Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the theories of quantum mechanics and general relativity. Loop quantum gravity suggests that space can be viewed as an extremely fine fabric or network "weaved" of finite quantised loops of excited gravitational fields called spin networks. When viewed over time, these spin networks are called spin foam, which should not be confused with quantum foam. A major contender with string theory, loop quantum gravity incorporates general relativity and does not require string theory's higher dimensions.
LQG preserves many of the important features of general relativity, while at the same time employing quantization of both space and time at the Planck scale in the tradition of quantum mechanics. The technique of loop quantization was developed for the nonperturbative quantization of diffeomorphisminvariant gauge theory. Roughly, LQG tries to establish a quantum theory of gravity in which the very space where all other physical phenomena occur becomes quantized.
LQG is one of a family of theories called canonical quantum gravity. The LQG theory includes also matter and forces, but the theory does not address the problem of the unification of all physical forces, as do some other quantum gravity theories such as string theory.
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In 1986, Abhay Ashtekar reformulated Einstein's field equations of general relativity, using what have come to be known as Ashtekar variables, a particular flavor of EinsteinCartan theory with a complex connection. In 1988, Carlo Rovelli and Lee Smolin used this formalism to introduce the loop representation of quantum general relativity, which was soon developed by Ashtekar, Rovelli, Smolin and many others. In the Ashtekar formulation, the fundamental objects are a rule for parallel transport (technically, a connection) and a coordinate frame (called a vierbein) at each point. Because the Ashtekar formulation was backgroundindependent, it was possible to use Wilson loops as the basis for a nonperturbative quantization of gravity. Explicit (spatial) diffeomorphism invariance of the vacuum state plays an essential role in the regularization of the Wilson loop states.
Around 1990, Rovelli and Smolin obtained an explicit basis of states of quantum geometry, which turned out to be labelled by Roger Penrose's spin networks, and showed that the geometry is quantized, that is, the (nongaugeinvariant) quantum operators representing area and volume have a discrete spectrum. In this context, spin networks arose as a generalization of Wilson loops necessary to deal with mutually intersecting loops. Mathematically, spin networks are related to group representation theory and can be used to construct knot invariants such as the Jones polynomial.
In the framework of quantum field theory, and using the standard techniques of perturbative calculations, one finds that gravitation is nonrenormalizable in contrast to the electroweak and strong interactions of the Standard Model of particle physics. This technical term implies that there are infinitely many free parameters in the theory and thus that it cannot be predictive.
In general relativity, the Einstein field equations assign a geometry (via a metric) to spacetime. Before this, there is no physical notion of distance or time measurements. In this sense, general relativity is said to be background independent. An immediate conceptual issue that arises is that the usual framework of quantum mechanics, including quantum field theory, relies on a reference (background) spacetime. Therefore, one approach to finding a quantum theory of gravity is to understand how to do quantum mechanics without relying on such a background; this is the approach of the canonical quantization/loop quantum gravity/spin foam approaches.
Starting with the initialvalueformulation of general relativity (cf. the section on General relativity#Evolution equations), the result is an analogue of the Schrödinger equation called the WheelerdeWitt equation, which some argue is illdefined.^{[1]} A major breakthrough came with the introduction of what are now known as Ashtekar variables, which represent geometric gravity using mathematical analogues of electric and magnetic fields.^{[2]} The resulting candidate for a theory of quantum gravity is Loop quantum gravity, in which space is represented by a network structure called a spin network, evolving over time in discrete steps.^{[3]}
Though not proven, it may be impossible to quantize gravity in 3+1 dimensions without creating matter and energy artifacts. Should LQG succeed as a quantum theory of gravity, the known matter fields will have to be incorporated into the theory a posteriori. Many of the approaches now being actively pursued (by Renate Loll, Jan Ambjørn, Lee Smolin, Sundance BilsonThompson, Laurent Freidel, Mark B. Wise and others^{[4]}) combine matter with geometry.
The main successes of loop quantum gravity are:
These claims are not universally accepted among the physics community, which is presently divided between different approaches to the problem of quantum gravity. LQG may possibly be viable as a refinement of either gravity or geometry. Many of the core results are rigorous mathematical physics, their physical interpretations remain speculative. Three speculative physical interpretations of LQG's core mathematical results are loop quantization, Lorentz invariance, General covariance and background independence, discussed below. Another physical test for LQG is to reproduce the physics of general relativity coupled with quantum field theory, discussed under problems.
At the core of loop quantum gravity is a framework for nonperturbative quantization of diffeomorphisminvariant gauge theories, which one might call loop quantization. While originally developed in order to quantize vacuum general relativity in 3+1 dimensions, the formalism can accommodate arbitrary spacetime dimensionalities, fermions,^{[5]} an arbitrary gauge group (or even quantum group), and supersymmetry,^{[6]} and results in a quantization of the kinematics of the corresponding diffeomorphisminvariant gauge theory. Much work remains to be done on the dynamics, the classical limit and the correspondence principle, all of which are necessary in one way or another to make contact with experiment.
In a nutshell, loop quantization is the result of applying C*algebraic quantization to a noncanonical algebra of gaugeinvariant classical observables. Noncanonical means that the basic observables quantized are not generalized coordinates and their conjugate momenta. Instead, the algebra generated by spin network observables (built from holonomies) and field strength fluxes is used.
Loop quantization techniques are particularly successful in dealing with topological quantum field theories, where they give rise to statesum/spinfoam models such as the TuraevViro model of 2+1 dimensional general relativity. A much studied topological quantum field theory is the socalled BF theory in 3+1 dimensions. Since classical general relativity can be formulated as a BF theory with constraints, scientists hope that a consistent quantization of gravity may arise from the perturbation theory of BF spinfoam models.
LQG is a quantization of a classical Lagrangian field theory which is equivalent to the usual EinsteinCartan theory in that it leads to the same equations of motion describing general relativity with torsion. As such, it can be argued that LQG respects local Lorentz invariance. Global Lorentz invariance is broken in LQG just as in general relativity. A positive cosmological constant can be realized in LQG by replacing the Lorentz group with the corresponding quantum group.
General covariance, also known as "diffeomorphism invariance", is the invariance of physical laws under arbitrary coordinate transformations. An example of this are the equations of general relativity, where this symmetry is one of the defining features of the theory. LQG preserves this symmetry by requiring that the physical states remain invariant under the generators of diffeomorphisms. The interpretation of this condition is well understood for purely spatial diffemorphisms. However, the understanding of diffeomorphisms involving time (the Hamiltonian constraint) is more subtle because it is related to dynamics and the socalled problem of time in general relativity.^{[7]} A generally accepted calculational framework to account for this constraint is yet to be found.^{[8]}^{[9]}
Whether or not Lorentz invariance is broken in the lowenergy limit of LQG, the theory is formally background independent. The equations of LQG are not embedded in, or presuppose, space and time, except for its invariant topology. Instead, they are expected to give rise to space and time at distances which are large compared to the Planck length.
While there has been a recent proposal relating to observation of naked singularities,^{[10]} and doubly special relativity, as a part of a program called loop quantum cosmology, as of now there is no experimental observation for which loop quantum gravity makes a prediction not made by the Standard Model or general relativity (a problem that plagues all current theories of quantum gravity).
Making predictions from the theory of LQG has been extremely difficult computationally, also a recurring problem with modern theories in physics.
Another problem is that a crucial free parameter in the theory known as the Immirzi parameter can only be computed by demanding agreement with Bekenstein and Hawking's calculation of the black hole entropy. Loop quantum gravity predicts that the entropy of a black hole is proportional to the area of the event horizon, but does not obtain the BekensteinHawking formula S = A/4 unless the Immirzi parameter is chosen to give this value. A prediction directly from theory would be preferable.
Presently, no semiclassical limit recovering general relativity has been shown to exist. What this means is it remains unproven that LQG's description of spacetime at the Planckian scale has the right continuum limit, described by general relativity with possible quantum corrections.
According to Rovelli, there has been a shift in LQG research from the canonical formulation as discussed above towards the path integral spinfoam models with the hope to better generate a classical spacetime that matches general relativity.^{[11]}
Recent results with the Fermi telescope show the beginning of empirical tests that could either falsify loop quantum gravity or support it by verifying its predictions of violation of the constancy of the speed of light.^{[12]}

