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In physics, the graviton is a hypothetical elementary particle that transmits the force of gravity in most quantum gravity systems. In order to do this gravitons have to be always-attractive (gravity never pushes), work over any distance (gravity is universal) and come in unlimited numbers (to provide high strengths near stars). In quantum theory, this defines an even- spin (spin 2 in this case) boson with a rest mass of zero.Gravitons are postulated simply because quantum theory has been so successful in other fields. For instance, the electromagnetic interaction can be very well explained by the application of quantization to photons, a science known as quantum electrodynamics. In this case photons are being continually created and destroyed by all charged particles, and the interactions between these photons produce the macroscopic forces we are familiar with, like magnetism.
Given the widespread success of quantum theory in describing the vast majority of basic forces in the universe, it seemed only natural that the same methods would work well on gravity as well. Many attempts finally led to introduction of a so-far unseen graviton, which would work in a fashion somewhat similar to the photon. It was hoped that this would quickly lead to a quantum gravity theory, although the mathematics became convoluted and no internally consistant theory has yet emerged.
1 Gravitons and models of quantum gravity
While the classical theory (i.e. the tree diagrams) and semiclassicalIn physics, the adjective semiclassical has different precise meanings depending on the context. All these meanings usually refer to some approximation, limit or situation that combines quantum and classical aspects in a given problem. The plurality of me corrections ( one-loop diagrams) behaved as expected, the Feynman diagramA Feynman diagram is a bookkeeping device for performing calculations in quantum field theory, invented by American physicist Richard Feynman. The problem of calculating scattering cross sections in particle physics reduces to summing over the amplitudess with two (or more) loops led to ultraviolet divergenceIn physics, an ultraviolet divergence is a situation in which an integral, for example a Feynman diagram, diverges because of contributions of objects with very high energy approaching infinity, or, equivalently, because of physical phenomena at very shors - i.e. infinite results that could not be removed because the quantized general relativityGeneral relativity (GR or general relativity theory (GRT is the theory of gravitation published by Albert Einstein in 1915. The conceptual core of general relativity, from which its other consequences largely follow, is the Principle of Equivalence which was not renormalizableIn physics, the adjective renormalizable refers to a theory (usually a quantum field theory) in which all ultraviolet divergences, infinities and other seemingly meaningless results can be cured by the process of renormalization. This means that the numbe, unlike Quantum electrodynamics. In popular terms, the discreteThe word discrete comes from the latin word discretus which means separate''. It is used with different meanings in different contexts: In perception a discrete entity is something that can be perceived individually and not as connected to, or part of somness of quantum theory is not compatible with the smoothnessIn mathematics, a smooth function is one that is infinitely differentiable, i. has derivatives of all finite orders. A function is called C1 if it has a derivative that is a continuous function; such functions are also called continuously differentiable . of Einstein's general relativity. These problems, together with some conceptual puzzles, led many physicists to believe that a theory more complete than just general relativity must regulate the behavior near the Planck length. Superstring theory finally emerged as the most promising solution; it is the only known theory in which the quantum corrections of any order to graviton scattering are finite.
String theory predicts the existence of gravitons and their well-defined interactions which represents one of its most important triumphs. A graviton in perturbative string theory is a closed string in a very particular low-energy vibrational state. The scattering of gravitons in string theory can also be computed from the correlation functions in conformal field theory, as dictated by the AdS/CFT correspondence, or from Matrix theory.
An interesting feature of gravitons in string theory is that, as closed strings without endpoints, they would not be bound to branes and could move freely between them; this "leakage" of gravitons from our brane into higher-dimensional space could explain why gravity is such a weak force, and gravitons from other branes adjacent to our own could provide a potential explanation for dark matter. See brane cosmology for more details.
It should be noted that there exist proposed quantum theories of gravity that do not predict a graviton; for instance, loop quantum gravity has no analogous particle.
