A 'gravitational lens' is formed when the light from a very distant, bright source (such as a
quasar) is "bent" around a massive object (such as a massive
galaxy) between the source object and the observer. The process is known as 'gravitational lensing', and is one of the
predictions of
Albert Einstein's
general theory of relativity.
Although O. Chwolson is credited as being the first to discuss the effect in print (in 1924), the effect is more usually associated with Einstein, who published a more famous article on the subject in 1936.
Fritz Zwicky posited in 1937 that the effect could allow galaxy clusters to act as gravitational lenses. It was not until 1979 that this effect was confirmed by observation of the so-called "
Twin Quasar" '
Q0957+561'.
Description
Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.
In a gravitational lens, the
gravity from the massive object (such as a massive
galaxy) bends
light, but unlike an optical
lens maximum 'bending' occurs closest to, and minimum 'bending' furthest from, the center of the lens. Consequently a gravitational lens has no single
focal point, but a focal line instead. As a result of this, if the source, massive lensing object, and the observer lie in a straight line, the source will appear as a ring behind the massive object. This phenomenon was first mentioned in 1924 by the
St. Petersburg physicist
Orest Chwolson [1], and quantified by
Einstein in
1936. It is usually referred to in the literature as an '
Einstein ring', since Chwolson did not concern himself with the flux or radius of the ring image. More commonly, the massive
galaxy is off-center, creating a number of images according to the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing galaxy. In addition, the observer may see sources duplicated about a simple gravitational lens, although the lensed image will always be distorted when compared to the source.
There are three classes of gravitational lensing:
# Strong lensing: where there are easily visible distortions such as the formation of
Einstein rings, arcs, and multiple images.
# Weak lensing: where the distortions of background objects are much smaller and can only be detected by analyzing large numbers of objects to find distortions of only a few percent. The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the center of the lens.
#
Microlensing: where no distortion in shape can be seen but the amount of light received from a background object changes in time. The background source and the lens may be stars in the
Milky Way in one typical case, and stars in a remote galaxy and an even more distant
quasar in another case.
The effect is small, such that (in the case of strong lensing) a galaxy having a mass of over 100 billion
solar masses will produce multiple images separated by only a few
arcseconds.
Galaxy clusters can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of
megaparsecs away from our Galaxy.
Gravitational lenses act equally on all kinds of
electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the
cosmic microwave background as well as
galaxy surveys. Strong lenses have been observed in
radio and
x-ray regimes as well. If a strong lens produces multiple images, there will be a relative time delay between two paths: that is, in one image the lensed object will be observed before the other image.
Simulation
Simulated gravitational lensing (black hole going past a background galaxy).
To the right is a simulation of gravitational lensing caused by a
Schwarzschild black hole passing in front of a background galaxy. A secondary image of the galaxy can be seen within the black hole's
Einstein ring on the side opposite the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. The
surface brightness of the two images remains constant, but their
angular sizes vary, hence producing an amplification of the galaxy luminosity as seen by a distant observer. Maximum amplification occurs when the galaxy (or in this case a bright part of it) is exactly behind the black hole.
History
According to general relativity,
mass "warps"
space-time to create
gravitational fields and therefore bend
light as a result. This theory was confirmed in
1919 during a
solar eclipse, when
Arthur Eddington observed the light from
stars passing close to the
sun was slightly bent, so that stars appeared slightly out of position.
Einstein realized that it was also possible for
astronomical objects to bend light, and that under the correct conditions, one would observe multiple images of a single source, called a 'gravitational lens' or sometimes a '
gravitational mirage'.
However, as he only considered gravitational lensing by single stars, he concluded that the phenomenon would most likely remain unobserved for the foreseeable future. In 1937,
Fritz Zwicky first considered the case where a
galaxy could act as a lens, something that according to his calculations should be well within the reach of observations.
It was not until 1979 that the first gravitational lens would be discovered. It became known as the "
Twin Quasar" since it initially looked like two identical quasars; it is officially named '
Q0957+561'. This gravitational lens was discovered accidentally by
Dennis Walsh,
Bob Carswell, and
Ray Weymann using the
Kitt Peak National Observatory 2.1 meter
telescope.
The study of gravitational lenses is an important part of the future of
astronomy and astrophysics.
Explanation in terms of space-time curvature
In general relativity, gravity is not construed as a force; hence, if the net force of non-gravitational interactions is zero or negligible, the law that determines motion is
Newton's First Law rather than
Newton's Second Law. Newton's First Law parameterizes displacement in terms of time in non-relativistic mechanics, but in general relativity the law is rewritten to demand motion along a space-time geodesic. This causes the observable path of an object subject to no significant net gravitational interaction to deviate from the straight lines expected from
Euclidean intuition; and, in particular, the path curves in exactly the same way as the geodesics in space-time.
In the case of gravitational lensing,
electromagnetic radiation is subject to ''no'' non-gravitational effects (except due to interference with an
electromagnetic field), and so the path of light is bent in the exact same way as geodesics. This is why, in the special case of no gravitational lensing, light travels in straight lines. The fact that this does not happen when gravitational lensing applies is due to the distinction between the straight lines imagined by Euclidean intuition and the geodesics of space-time. In fact, just as distances and lengths in
special relativity can be defined in terms of the motion of electromagnetic radiation in a vacuum, so can the notion of a straight geodesic in general relativity.
Since the speed of electromagnetic radiation in a vacuum is invariant in both theories of relativity, lensing involves deflection as the only source of change in velocity. Weak lensing and micro-lensing in particular cause deflection in a single direction, and through an angle given by GM/rc
2 for a mass M at a distance r from the affected radiation. Some care needs to be taken in defining this distance because gravity is not instantaneous (it expands at speed c, the same as the light itself). The path of the gravitational wave and the electromagnetic radiation intersect at specific space-time co-ordinates, and the lensing is due to the unique direction of gravitational wave perpendicular at the time to the direction of the electromagnetic radiation's motion. This uniquely defined the radius r relevant to the lensing. Finally, it should be noted that, as with all known gravitational interactions, the deflection is towards the responsible mass.
Cosmological applications
The most important application of gravitational lensing in cosmology is the weak lensing of galaxies. By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the
shear of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of
dark matter can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys. These weak lensing surveys must carefully avoid a number of important sources of
systematic error: the intrinsic shape of galaxies, the tendency of a camera's
point spread function to distort the shape of a galaxy and the tendency of
atmospheric seeing to distort images must be understood and carefully accounted for. The results of these surveys are important for cosmological parameter estimation, to better understand and improve upon the
Lambda-CDM model, and to provide a consistency check on other cosmological observations. They may also provide an important future constraint on
dark energy.
Strong gravitational lenses may be used to examine objects at distances at which they would not normally be visible, providing information from further back in time than otherwise possible. Also, not just the object being lensed but the lens itself can provide useful information. By
inverting the lens equations information can be gathered on the mass and distribution of the lensing body.
The statistics of strong gravitational lenses can also be used to measure values of cosmological parameters such as the
cosmological constant and the mean density of matter in the universe. Presently, the statistics do not place very strong limits on cosmological parameters, partly because the number of strong lenses found is relatively small (less than a hundred).
Another parameter that may come out of the study of gravitational lenses is
Hubble's constant which encodes the age and size of the universe. Theoretically, it can be determined by measuring two quantities: the angular separation between two images, and the time delay between these images.
There are two contributions to the time delay:
#the first is the obvious delay due to the difference in
optical path length between the two rays.
#the second is a ''general relativistic'' effect, the
Shapiro effect, that describes light rays as taking longer to traverse a region of stronger gravitation, (''see:''
gravity well,
gravitational time dilation).
Because the two rays travel through different parts of the potential well created by the deflector, the clocks carrying the source's signal will differ by a small amount.
Astronomical applications
Actual gravitational lensing effects as observed by the Hubble Space Telescope in
Abell 1689 - Enlarge the image to see the lensing arcs
Gravitational lenses can be used as ''gravitational telescopes'', because they concentrate the light from objects seen behind them, making very faint objects appear brighter and therefore more easily studied. Researchers at
Caltech have used the gravitational lensing afforded by the
Abell 2218 cluster of galaxies to detect the most distant galaxy known (
February 15,
2004) through imaging with the
Hubble Space Telescope.
Gravitational microlensing can provide information on comparatively small astronomical objects, such as
MACHOs within our own galaxy, or
extrasolar planets (planets beyond the solar system). Three extrasolar planets have been found in this way, and this
technique has the promise of finding Earth-mass planets around sunlike stars within the
21st century.
Gravitational lensing can be used to calculate an estimate of the amount of dark matter contained in the lensing body.
Historical papers and references
★
Über eine mögliche Form fiktiver Doppelsterne, , O, Chwolson, Astronomische Nachrichten, 1924
★
Lens-like Action of a Star by the Deviation of Light in the Gravitational Field, , Albert, Einstein, Science, 1936
★
The Origin of Gravitational Lensing: A Postscript to Einstein's 1936 Science paper, , Jürgen, Renn, Science, 1997
References
★ "''
XFGLenses''". A Computer Program to visualize Gravitational Lenses, Francisco Frutos-Alfaro
★ "''
G-LenS''". A Point Mass Gravitational Lens Simulation, Mark Boughen.
★ Newbury, Pete, "''
Gravitational Lensing''". Institute of Applied Mathematics, The University of British Columbia.
★ Cohen, N., "Gravity's Lens: Views of the New Cosmology", Wiley and Sons, 1988.
★ "''
Q0957+561 Gravitational Lens''". Harvard.edu.
★ "''
Gravitational lensing''". Gsfc.nasa.gov.
★ Bridges, Andrew, "''
Most distant known object in universe discovered''".
Associated Press.
February 15,
2004. (Farthest galaxy found by gravitational lensing, using Abell 2218 and Hubble Space Telescope.)
★
Analyzing Corporations ... and the Cosmos An unusual career path in gravitational lensing.
★ "''
HST images of strong gravitational lenses''". Harvard-Smithsonian Center for Astrophysics.
★ "''
A planetary microlensing event''" and "''
A Jovian-mass Planet in Microlensing Event OGLE-2005-BLG-071''" , the first
extra-solar planet detections using microlensing.
★
Gravitational lensing on arxiv.org
Further reading
★
العربية
中国
Français
Deutsch
Ελληνική
हिन्दी
Italiano
日本語
Português
Русский
Español
