Why is light bent when it passess a massive body in space?

A neutron star with gravitational lensing
What causes gravitational lensing?

In our everyday life, we experience light only in straight lines – unaffected by gravity. In space, however, this way of thinking is wrong. Particularly massive objects such as stars, galaxies and black holes attract passing light rays and even bend or absorb them. For the first time, scientists were able to experimentally prove this phenomenon, known as the gravitational lens effect, during a solar eclipse in 1919.

The solar eclipse made it possible to observe those stars near the edge of the sun that were not visible otherwise. The result: The position of the marginal stars, or rather the light rays emanating from them, were slightly shifted. But how can gravity affect photons (light particles) without their own mass? The answer to this is provided by Albert Einstein’s general theory of relativity.

What is gravitational lensing?

According to Einstein’s famous theory, we should not understand gravity as a force that attracts mass in a straight direction and moves it through space. Instead, the gravitation of very heavy objects causes a curvature of space-time. As three-dimensional beings, however, we humans cannot see this extra dimension and can hardly imagine it.

A thought aid: If you place a ball (mass) on a cloth (space) stretched in the air, the ball sags and deforms the tissue – the space curves. If you add smaller objects, they will follow the cloth towards the ball. In other words, they follow the curvature of the space.

What is the concrete situation in space? Earth also follows the space-time curvature in its orbit around the sun. Actually, the earth moves in a straight path, only in a curved space. The space-time curvature is now not only valid for planets, asteroids and other objects with mass. Light is bent by gravity because it also follow the curvature in space, although photons have no mass of their own.

How much does earth’s gravity bend light? Light travels very fast (299,792,458 m/s). The mass of the earth is far from sufficient to curve space in a way needed to prevent light from escaping it. It only would be bent by 0.0006 arc-seconds. Only a black hole has such a high mass that light cannot escape its corresponding curvature and is “swallowed”.

Related articles:

What are stars? A short introduction.

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