General relativity predicts that light grazing a mass bends by α = 4GM/c²b, exactly twice the Newtonian figure, and that factor of two is what the 1919 eclipse expedition confirmed. In perfect alignment the two images close into an Einstein ring; slightly off-axis, the ring breaks into a double image; a heavier lens grows the ring radius as the square root of mass. A foreground body drifting across a background star leaves the symmetric microlensing curve, while weak lensing shears whole fields of galaxies into aligned ellipses.
General relativity predicts that light grazing a mass bends by α = 4GM/c²b, exactly twice the Newtonian figure, and that factor of two is what the 1919 eclipse expedition confirmed. In perfect alignment the two images close into an Einstein ring; slightly off-axis, the ring breaks into a double image; a heavier lens grows the ring radius as the square root of mass. A foreground body drifting across a background star leaves the symmetric microlensing curve, while weak lensing shears whole fields of galaxies into aligned ellipses.
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