. A 'Hypercube' is a four dimensional cube. A Hypercube is to a cube what a cube is to a square. It is extremely difficult to visualise a four dimensional object , although one author claims it is possible. The following animation shows how a rotating Hypercube would appear if it interesected our 3D Space.

What a rotating 4 Dimensional Cube would look like

There is some interesting literature on the theme of higher spatial dimensions. The most famous is 'Flatland' by Edward Abott Abott. The book describes a world where 2D creatures roam a universe that occupies a plane. They have no knowledge of a third dimension. The author describes the appearance to them of 3D objects inserted into their 2D universe. By analogy it gives us some hints of the properties of 4D objects.
 

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