Theory of everything

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.

General relativity and quantum mechanics have been repeatedly validated in their separate fields of relevance. Since the usual domains of applicability of general relativity and quantum mechanics are so different, most situations require that only one of the two theories be used. The two theories are considered incompatible in regions of extremely small scale – the Planck scale – 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 the incompatibility, a theoretical framework revealing a deeper underlying reality, unifying gravity with the other three interactions, must be discovered to harmoniously integrate the realms of general relativity and quantum mechanics into a seamless whole: a theory of everything may be defined as a comprehensive theory that, in principle, would be capable of describing all physical phenomena in the universe.

In pursuit of this goal, quantum gravity has become one area of active research. One example is string theory, which evolved into a candidate for the theory of everything, but not without drawbacks (most notably, its apparent lack of currently testable predictions) and controversy. String theory posits that at the beginning of the universe (up to 10⁻⁴³ seconds after the Big Bang), the four fundamental forces were once a single fundamental force. According to string theory, every particle in the universe, at its most ultramicroscopic level (Planck length), consists of varying combinations of vibrating strings (or strands) with preferred patterns of vibration. String theory further claims that it is through these specific oscillatory patterns of strings that a particle of unique mass and force charge is created (that is to say, the electron is a type of string that vibrates one way, while the up quark is a type of string vibrating another way, and so forth). String theory/M-theory proposes six or seven dimensions of spacetime in addition to the four common dimensions for a ten- or eleven-dimensional spacetime.

The scientific use of the term theory of everything occurred in the title of an article by physicist John Ellis in 1986 but it was mentioned by John Henry Schwarz in a conference proceedings in 1985.

Archimedes was possibly the first philosopher to have described nature with axioms (or principles) and then deduce new results from them. Once Isaac Newton proposed his universal law of gravitation, mathematician Pierre-Simon Laplace suggested that such laws could in principle allow deterministic prediction of the future state of the universe. Any "theory of everything" is similarly expected to be based on axioms and to deduce all observable phenomena from them.

In the late 17th century, Isaac Newton's description of the long-distance force of gravity implied that not all forces in nature result from things coming into contact. Newton's work in his Mathematical Principles of Natural Philosophy dealt with this in a further example of unification, in this case unifying Galileo's work on terrestrial gravity, Kepler's laws of planetary motion and the phenomenon of tides by explaining these apparent actions at a distance under one single law: the law of universal gravitation. Newton achieved the first great unification in physics, and he further is credited with laying the foundations of future endeavors for a grand unified theory.

An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.