Why does the mass bend spacetime

General Theory of Relativity / Beginners Tour Part 4: Bent light rays

In contrast to Newton's picture of gravity, the general theory of relativity directly shows that light is also influenced by gravity. Just like the movement of freely falling matter, the propagation of light is also determined by geodesics: light follows the straightest possible space-time orbits in a curved space-time.

For example, rays of light that lead past a massive body are bent towards this mass, and the stronger the closer the light comes to the body. This is shown in the following figure; A solid body is drawn in yellow, the paths of light coming from the left in red:

Light deflection in the gravitational field of the sun

The first evidence of the relativistic deflection of light was made in 1919 by British astronomers. They take advantage of the fact that the deflection effect becomes apparent during astronomical observations: Wherever we observe a star on the celestial sphere, this is an expression of the direction from which the star's light reaches us. For light from a star that passes near the edge of the sun, the deflection effect shown above occurs: the path of light bends slightly, and the direction from which the light of the star in question reaches us changes accordingly, and its position on the celestial sphere also appears shifted something. This changed position could of course only be observed during a solar eclipse, since the light of the sun otherwise clearly outshines a star at the edge of the sun. If one compares photographic recordings of such a star at the edge of the sun with photographs of the same star and the same region of the sky, which were taken when the sun was at a completely different location on the celestial sphere, the effect of light deflection can be measured. The fact that the observation data obtained in this way - admittedly not extremely precise - corresponded to Einstein's prediction, was a great success of general relativity (and at the same time the beginning of Einstein's worldwide fame).

An important application of the deflection effect are so-called gravitational lenses, in which the deflection of light by a mass means that astronomers can detect two or more images of one and the same astronomical object in the sky. In the following illustration, there are not four objects arranged in a cross around a center, but four images of one and the same object:

Gravitational lenses create Einstein's cross, image taken by the Hubble space telescope
© NASA / ESA / STScI