Graviton

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Graviton

describing graviton interactions, the classical theory

Classical theory

Classical theory has at least two distinct meanings in Physics:#In the context of quantum mechanics, "classical theory" refers to theory of physics that do not use the Quantization paradigm, particularly Newtonian mechanics ....
(i.e. the tree diagram

Tree diagram

The term tree diagrams is used in different ways in different disciplines.* In mathematics and statistical methods, a tree diagram is used to determine the probability of getting specific results where the possibilities are nested....
s) and semiclassical

Semiclassical

In 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 mechanics and classical mechanics aspects in a given problem....
corrections (one-loop diagrams) behave normally, but Feynman diagram

Feynman diagram

In quantum field theory a Feynman diagram is an intuitive graphical representation of a contribution to the transition amplitude or correlation function of a quantum mechanical or statistical field theory....
s with two (or more) loops lead to ultraviolet divergence

Ultraviolet divergence

In 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 , or, equivalently, because of physical phenomena at very short distances....
s; that is, infinite results that cannot be removed because the quantized general relativity

General relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916....
is not renormalizable, unlike quantum electrodynamics

Quantum electrodynamics

Quantum electrodynamics is a relativity theory quantum field theory of electrodynamics. QED was developed by a number of physicists, beginning in the late 1920s....
. In popular terms, the discrete

Discrete mathematics

Discrete mathematics, also called finite mathematics, is the study of mathematical structures that are fundamentally discrete in the sense that its objects can assume only distinct, separate values, rather than a values on a continuum ....
ness of quantum theory is not compatible with the smoothness

Smooth function

In mathematical analysis, a differentiability class is a classification of function according to the properties of their derivatives. Higher order differentiability classes correspond to the existence of more derivatives....
of Einstein

Albert Einstein

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Gravitons and models of quantum gravity

When describing graviton interactions, the classical theory

Classical theory

Tree diagram

Semiclassical

Feynman diagram

Ultraviolet divergence

General relativity

Quantum electrodynamics

Discrete mathematics

Smooth function

Albert Einstein

Albert Einstein was a Germany-born theoretical physics. He is best known for his theory of relativity and specifically mass?energy equivalence, expressed by the equation E = mc2....
'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 scale

Planck scale

In particle physics and physical cosmology, the Planck scale is an energy scale around 1.22 ? 1028 eV at which quantum mechanics of gravity become strong....
. Superstring theory

Superstring theory

Superstring theory is an attempt to explain all of the Elementary particle and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetry strings....
finally emerged as the most promising solution; it is the only known theory with finite corrections to graviton scattering

Scattering

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles,are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass....
at all orders.

String theory

String theory

String theory is a developing branch of theoretical physics that combines quantum mechanics and general relativity into a quantum gravity. The String s of string theory are one-dimensional oscillating lines, but they are no longer considered fundamental to the theory, which can be formulated in terms of points or surfaces too....
predicts the existence of gravitons and their well-defined interaction

Interaction

Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect....
s 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

Correlation function (quantum field theory)

In quantum field theory, correlation functions generalize the concept of correlation functions in statistics. In the quantum mechanics context they are computed as the matrix element of a product of operator inserted between two vectors, usually the vacuum states....
in conformal field theory

Conformal field theory

A conformal field theory is a quantum field theory that is invariant under conformal symmetry. Conformal field theory is often studied in two-dimensional geometry dimensions where there is an infinite-dimensional group of local conformal transformations, described by the holomorphic functions....
, 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 brane

Brane

In theoretical physics, a membrane, brane, or p-brane is a spatially extended mathematical concept that appears in string theory and its relatives that exists in a static number of dimensions....
s and could move freely between them. 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

Dark matter

In astronomy and physical cosmology, dark matter is Hypothesis matter that is undetectable by its emitted electromagnetic radiation, but whose presence can be inferred from gravity effects on visible matter....
. See brane cosmology

Brane cosmology

Brane cosmology refers to several theories in particle physics and physical cosmology motivated by, but not exclusively derived from, superstring theory and M-theory....
for more details.

Experimental observation

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is simply the extremely low cross section

Cross section (physics)

In nuclear physics and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a surface....
for the interaction of gravitons with matter. For example, a detector the mass of Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the Solar system by size planet within the Solar System. It is two and a half times as massive as all of the other planets in our Solar System combined....
with 100% efficiency, placed in close orbit around a neutron star

Neutron star

A neutron star is a type of compact star that can result from the gravitational collapse of a massive star during a Type II supernova, Type Ib and Ic supernovae supernova event....
, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background of neutrino

Neutrino

Neutrinos are elementary particles that travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect....
s, and it would be impossible to shield the neutrinos without the shielding material collapsing into a black hole

Black hole

In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation , can escape its pull after having fallen past its event horizon....
.

However, experiments to detect gravitational wave

Gravitational wave

In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave#Traveling wave, traveling outward from a moving object or system of objects....
s, which may be viewed as coherent state

Coherent state

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 of a classical harmonic oscillator system....
s of many gravitons, are already underway (e.g. LIGO

LIGO

LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory, is a large physics experiment which is attempting to directly detect gravitational waves....
and VIRGO

Virgo

Virgo may refer to:8 beautiful* Virgo , an astrological sign* Virgo , a musical project between Andre Matos and Sascha Paeth* Virgo , a constellation...
). 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

Speed of light

The speed of light in an free space is an important physical constant usually written as c, with a value of 299,792,458 metres per second....
in a vacuum), that would imply that the graviton has mass.

Comparison with other forces

Unlike the force carrier

Force carrier

In particle physics, the quantum field theory called the Standard Model describes the strong interaction, weak nuclear force and electromagnetism fundamental forces....
s of the other forces

Fundamental interaction

In physics, a fundamental interaction or fundamental force is a process by which elementary particles interact with each other. An interaction is often described as a field , and is mediated by the exchange of gauge bosons between particles....
, gravitation plays a special role in general relativity

General relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916....
in defining the spacetime

Spacetime

In physics, spacetime is any mathematical model that combines space and Time in physics into a single continuum . Spacetime is usually interpreted with space being Three-dimensional space and time playing the role of a fourth dimension that is of a different sort than the spatial dimensions....
in which events take place. Because it does not depend on a particular spacetime background, general relativity is said to be background independent. In contrast, the Standard Model is not background independent. In other words, general relativity and the Standard Model are mutually incompatible. A theory of quantum gravity

Quantum gravity

Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the Fundamental interaction , with general relativity, the theory of the fourth fundamental force: Gravitation....
is needed in order to reconcile these differences. Whether this theory should be background independent or not is an open question. The answer to this question will determine if gravity plays a "special role" in the universe.

Graviton

Gravitons and models of quantum gravity

Experimental observation

Comparison with other forces

See also