In physics, the graviton is a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made Gravitation is a natural Phenomenon by which objects with Mass attract one another In quantum field theory (QFT the forces between particles are mediated by other particles If it exists, the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2 (because gravity is a second-rank tensor field). The term Mass in Special relativity usually refers to the Rest mass of the object which is the Newtonian mass as measured by an observer moving along with In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin In Mathematics, Physics and Engineering, a tensor field is a very general concept of variable geometric quantity

Gravitons are postulated because of the great success of the quantum field theory (in particular, the Standard Model) at modeling the behavior of all other forces of nature with similar particles: electromagnetism with the photon, the strong interaction with the gluons, and the weak interaction with the W and Z bosons. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and Gluons ( Glue and the suffix -on) are Elementary particles that cause Quarks to interact and are indirectly responsible for the The weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four Fundamental interactions of nature The W and Z bosons are the Elementary particles that mediate the Weak force. In this framework, the gravitational interaction is mediated by gravitons, instead of being described in terms of curved spacetime as in general relativity. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In the classical limit, both approaches give identical results, including Newton's law of gravitation. The classical limit is the ability of a physical theory to approximate or "recover" Classical mechanics when considered over special values of its parameters Newton 's law of universal Gravitation is a physical law describing the gravitational attraction between bodies with mass ^[1][2][3]

However, attempts to extend the Standard Model with gravitons run into serious theoretical difficulties at high energies (processes with energies close to or above the Planck scale) because of infinities arising due to quantum effects (in technical terms, gravitation is nonrenormalizable. In Particle physics and Physical cosmology, the Planck scale is an Energy scale around 1 In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection ) Some proposed theories of quantum gravity (in particular, string theory) address this issue. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings In string theory, gravitons (as well as the other particles) are states of strings rather than point particles, and then the infinities do not appear, while the low-energy behavior can still be approximated by a quantum field theory of point particles. In that case, the description in terms of gravitons serves as a low-energy effective theory. In Physics, an effective field theory is an approximate theory (usually a Quantum field theory) that includes appropriate degrees of freedom to describe

Gravitons and models of quantum gravity

When describing graviton interactions, the classical theory (i. Classical theory has at least two distinct meanings in Physics: In the context of Quantum mechanics, "classical theory" refers to e. the tree diagrams) and semiclassical corrections (one-loop diagrams) behave normally, but Feynman diagrams with two (or more) loops lead to ultraviolet divergences; that is, infinite results that cannot be removed because the quantized general relativity is not renormalizable, unlike quantum electrodynamics. In Physics, the adjective semiclassical has different precise meanings depending on the context In Physics, a one-loop Sander-Feynman diagram is a connected Feynman diagram with only one cycle ( Unicyclic) Motivation and history When calculating Scattering cross sections in Particle physics, the interaction between particles can be described In Physics, an ultraviolet divergence is a situation in which an Integral, for example a Feynman diagram, diverges because of contributions of objects with General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics. In popular terms, the discreteness of quantum theory is not compatible with the smoothness of Einstein's general relativity. Discrete mathematics, also called finite mathematics, is the study of mathematical structures that are fundamentally discrete in the sense of not supporting or requiring the In Mathematical analysis, a differentiability class is a classification of functions according to the properties of their Derivatives Higher order differentiability Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical 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 scale. In Particle physics and Physical cosmology, the Planck scale is an Energy scale around 1 Superstring theory finally emerged as the most promising solution; it is the only known theory with finite corrections to graviton scattering at all orders. See also String theory Superstring theory is an attempt to explain all of the particles and Fundamental forces of nature in one theory by modelling Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from

String theory predicts the existence of gravitons and their well-defined interactions which represents one of its most important triumphs. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings Interaction is a kind of action that occurs as two or more objects have an Effect upon one another A graviton in perturbative string theory is a closed string in a very particular low-energy vibrational state. In Quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system A string is one of the main objects of study in String theory, a branch of Theoretical physics. 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. In Quantum field theory, correlation functions generalize the concept of Correlation functions in statistics A conformal field theory (CFT is a Quantum field theory (or Statistical mechanics model at the Critical point) that is Invariant under For the relation of the AdS/CFT correspondence to the general context of string theory see String theory. In Physics, matrix string theory is the first known set of equations that describe Superstring theory in a non-perturbatively complete and consistent framework

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. In Theoretical physics, a membrane, brane, or p -brane is a spatially extended mathematical concept that appears in String theory If we live on a brane (as hypothesized by some theorists) this "leakage" of gravitons from the 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. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from See brane cosmology for more details. Brane cosmology refers to several theories in Particle physics and cosmology motivated by but not exclusively derived from Superstring theory and

Some proposed quantum theories of gravity do not predict a graviton.

Experimental observation

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. ^[4] The reason is simply the extremely low cross section for the interaction of gravitons with matter. In nuclear and Particle physics, the concept of a cross section is used to express the likelihood of interaction between particles For example, a detector the mass of Jupiter with 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type It would be impossible to discriminate these events from the background of neutrinos, and it would be impossible to shield the neutrinos without the shielding material collapsing into a black hole. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e ^[4]

However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are already underway (e. In Physics, a gravitational wave is a Fluctuation in the Curvature of Spacetime which propagates as a wave, traveling outward from In Quantum mechanics a coherent state is a specific kind of quantum state of the Quantum harmonic oscillator whose dynamics most closely resemble the oscillating behaviour g. LIGO and VIRGO). For the Latvian holiday Ligo see Jāņi. LIGO stands for Laser Interferometer Gravitational-Wave Observatory. The Virgo detector for Gravitational waves consists mainly in a Michelson Laser Interferometer made of two orthogonal arms being each 3 kilometers Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass. ^[5]

Is gravity like the other forces?

Some question the analogy which motivates the introduction of the graviton. Unlike the other forces, gravitation plays a special role in general relativity in defining the spacetime in which events take place. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS Because it does not depend on a particular spacetime background, general relativity is said to be background independent. Background independence is a condition in theoretical physics especially in Quantum gravity (QG that requires the defining equations of a theory to be independent of the actual In contrast, the Standard Model is not background independent. In other words, general relativity and the standard model are incompatible. A theory of quantum gravity is needed in order to reconcile these differences. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature Whether this theory should itself be background independent, or whether the background independence of general relativity arises as an emergent property is an open question. For other uses see Emergence (disambiguation, Emergent, and Emergency. The answer to this question will determine whether gravity plays a "special role" in this underlying theory similar to its role in general relativity.

See also

• Gravitomagnetism

References