The light bends

From thought experiment to expedition : Light on crooked paths

Albert Einstein is famous for his thought experiments. On the way to the general theory of relativity, he imagined a cabin somewhere in space. On its inner wall there is a light source with which a physicist sends a beam parallel to the floor into the room. The light beam strikes exactly opposite the starting point as long as the cabin, seen from the outside, is moving upwards at a constant speed. This changes when an external drive accelerates the car upwards. Then the light beam bends a little downwards and describes a slightly curved path.

The physicist can also interpret this fact differently. As an occupant, it seems to him that the cabin is at rest, but in a gravitational field. If the light beam is deflected from its straight path, then the cause should be found in the gravitational field. According to Einstein's "equivalence principle", the effect of an accelerated movement cannot be distinguished from that in the gravitational field. Relying on it, Einstein predicted that light in the sun's gravitational field would drift off course. He calculated a tiny change in angle for a ray of light passing directly on the sun and hoped that astronomers would test his hypothesis. In the case of a total solar eclipse, when the moon moves in front of the solar disk. Then those stars in the immediate vicinity of the sun, whose light would otherwise be outshone by it, become briefly visible. During the eclipse one can photograph the positions of these near-sun stars and compare the picture with a photo of the same group of stars taken at night, when the starlight does not pass close to the sun on its way to earth. The positions should differ a little. A solar eclipse expedition by British researchers to the equatorial region in 1919 confirmed Einstein's prediction and made its creator famous.

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