Gravitational lensing: strong, weak and micro

The observation, in 1919 by A. S. Eddington and collaborators, of the gra- tational de? ection of light by the Sun proved one of the many predictions of Einstein’s Theory of General Relativity: The Sun was the ? rst example of a gravitational lens. In 1936, Albert Einstein published an article in which he suggested - ing stars as gravitational lenses. A year later, Fritz Zwicky pointed out that galaxies would act as lenses much more likely than stars, and also gave a list of possible applications, as a means to determine the dark matter content of galaxies and clusters of galaxies. It was only in 1979 that the ? rst example of an extragalactic gravitational lens was provided by the observation of the distant quasar QSO 0957+0561, by D. Walsh, R. F. Carswell, and R. J. Weymann. A few years later, the ? rst lens showing images in the form of arcs was detected. The theory, observations, and applications of gravitational lensing cons- tute one of the most rapidly growing branches of astrophysics. The gravi- tional de? ection of light generated by mass concentrations along a light path producesmagni? cation, multiplicity, anddistortionofimages, anddelaysp- ton propagation from one line of sight relative to another. The huge amount of scienti? c work produced over the last decade on gravitational lensing has clearly revealed its already substantial and wide impact, and its potential for future astrophysical applications.