Fundamental interactions in the context of "Fifth force"

Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay are alpha, beta, and gamma decay. The weak force is the mechanism that is responsible for beta decay, while the other two are governed by the electromagnetic and nuclear forces.

Radioactive decay is a random process at the level of single atoms. According to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. However, for a significant number of identical atoms, the overall decay rate can be expressed as a decay constant or as a half-life. The half-lives of radioactive atoms have a huge range: from nearly instantaneous to far longer than the age of the universe.

A theory of everything (TOE) or final theory is a hypothetical coherent theoretical framework of physics containing all physical principles. The scope of the concept of a "theory of everything" varies. The original technical concept referred to unification of the four fundamental interactions: electromagnetism, strong and weak nuclear forces, and gravity.Finding such a theory of everything is one of the major unsolved problems in physics. Numerous popular books apply the words "theory of everything" to more expansive concepts such as predicting everything in the universe from logic alone, complete with discussions on how this is not possible.

Starting with Isaac Newton's unification of terrestrial gravity, responsible for weight, with celestial gravity, responsible for planetary orbits, concepts in fundamental physics have been successively unified. The phenomena of electricity and magnetism were combined by James Clerk Maxwell's theory of electromagnetism and Albert Einstein's theory of relativity explained how they are connected. By the 1930s, Paul Dirac combined relativity and quantum mechanics and, working with other physicists, developed quantum electrodynamics that combines quantum mechanics and electromagnetism.Work on nuclear and particle physics lead to the discovery of the strong nuclear and weak nuclear forces which were combined in the quantum field theory to implemented the Standard Model of physics, a unification of all forces except gravity. The lone fundamental force not built into the Standard Model is gravity. General relativity provides a theoretical framework for understanding gravity across scales from the laboratory to planets to the complete universe, but it has not been successfully unified with quantum mechanics.

In physical cosmology, the grand unification epoch is a poorly understood period in the evolution of the early universe following the Planck epoch and preceding inflation. This places it between about 10 seconds after the Big Bang and 10 seconds, when the temperature of the universe was comparable to the characteristic temperatures of grand unified theories. However, these theories have not been successful producing quantitative agreement with the results of modern astrophysical observations.

If the grand unification energy is taken to be 10 GeV, this corresponds to temperatures higher than 10 K. During this period, three of the four fundamental interactions—electromagnetism, the strong interaction, and the weak interaction—were unified as the electronuclear force. Gravity had separated from the electronuclear force at the end of the Planck era. During the grand unification epoch, physical characteristics such as mass, charge, flavour and colour charge were meaningless.