fundamental forces

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fundamental forces

The four basic forces of nature: the gravitational force, the electromagnetic force (both long established), and the two nuclear forces, the strong force and the weak force. These forces act between elementary particles, i.e. the basic building blocks of matter. All the visible matter in the Universe and its behavior may be described in terms of these particles and the forces acting between them. The standard model of elementary particles uses gauge theories (based on the idea of symmetry) to describe the electromagnetic, strong, and weak forces: each is generated by the interaction between particles, which involves the exchange of an intermediate particle known as a gauge boson. These intermediates effectively ‘carry’ the force from one particle to another. Different gauge bosons are exchanged in the three types of interaction: the electromagnetic force is carried by photons, the strong force by gluons (see quark), and the weak force by neutral Z and charge W particles.

The gravitational force occurs between all particles, the electromagnetic force between charged particles, e.g. electrons and protons. The strong force arises between hadrons, e.g. protons and neutrons, and between the constituents of hadrons, i.e. quarks, and is the force binding together particles in the nuclei of atoms. The weak force occurs between both leptons and hadrons and is responsible for radioactivity. The four forces vary greatly in range: the gravitational and electromagnetic forces have an infinite range whereas the other two have an extremely short range. They also vary greatly in strength: the gravitational force is the weakest over very short distances (˜10–39 times the strength of the strong force) but on a cosmic scale it dominates the others.

Attempts to construct a single theory unifying the four forces have been progressing. Steven Weinberg and Abdus Salam in the late 1960s found a mathematical description – the electroweak theory – that successfully unified the electromagnetic and weak forces so that they could be regarded as two aspects of the same phenomenon. There are various grand unified theories (GUTs) that aim to provide a mathematical framework in which the strong, electromagnetic, and weak forces emerge as parts of a single unified force. A symmetry is said to relate one force to another. Since the forces are very different in strength and character this symmetry is broken in the present-day Universe. GUTs predict that the symmetry holds only when the temperature is greater than about 1027 K, when particles would have extremely high energies, above 1024 electronvolts. Such extremes of temperature and energy occurred in the very young Universe immediately after the Big Bang. GUTs are thus of great importance to cosmology.

The ultimate goal is to incorporate the gravitational force into the unification by formulating a satisfactory theory of quantum gravitation. This would produce a theory of everything, (TOE); one candidate is string theory.