The forces

Interactions between matter can be explained by four types of forces:

Electromagnetic force concerns electrically charged particles. Two particles of opposite charge attract and two particles of like charge repel each other. It varies according to the equation $\frac {1}{r^2}$. It is responsible for binding atoms: the protons ^? of the nucleus and the electrons attract. It is no longer significant on a large scale, because there are the same number of protons ^? as electrons, and matter is said to be “neutral” at large scales.

The strong interaction force: his is extremely intense and acts at very short range ($10^{-15}\space\text{metres}$). It concerns protons ^? and neutrons. It enables the nuclei of atoms to hold together even though the protons ^? are repelling each other through electromagnetic force.

The weak interaction force acts on all particles at very short distances ($10^{18}\space\text{metres}$), and is responsible for beta radioactivity.

Gravitational force concerns all masses, and therefore all particles. It is perhaps a geometrical property of space-time-matter. It varies according to the equation $\frac {1}{r^2}$ up to infinity. This force is responsible for the large-scale structure of the Universe.

Physicists are seeking to unify these forces in a single theory which would encompass general relativity and quantum mechanics. Electroweak theory combines electromagnetism and weak interaction, but as yet no complete theory exists.