What pulls us down the earth

Aha : Why is the earth attractive?

Sometimes it makes you dizzy to think that the earth is a sphere. Why don't you fall down on the other side, in Australia? The thought that there is a force at work everywhere, which is always directed towards the center of the earth, is not exactly obvious. However, the force can be made visible with a simple instrument: the plumb line, a string with a weight dangling from it. It always points to the center of the earth.

We have known since the 17th century that gravity also keeps the moon in orbit. On the other hand, it attracts the earth and its water masses, from which ebb and flow result. However, the moon is smaller than the earth and has a significantly lower mass. Since the latter is crucial for the strength of the attraction, the lunar astronauts were able to make great leaps.

The attraction of the masses is a universal principle. Albert Einstein tried like no other to understand it. He formulated the law of the equivalence of mass and energy and came to a geometric interpretation: According to his general theory of relativity, every mass in space causes space-time to bend.

"If there are no masses, then spacetime is as flat as a taut trampoline," explains Axel Kleinschmidt from the Max Planck Institute for Gravitational Physics in Golm. On the other hand, where there are stars or planets, a hollow is created in the spacetime cloth. "And other bodies also move into this hollow."

In modern physics, gravity is one of four fundamental forces through which matter particles in space can perceive each other. It is weak compared to electromagnetism. It is only noticeable in large masses such as moons, planets and stars. Nobody can say where it comes from. For now, researchers have to be content to say that they exist. “It is, so to speak, the lowest level of description that we have in physics,” emphasizes Kleinschmidt. So far no Archimedean point has been found from which one could unhinge gravity itself. Thomas de Padova

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