A theory of everything (ToE) or final theory is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle.
Many candidate theories of everything have been proposed by theoretical physicists during the twentieth century, but none have been confirmed experimentally. The primary problem in producing a TOE is that general relativity and quantum mechanics are hard to unify. This is one of the unsolved problems in physics.
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 Tichya character from a cycle of Stanisław Lem's science fiction stories of the 1960swas 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 Over time, the term stuck in popularizations of quantum physics to describe a theory that would unify or explain through a single model the theories of all fundamental interactions and of all particles of nature: general relativity for gravitation, and the standard model of elementary particle physics which includes quantum mechanics for electromagnetism, the two nuclear interactions, and the known elementary particles.
"An Exceptionally Simple Theory of Everything is a physics preprint proposing a basis for a unified field theory, very often referred to as "E8 Theory, which attempts to describe all known fundamental interactions in physics and to stand as a possible theory of everything. The paper was posted to the physics arXiv by Antony Garrett Lisi on November 6, 2007, and was not submitted to a peer-reviewed scientific journal. The title is a pun on the algebra used, the Lie algebra of the largest "simple," "exceptional" Lie group, E 8. The paper's goal is to describe how the combined structure and dynamics of all gravitational and Standard Model particle fields, including fermions, are part of the E8 Lie algebra. In the paper, Lisi states that all three generations of fermions do not directly embed in E 8 with correct quantum numbers and spins, but that they might be described via a triality transformation, noting that the theory is incomplete and that a correct description of the relationship between triality and generations, if it exists, awaits a better understanding.
The theory received accolades from a few physicists amid a flurry of media coverage, but also met with widespread skepticism. Scientific American reported in March 2008 that the theory was being "largely but not entirely ignored" by the mainstream physics community, with a few physicists picking up the work to develop it further.
In a follow-up paper, Lee Smolin proposes a spontaneous symmetry breaking mechanism for obtaining the classical action in Lisi's model, and speculates on the path to its quantization.
In July 2009 Jacques Distler and Skip Garibaldi published a critical paper in Communications in Mathematical Physics called "There is no 'Theory of Everything' inside E8, arguing that Lisi's theory, and a large class of related models, cannot work. They offer a direct proof that it is impossible to embed all three generations of fermions in E 8, or to obtain even the one-generation Standard Model without the presence of an antigeneration. In response to Distler and Garibaldi's paper, Lisi argues, in a new paper "An Explicit Embedding of Gravity and the Standard Model in E8, peer reviewed and published in a conference proceedings, that some assumptions about fermion embeddings are unnecessary and that the antigeneration is not per se a problem sufficient to rule out the one-generation Standard Model. In December 2010 and May 2011 Lisi wrote in the popular magazine Scientific American a feature article on the E8 Theory of Everything and an entry in the blog section of the magazine addressing some of the criticism of his theory and how it has progressed, noting that the theory is still incomplete and makes only tenuous predictions, with the three generation issue remaining as a significant problem.