There are four fundamental forces in the universe, sometimes called "interactions". These forces are gravity, electromagnetism, the weak force, the forces can be known as SNF (strong nuclear forces) and WNF (weak nuclear forces) and the strong force. They are called 'fundamental' because they cannot be reduced to anything simpler. Gravity and to some extent electromagnetism are experienced by people in their everyday life. A complete description of the forces requires advanced physics.
- The gravitational force is due to the curvature of spacetime, described by Einstein's general theory of relativity.
- Electromagnetic force is due to electric charge. Charge causes electric force and movement of charge causes magnetic force.
- The strong force is the force that binds protons and neutrons together, keeping atoms intact.
- The weak interaction force causes beta decay.
It is thought by most physicists that three of these forces (electromagnetism, the weak force, and the strong force) become a single force under very high temperatures. This idea is known as the grand unification theory.