Einstein's 1915 theory of gravitation is purely geometrical, and describes gravity in terms of space-time geometry, specifically the curvature of space-time. To understand what this means, take a flexible plastic ruler and draw a straight line on a sheet of paper on your desk. Now go find a curved surface, like the fender of your car; take another sheet and smooth it out on that surface, then smooth out the ruler on that surface and draw another "straight" line. Well, it isn't straight, but it is the straightest possible line that lies on that surface.

Einstein found that, while empty space-time is "flat" like your table, space-time containing matter is "curved" like your car's fender. A "straight path," like the path of a ray of light, is "bent" in the curved space-time, compared to its path in "flat" space. The more matter per unit volume, the more acute is the bend. Einstein predicted that stars visible around the edge of the sun (for example, during a total eclipse) would therefore be seen displaced outward from the center of the sun, compared to their normal positions, as the animation indicates.

Einstein's prediction of the bending of starlight around the Sun has been confirmed often, beginning about the time of World War I. But Einstein went on to point out that large concentrations of matter would act as lenses to reveal views of objects which happened to lie behind the concentrations as seen from earth. Because the curvature of space-time produced by gravity is "positive," the effect of gravity is always to converge light beams, just as does the usual desktop magnifying glass. Einstein's prediction of the gravitational lens effect has also been beautifully confirmed over the past decade, particularly through images from the Hubble Space Telescope. Hit "reload" to restart the animation below.

When a galaxy lies behind another galaxy or cluster of galaxies, the object behind is often visible, not just as a distorted single image, but sometimes as a ring, two images, four images, or more. The most famous single example from Hubble is shown below. A blue ring-shaped galaxy is imaged many times in various orientations and distortions by the massive cluster of yellowish galaxies seen in the center of the image.

For more information on the lens effect, click the animated image above or consult these sites: Soper | Newbury | Tutorial | Learn how to make your very own optical analog of a gravitational lens here.

Other examples of gravitational lensing effects, from the good old "Astronomy Picture of the Day" website: 1, 2, 3, and 4.