# Why does the light bend towards the gravitational field

### Question:

What causes gravity?

### Answer:

The force of gravity or gravitational force, i.e. the force of attraction between two massive bodies, comes from the curvature of the room. A vivid example of a crooked space, for the sake of simplicity only in two dimensions, is the surface of a sphere: Draw a triangle on a globe with one point on the north pole and the other two on the equator (the edges must each be be the shortest connections between two points), you can immediately see that there is a 90-degree angle at each corner; the sum of the angles cannot therefore be 180 degrees, as is the case in the plane. This is a direct result of the curvature of the surface. If, on the other hand, you take a sheet of paper and draw a triangle on it, this will always have a sum of angles of 180 degrees, however you bend the paper afterwards. In this sense, a curved paper is a non-curved (“flat”, or Euclidean) two-dimensional space.

Einstein showed that space is warped by energy. The greatest energy densities are precisely where there are masses (E = mc2!), E.g. B. Stars. If we now draw a star on our sheet of paper, i. H. a small disk, the space there must be curved, but everywhere else it must be flat. What is created is a kind of Chinese hat: the star has a rounded tip (a piece of the above globe) and a conical surface around it. If you now draw two rays of light (which always fly straight in flat space!) On the cone, which, starting from a point far away from the star, pass it just to the left or just to the right, you can see that they are behind the star again converge: they were deflected by the star's gravitational field! That is exactly what is meant by a gravitational lens.

And what works with light is also possible with massive bodies; only one then has to take into account that space is not two-dimensional, but a four-dimensional something that unites space and time in (almost) equal ways. A stone that is thrown up and down again moves in a perfectly straight line in this four-dimensional space. But because the space is not flat because of the near earth mass, the 'straight' line leads back to the earth, just as the 'straight' rays of light on the Chinese hat intersect again behind the star (the tip of the hat).