Curved spacetime

Curved spacetime is predicted by General relativity, in a technical sense from differential geometry, wherever there is a gravitational field.

At one level, this is a way of looking at classical, Newtonian ideas of gravity. A common example is that spherical masses deform spacetime in much the same way a lead ball would deform the surface of a rubber sheet. It is this deformation that causes the planets to orbit the Sun, and the Moon to orbit the Earth. In fact, all orbital motion is the result of bodies being affected by the curvature of the spacetime in which they move.

There has been experimental evidence for the curvature of spacetime since 1922, when observers set out to test certain predictions of general relativity. Knowing that during a solar eclipse, the light from stars in the same general area of the sky as the Sun are visible during the day, they proposed that if light from these stars is affected by the curvature of spacetime due to the Sun's mass, then this would be measurable as a deflection (or a change in location) of the star's position on the sky. The stars closer to the position of the Sun in the sky would suffer a larger deflection; in general the deflection would be in proportion to the star's distance from the Sun's location on the sky. This effect was observed for 15 stars during the solar eclipse of 1922 in Western Australia, and was interpreted as observational verification of the predictions of general relativity.

Since that time, astronomers have observed other instances of the curvature of spacetime near massive objects. One example is the deflection of radio waves from quasars which are occulted by the Sun every year (such as 3C 279). Another is the growing collection of gravitational lenses. A gravitational lens occurs when the light from a very distant object (often a quasar) is bent by a closer massive object (such as a galaxy) into multiple images. Some very impressive images of gravitational lenses have been taken.