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In this brief note, Einstein also presented formulae for the optical properties of a gravitational lens. They are the exact same formulae which he had derived some 24 years earlier, right down to the formula for the magnification factor. Mandl receives an honourable mention as the person who had asked Einstein to derive and publish these results. Right after Einstein's brief text had been published, it was followed by a number of articles by well-known scientists, who picked up where Einstein had left off. Fritz Zwicky , an astronomer at the California Institute of Technology, discussed the possibility of observing the lensing effect in the case of the recently discovered extragalactic nebula, in other words: other galaxies.

The typical masses, sizes and mutual distances of galaxies are such that double images of a distant galaxy should be significantly more frequent than double images of stars: The necessary near-alignment of a closer object, a more distant object and an observer here on Earth is much more probable for galaxies than for stars.

Henry Norris Russell, the astronomer from Princeton, published an article in which he speculated about the inhabitants of a hypothetical planet orbiting the White Dwarf companion of the star Sirius. What would they see during a total eclipse - on the occasion when, from the point of view of these inhabitants, the White Dwarf star would move in front of the more distant Sirius?

As White Dwarfs are very compact objects, light from Sirius passing close to the companion would be markedly deflected. Of course, astronomers on Earth would not be able to see this relativistic gala performance, and in fact Russell cites this scenario as a perfect test of relativity theory which, regrettably, is impossible to put into practice. Indisputably, Einstein's little publication had lent credibility to the idea of gravitational lensing, and the concept became part of the general knowledge of theoretical astronomers.

The discovery of quasars in the early s led to renewed interest in the theory of gravitational lensing, with notable contributions by Yu.

Gravitational lensing

Klimow, S. Liebes Jr. Quasars are extragalactic objects, and they are both very distant and extremely bright. These properties combined make a distant quasar as a light source paired with a less distant galaxy as the deflecting mass an ideal candidate for an observable gravitational lens. By that time, the models of gravitational lensing under discussion were already significantly more complex than for the early calculations. In fact, the description of a gravitational lens is simple only in the approximation of geometric objects, with light rays emitted by point-like sources deflected in a gravitational field with perfect spherical symmetry.

In contrast, in any realistic situation neither the light source nor the lens mass will exhibit perfect symmetry. For more general situations, there is a much greater variety of possible images. One example is a fourfold image evenly grouped around a central image however, the central image is not usually visible - the cosmic version of a four-leaf clover, called an "Einstein cross".

Other examples include multiple images, rings and similar shapes, or arc-like deformations.


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Einstein's simple derivation is woefully inadequate when dealing with these images - they are described using significantly more complicated models based on general relativity. Such models produce candidate shapes for possible gravitational lens images, and it is then up to astronomers to identify their real-life counterparts in the night sky.

The first gravitational lens was found in by Dennis Walsh, Robert F.


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Carswell and Ray J. The following illustration is a false color image of the object in question, based on radio observations:. As seen from Earth, the angular distance between the two images is roughly six seconds of arc - the same as the apparent size of a compact disc CD viewed from a distance of four kilometers. This first identification of a gravitational lens was soon followed by others.

It was discovered in Nowadays, astronomers know of many dozens of systems with multiple images, as well as of a few Einstein rings. In the twenty-first century, gravitational lensing is a highly active field of astrophysical research. The reason for the field's growth is that, today, gravitational lenses are much more than just an interesting general relativistic phenomenon.

Now that a significant number of lens systems has been identified, lensing is used more and more as an observation tool, allowing astronomers to answer astrophysical as well as cosmological questions, from estimates of the amount of dark matter contained in the lens mass to the determination of fundamental parameters of the big bang models. Introductory information about Einstein's theory of gravity can be found in the section General relativity of Elementary Einstein. Related spotlight texts on Einstein Online can be found in the category General relativity.

The Einstein notebook mentioned in the text has been published in M. Klein, A. Kox, J.

Fall-out of the article in "Science"

Renn und R. Schulman Hg.


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  • We would like to thank Princeton University Press for their permission to use the above image. Einstein's writings can be accessed online thanks to the website Einstein Archives Online. There, you will find both the entry in the notebook and the manuscript for the Science article. His main research interest are Einstein's writings about relativity and his unified field theories. Around , Einstein himself derived the math for how light is deflected as it passes through the Sun's gravitational field.

    His idea was subsequently tested during a total eclipse of the Sun in May by astronomers Arthur Eddington, Frank Dyson, and a team of observers stationed in cities across South America and Brazil. Their observations proved that gravitational lensing existed. While gravitational lensing has existed throughout history, it's fairly safe to say that it was first discovered in the early s.

    Gravitational lensing

    Today, it is used to study many phenomena and objects in the distant universe. Stars and planets can cause gravitational lensing effects, although those are hard to detect. The gravitational fields of galaxies and galaxy clusters can produce more noticeable lensing effects. And, it now turns out that dark matter which has a gravitational effect also causes lensing. Now that astronomers can observe lensing across the universe, they've divided such phenomena into two types: strong lensing and weak lensing. Strong lensing is fairly easy to understand — if it can be seen with the human eye in an image say, from Hubble Space Telescope , then it's strong.

    Astronomers have to use special techniques to observe and analyze the process. Due to the existence of dark matter, all distant galaxies are a tiny bit weak-lensed. Weak lensing is used to detect the amount of dark matter in a given direction in space. It's an incredibly useful tool for astronomers, helping them understand the distribution of dark matter in the cosmos. It also magnifies the light from very distant objects, such as the earliest galaxies, and often gives astronomers an idea of the galaxies' activity back in their youth.

    Another type of lensing called "microlensing" is usually caused by a star passing in front of another one, or against a more distant object. The shape of the object may not be distorted, as it is with stronger lensing, but the intensity of the light wavers. That tells astronomers that microlensing was likely involved.

    Interestingly, planets can also be involved in microlensing as they pass between us and their stars. Gravitational lensing occurs to all wavelengths of light, from radio and infrared to visible and ultraviolet, which makes sense, since they're all part of the spectrum of electromagnetic radiation that bathes the universe.

    What is Gravitational Lensing?

    The first gravitational lens other than the eclipse lensing experiment was discovered in when astronomers looked at something dubbed the "Twin QSO". QSO is shorthand for "quasi-stellar object" or quasar. Originally, these astronomers thought this object might be a pair of quasar twins.

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    Instead, they were actually two images of a more distant quasar that were produced as the quasar's light passed near a very massive gravity along the light's path of travel. That observation was made in optical light visible light and was later confirmed with radio observations using the Very Large Array in New Mexico.

    Since that time, many gravitationally lensed objects have been discovered. The most famous are Einstein rings, which are lensed objects whose light makes a "ring" around the lensing object. On the chance occasion when the distant source, the lensing object, and telescopes on Earth all line up, astronomers are able to see a ring of light.