# What is gravity according to the general theory of relativity

## Gravitation as geometry

After his thought experiment, it is clear to Einstein that gravity is not just a force that moves celestial bodies and everyday objects as if by threads. Because this does not explain why it can bend the light. What now follows is one of the great flashes of inspiration in science.

The gravity of the sun bends space-time - it creates a "hollow" © scinexx

Einstein's revolutionary idea: gravity does not just work between the objects in the universe, but is a direct result of the geometry of space and time. The effect of gravity can thus be expressed geometrically: the more mass a body has, the more it bends the cosmic matrix of spacetime. And the greater this distortion, the more it influences the trajectories of nearby objects: They are deflected towards this body - and that appears to us as an attraction.

In doing so, Einstein also turns the idea of ​​space-time upside down - the matrix of the cosmos that he himself introduced in his special theory of relativity. Because this is no longer just a passive background, but intervenes itself in what is happening and influences how the objects move in the universe.

Einstein's idea can be clarified with a stretched rubber blanket. If you put a steel ball in its center, it will make a deep hollow in the cloth. In a similar way, our earth's gravity, for example, bends space-time. If you put some table tennis balls on the rubber cloth, they roll in the direction of the steel ball until they reach them - always following the incline of the cloth. In the real world, for example, this would correspond to an object that is dropped from a great height: it is attracted and inevitably lands on the floor.

With enough momentum a space probe can escape the gravitational "hollow" of the earth. © MMCD NEW MEDIA GmbH

### Energy helps escape

But Einstein's theory of curved spacetime has another consequence: You can escape the gravitational “hollows” - if you are only fast enough. Because according to Einstein's formula E = mc² formulated a few years earlier, mass and energy are equivalent. Gravity acts on a high-energy and therefore fast object in the same way as it does on a correspondingly heavy one. At this point, Einstein's theory of gravity goes beyond Newton's.

Translated into our example, this means: With enough momentum, a table tennis ball can also be rolled over our rubber blanket in such a way that it is steered out of its path by the steel ball, but then continues to roll in the new direction. Its path was curved by the gravity of the steel ball. And this is exactly what happens in space: The gravity of our sun bends the space-time that surrounds it and thus influences the orbits of the planets. They move just so fast that their speed keeps them on stable orbits - they do not fall into the solar gravitational “hollow”, but they cannot escape either.

So far, so obvious. But if that were all, then Einstein's general theory of relativity would be little more than another way of describing the gravitational effects known since Newton. Their true explosive power goes much further.