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<-   The forces in the Universe   ->

The interactions of matter in the Universe are explained by four force types :

The electromagnetic force concerns electrically charged particles. Therefore, it is attractive between two particles of opposite signs, and repulsive between two particles of the same sign. It varies as 1/r^2. It is responsible for the cohesion between atoms : the protons of the nucleus and the electrons attract each other. It is not perceptable on large scales because there are as many protons as electrons in the Universe, the matter is "neutral" on large scales.

The strong force of interaction : this is very intense and acts on very small scales (10^(-15)meters). It concerns the protons and the neutrons. It enables the nuclei to remain "stuck" together, despite the repulsive effect of the electromagnetic force on the protons.

The weak force of interaction acts on all the particles on very small scales (10^(-18)meters), it is responsible for beta radioactivity.

The gravitational force concerns all masses, and therefore all particles. It can be understood as a geometric property of space-time matter. It varies as 1/r^2 until infinity. This force is responsible for the large scale structure of the Universe.

Physicists are searching to unifiy these forces in a comprehensive theory which could give an account of both general relativity and quantum mechanics. No complete theory exists now. However, weak interaction and electromagnetism are united in a single theory : the electroweak theory.