A theory of everything (ToE) or final theory, ultimate theory or master theory refers to the hypothetical presence of a single, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe. ToE is one of the major unsolved problems in physics. Over the past few centuries, two theoretical frameworks have been developed that, as a whole, most closely resemble a ToE. The two theories upon which all modern physics rests are General Relativity (GR) andQuantum Mechanics (QM). GR is a theoretical framework that only considers the force of gravity for understanding the universe in regions of both large scale and high-mass: stars, galaxies, clusters of galaxies, etc. QM is a theoretical framework for understanding the universe in regions of both small scale and low-mass: sub-atomic particles, atoms, molecules, etc. To achieve this means, QM implements the Standard Model to unify the interactions (so-called grand unified theory) between the three non-gravitational forces:weak, strong and electromagnetic force.
Through years of research, physicists have experimentally confirmed with tremendous accuracy virtually every prediction made by these two theories. In accordance with their findings, scientists have learned that GR and QM, as they are currently formulated, are mutually incompatible – they cannot both be right. Since the usual domains of applicability of GR and QM are so different, most situations require that only one of the two theories be used. The incompatibility between GR and QM is only an apparent issue in regions of both extremely small scale and high-mass, such as those that exist within a black hole or during the beginning stages of the universe (i.e., the moment immediately following the big bang). To resolve this conflict, a theoretical framework revealing a deeper underlying reality – unifying gravity with the other three interactions- must be discovered to harmoniously integrate the realms of GR and QM into a seamless whole- a single theory that, in principle, is capable of describing all phenomena. In pursuit of this goal, quantum gravity has recently become an area of active research.
Some scientists believe that, at the beginning of the universe (up to 10-43 second after the Big Bang), the four fundamental forces were one fundamental force. Over the past few decades, a single explanatory framework, called “string theory”, has emerged that has the potential to become the ultimate theory of the universe. Unlike other theories, String theory is able to incorporate each of the four fundamental forces for all matter in the universe. According to string theory, everything in the universe, at its most microscopic level, consists of varying combinations of vibrating strings (or strands) each with preferred patterns of vibration. String theory claims that it is through specific oscillatory patterns of strings that a particle of unique mass and force charge is created (that is to say, the electron is a string vibrating one way, while the up-quark is a string vibrating another way, and so on). In Stephen Hawking’s view, the discovery of a so-called ToE would provide us access into ‘the mind of god’.
Initially, the term theory of everything was used with an ironic connotation to refer to various overgeneralized theories. For example, a great-grandfather of Ijon Tichy — a character from a cycle of Stanisław Lem’s science fiction stories of the 1960s — was known to work on the “General Theory of Everything”. Physicist John Ellis claims to have introduced the term into the technical literature in an article in Nature in 1986. Over time, the term stuck in popularizations of theoretical physics research.