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unify electromagnetism and the weak and strong forces grand unification would imply the existence of an electronuclear force it is expected to set in at energies of the order of 10 16 gev far greater than could be reached by any currently feasible particle accelerator although the simplest grand unified theories have been experimentally ruled out the idea of a grand unified theory especially when linked with supersymmetry remains a favorite candidate in the theoretical physics community supersymmetric grand unified theories seem plausible not only for their theoretical beauty but because they naturally produce large quantities of dark matter and because the inflationary force may be related to grand unified theory physics although it does not seem to form an inevitable part of the theory yet grand unified theories are clearly not the final answer both the current standard model and all proposed guts are quantum field theories which require the problematic technique of renormalization to yield sensible answers this is usually regarded as a sign that these are only effective field theories omitting crucial phenomena relevant only at very high energies 8 the final step in the graph requires resolving the separation between quantum mechanics and gravitation often equated with general relativity numerous researchers concentrate their efforts on this specific step nevertheless no accepted theory of quantum gravity and thus no accepted theory of everything has emerged with observational evidence it is usually assumed that the theory of everything will also solve the remaining problems of grand unified theories in addition to explaining the forces listed in the graph a theory of everything may also explain the status of at least two candidate forces suggested by modern cosmology an inflationary force and dark energy furthermore cosmological experiments also suggest the existence of dark matter supposedly composed of fundamental particles outside the scheme of the standard model however the existence of these forces and particles has not been proven string theory and m theory edit unsolved problem in physics is string theory superstring theory or m theory or some other variant on this theme a step on the road to a theory of everything or just a blind alley more unsolved problems in physics since the 1990s some physicists such as edward witten believe that 11 dimensional m theory which is described in some limits by one of the five perturbative superstring theories and in another by the maximally supersymmetric eleven dimensional supergravity is the theory of everything there is no widespread consensus on this issue one remarkable property of string m theory is that seven extra dimensions are required for the theory s consistency on top of the four dimensions in our universe in this regard string theory can be seen as building on the insights of the kaluza klein theory in which it was realized that applying general relativity to a 5 dimensional universe with one space dimension small and curled up looks from the 4 dimensional perspective like the usual general relativity together with maxwell s electrodynamics this lent credence to the idea of unifying gauge and gravity interactions and to extra dimensions but did not address the detailed experimental requirements another important property of string theory is its supersymmetry which together with extra dimensions are the two main proposals for resolving the hierarchy problem of the standard model which is roughly the question of why gravity is so much weaker than any other force the extra dimensional solution involves allowing gravity to propagate into the other dimensions while keeping other forces confined to a 4 dimensional spacetime an idea that has been realized with explicit stringy mechanisms 22 research into string theory has been encouraged by a variety of theoretical and experimental factors on the experimental side the particle content of the standard model supplemented with neutrino masses fits into a spinor representation of so 10 a subgroup of e 8 that routinely emerges in string theory such as in heterotic string theory 23 or sometimes equivalently in f theory 24 25 string theory has mechanisms that may explain why fermions come in three hierarchical generations and explain the mixing rates between quark generations 26 on the theoretical side it has begun to address some of the key questions in quantum gravity such as resolving the black hole information paradox counting the correct entropy of black holes 27 28 and allowing for topology changing processes 29 30 31 it has also led to many insights in pure mathematics and in ordinary strongly coupled gauge theory due to the gauge string duality in the late 1990s it was noted that one major hurdle in this endeavor is that the number of possible 4 dimensional universes is incredibly large the small curled up extra dimensions can be compactified in an enormous number of different ways one estimate is 10 500 each of which leads to different properties for the low energy particles and forces this array of models is known as the string theory landscape 15 347 one proposed solution is that many or all of these possibilities are realized in one or another of a huge number of universes but that only a small number of them are habitable hence what we normally conceive as the fundamental constants of the universe are ultimately the result of the anthropic principle rather than dictated by theory this has led to criticism of string theory 32 arguing that it cannot make useful i e original falsifiable and verifiable predictions and regarding it as a pseudoscience philosophy others disagree 33 and string theory remains an active topic of investigation in theoretical physics 34 loop quantum gravity edit current research on loop quantum gravity may eventually play a fundamental role in a theory of everything but that is not its primary aim 35 loop quantum gravity also introduces a lower bound on the possible length scales there have been recent claims that loop quantum gravity may be able to reproduce features resembling the standard model so far only the first generation of fermions leptons and quarks with correct parity properties have been modelled by sundance bilson thompson using preons constituted of braids of spacetime as the building blocks 36 however there is no derivation of the lagrangian that would describe the interactions of such particles nor is it possible to show that such particles are fermions nor that the gauge groups or interactions of the standard model are realised use of quantum computing concepts made it possible to demonstrate that the particles are able to survive quantum fluctuations 37 this model leads to an interpretation of electric and color charge as topological quantities electric as number and chirality of twists carried on the individual ribbons and colour as variants of such twisting for fixed electric charge bilson thompson s original paper suggested that the higher generation fermions could be represented by more complicated braidings although explicit constructions of these structures were not given the electric charge color and parity properties of such fermions would arise in the same way as for the first generation the model was expressly generalized for an infinite number of generations and for the weak force bosons but not for photons or gluons in a 2008 paper by bilson thompson hackett kauffman and smolin 38 present status edit at present there is no candidate theory of everything that includes the standard model of particle physics and general relativity and that at the same time is able to calculate the fine structure constant or the mass of the electron 3 most particle physicists expect that the outcome of ongoing experiments the search for new particles at the large particle accelerators and for dark matter are needed in order to provide further input for a theory of everything other proposals edit the search for a theory of everything is hindered by fundamental incompatibility between the noncommutative and discrete operator algebra structures underlying quantum mechanics and the commutative continuous geometric nature of classical spacetime in general relativity reconciling the background independent diffeomorphism invariant formulation of gravity with the fixed background time ordered framework of quantum theory raises profound conceptual issues such as the problem of time and quantum measurement 39 while a fully successful and experimentally confirmed unified field theory remains elusive several recent proposals have been advanced each employing distinct mathematical structures and physical assumptions twistor theory developed by roger penrose reinterprets the structure of spacetime and fundamental particles through complex geometric objects called twistors instead of treating spacetime points as fundamental twistor theory encodes physical fields and particles into complex projective spaces aiming to unify quantum theory and general relativity in a geometric framework twistors provide potential descriptions of massless fields and scattering amplitudes and have influenced modern approaches in mathematical physics and quantum field theory including advances in scattering amplitude calculations twistor theory has not yet yielded a complete unified field theory 40 non primary source needed alain connes developed a geometric framework known as noncommutative geometry in which spacetime is extended via noncommutative operator algebras when combined with spectral triples this approach can reproduce features of the standard model including the higgs field from purely geometric data 41 42 asymptotic safety a concept developed by steven weinberg in 1976 and also known as quantum einstein gravity and nonperturbative renormalizability suggests that gravity could find a role in quantum theory if its behavior at very high energies becomes stabilized into a nontrivial ultraviolet uv fixed point 43 this form has been studied through functional renormalization group methods 44 and on the lattice 45 and applied in cosmology particle physics black hole physics and quantum gravity 46 whereas overwhelming numerical evidence does exist that such a fixed point does occur in lower dimensional constructions and in the numerics 47 a rigorous proof even for four dimensional spacetime remains to be found 48 arguments against edit in parallel to the intense search for a theory of everything various scholars have debated the possibility of its discovery gödel s incompleteness theorem edit a number of scholars claim that gödel s incompleteness theorem suggests that attempts to construct a theory of everything are bound to fail gödel s theorem informally stated asserts that any formal theory sufficient to express elementary arithmetical facts and strong enough for them to be proved is either inconsistent both a statement and its denial can be derived from its axioms or incomplete in the sense that there is a true statement that can t be derived in the formal theory the benedictine priest and science writer stanley jaki in his 1966 book the relevance of physics suggested that gödel s theorem dooms searches for a deterministic theory of everything at least as a consistent non trivial mathematical theory 49 freeman dyson has stated that gödel s theorem implies that pure mathematics is inexhaustible no matter how many problems we solve there will always be other problems that cannot be solved within the existing rules because of gödel s theorem physics is inexhaustible too the laws of physics are a finite set of rules and include the rules for doing mathematics so that gödel s theorem applies to them 50 stephen hawking originally believed that a theory of everything could be found but after considering gödel s theorem he concluded that one was not obtainable some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles i used to belong to that camp but i have changed my mind 51 jürgen schmidhuber 1997 has argued against this view he asserts that gödel s theorems are irrelevant for computable physics 52 in 2000 schmidhuber explicitly constructed limit computable deterministic universes whose pseudo randomness based on undecidable gödel like halting problems is extremely hard to detect but does not prevent formal theories of everything describable by very few bits of information 53 related critique was offered by solomon feferman 54 and others douglas s robertson offers conway s game of life as an example 55 the underlying rules are simple and complete but there are formally undecidable questions about the game s behaviors analogously it may or may not be possible to completely state the underlying rules of physics with a finite number of well defined laws but there is little doubt that there are questions about the behavior of physical systems which are formally undecidable on the basis of those underlying laws fundamental limits in accuracy edit no physical theory to date is believed to be precisely accurate instead physics has proceeded by a series of successive approximations allowing more and more accurate predictions over a wider and wider range of phenomena some physicists believe that it is therefore a mistake to confuse theoretical models with the true nature of reality and hold that the series of approximations will never terminate in the truth 56 einstein himself expressed this view on occasions 57 definition of fundamental laws edit there is a philosophical debate within the physics community as to whether a theory of everything deserves to be called the fundamental law of the universe 58 one view is the hard reductionist position that the theory of everything is the fundamental law and that all other theories that apply within the universe are a consequence of the theory of everything another view is that emergent laws which govern the behavior of complex systems should be seen as equally fundamental examples of emergent laws are the second law of thermodynamics and the theory of natural selection the advocates of emergence argue that emergent laws especially those describing complex or living systems are independent of the low level microscopic laws in this view emergent laws are as fundamental as a theory of everything impossibility of calculation edit weinberg 59 points out that calculating the precise motion of an actual projectile in the earth s atmosphere is impossible so how can we know we have an adequate theory for describing the motion of projectiles weinberg suggests that we know principles newton s laws of motion and gravitation that work well enough for simple examples like the motion of planets in empty space these principles have worked so well on simple examples that we can be reasonably confident they will work for more complex examples for example although general relativity includes equations that do not have...