ABSTRACT
The universe appears to be governed by four kinds of forces: strong, electromagnetic, weak and gravitational. Strong forces – also known as nuclear forces – act only at very short distances; they bind quarks together to make nucleons (protons and neutrons) and bind nucleons together to make nuclei. Electromagnetic forces provide the attractions between electrons and nuclei that build atoms and molecules; they control chemistry and the physics of materials. Weak forces lie behind processes like beta-decay, which allows protons to transmute into neutrons and vice versa; they are vital for the synthesis of heavy elements in the early universe and in stellar cores and for the fusion power cycles in stars. Gravitational forces are by far the weakest; they are important for large bodies but negligible for nuclear and sub-nuclear particles, compared to the other forces.
