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As light travels through space, its path is affected by the gravity of the objects that it passes. This effect had been theorised as a long ago as the eighteenth century by Newton, but it was formalised by Albert Einstein, who was able to predict exactly how the light would be affected. Observations of the a total solar eclipse in May 1919 showed that Einstein's calculations were correct, and that the positions of stars close to the Sun's limb appeared to be displaced, as their light was affected by the mass of the Sun by precisely the degree that Einstein had predicted.
For truly massive objects with strong gravitational fields, such as the largest galaxies, this distortion effect can be unmistakable. A massive foreground object like this can bend the light of its background to the extent that more distant objects appear to be replicated as their light is split and bent by gravity. In other cases, the light from background objects creates a circular distortion affect around the massive foreground object, a phenomenon known as an 'Einstein ring'. These are 'strong' effects, but less massive bodies also cause a lensing effect, though this 'weak' lensing requires statistical analysis to detect.
Even comparatively low-mass objects such as planets can produce a tiny by detectable lensing effect. As a planet passes in front of a star, it causes a minuscule distortion in that star's light, so that it appears fractionally brighter than usual as seen from Earth. This 'microlensing' phenomenon has been used as a technique to detect extrasolar planets in orbit around other stars, as well as rogue planets - those without their own parent star - as they affect the light of background stars far beyond. The same approach can be used to measure the gravitational effects of otherwise undetectable masses, such as that of dark matter.
While the Sun's gravitational lensing effect was demonstrated as long ago as 1919, it was later calculated that the Sun could in theory be used as a true observational aid, giving rise to the possibility of constructing a 'solar gravitational lens' to observe distant objects with extreme precision. The technical challenges of such a project would be formidable, not least because the focal point of such a lens lies more than five hundred Astronomical Units from the Sun (some eighteen times more distant than Neptune). In principle, however, if an observing probe could be positioned at this immense distance, it would be able to use the Sun as a gravitational lens to produce observations of unprecedented detail.
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