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Loop quantum gravity
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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. It 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 diffeomorphism-invariant gauge theory.
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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. It 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 diffeomorphism-invariant 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. A list of quantum gravity theories can be found on the quantum gravity page. The LQG theory includes also matter and forces, but the theory does not address the problem of the unification of all physical forces, as other tentative quantum gravity theories do (for instance string theory).
Loop quantum gravity in general, and its ambitions
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 Bilson-Thompson, Laurent Freidel, Mark B. Wise and others) combine matter with geometry. Several of these current efforts would be proven wrong if evidence were found of extra spatial dimensions.
The main successes of loop quantum gravity are:
- It is a nonperturbative quantization of 3-space geometry, with quantized area and volume operators.
- It includes a calculation of the entropy of black holes.
- It replaces the Big Bang spacetime singularity with a Big Bounce.
These claims are not universally accepted among the physics community, which is presently divided between different approaches to the problem of quantum gravity. Many of the core results are rigorous mathematical physics, their physical interpretations remain speculative. LQG may possibly be viable as a refinement of either gravity or geometry. It has not been shown that loop quantum gravity reproduces general relativity as a low energy limit.
There are several other approaches to quantum gravity, such as spin foam models, which are closely related to loop quantum gravity.
The apparent incompatibility between quantum mechanics and general relativity
In general relativity, the Einstein field equations assign a geometry (via a metric) to space-time. 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) space-time. 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.
Furthermore, in the framework of quantum field theory, and using the standard techniques of perturbative calculations, one finds that gravitation is non-renormalizable 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.
Another interface between general relativity and quantum mechanics occurs in quantum field theory studied on curved (non-Minkowskian) backgrounds. The vacuum, when it exists, is shown in general relativity to depend on the path of the observer through space-time (see Unruh effect). The Unruh effect can be described semi-classically in the case of a fixed background geometry on which propagate non-gravitational quantum mechanical particles (quanta). It's then natural to wonder about the inclusion of quantum gravitational effects, including interactions with the graviton (in analogy with the electron's interactions with the electromagnetic field of the nucleus within an atom).
History of LQG
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 Einstein-Cartan 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 background-independent, 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, Carlo Rovelli and Lee Smolin obtained an explicit basis of states of quantum geometry, which turned out to be labelled by Penrose's spin networks, and showed that the geometry is quantized, that is, the (non-gauge-invariant) 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.
The ingredients of loop quantum gravity
Loop quantization
At the core of loop quantum gravity is a framework for nonperturbative quantization of diffeomorphism-invariant 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, an arbitrary gauge group (or even quantum group), and supersymmetry, and results in a quantization of the kinematics of the corresponding diffeomorphism-invariant 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 non-canonical algebra of gauge-invariant classical observables. Non-canonical 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 state-sum/spin-foam models such as the Turaev-Viro model of 2+1 dimensional general relativity. A much studied topological quantum field theory is the so-called 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 spin-foam models.
This discrete structure may require modifications of quantum mechanics, and a line of research called polymer quantum mechanics has been pursued.
Lorentz invariance
LQG is a quantization of a classical Lagrangian field theory which is equivalent to the usual Einstein-Cartan 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 and background independence
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 so-called problem of time in general relativity. A generally accepted calculational framework to account for this constraint is yet to be found.
Whether or not Lorentz invariance is broken in the low-energy 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. At present, 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.
Problems
While there has been a recent proposal relating to observation of naked singularities, 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. This problem 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 Bekenstein-Hawking 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.
See also
Bibliography
- Topical Reviews
- Carlo Rovelli, Loop Quantum Gravity, 1, (1998), 1, , 2001 15 August version.
- Thomas Thiemann, Lectures on loop quantum gravity, e-print available as
- Abhay Ashtekar and Jerzy Lewandowski, Background independent quantum gravity: a status report, e-print available as
- Carlo Rovelli and Marcus Gaul, Loop Quantum Gravity and the Meaning of Diffeomorphism Invariance, e-print available as .
- Lee Smolin, The case for background independence, e-print available as .
- Alejandro Corichi, Loop Quantum Geometry: A primer, e-print available as .
- Alejandro Perez, Introduction to loop quantum gravity and spin foams, e-print available as .
- Hermann Nicolai and Kasper Peeters Loop and spin foam quantum gravity: A Brief guide for beginners., e-print available as .
- Popular books:
- Lee Smolin, Three Roads to Quantum Gravity
- Carlo Rovelli, Che cos'è il tempo? Che cos'è lo spazio?, Di Renzo Editore, Roma, 2004. French translation: Qu'est ce que le temps? Qu'est ce que l'espace?, Bernard Gilson ed, Brussel, 2006. English translation: What is Time? What is space?, Di Renzo Editore, Roma, 2006.
- Julian Barbour, The End of Time
- – Focuses on string theory but has an extended discussion of loop gravity as well.
- Magazine articles:
- Easier introductory, expository or critical works:
- Abhay Ashtekar, Gravity and the quantum, e-print available as (2004)
- John C. Baez and Javier Perez de Muniain, Gauge Fields, Knots and Quantum Gravity, World Scientific (1994)
- Carlo Rovelli, A Dialog on Quantum Gravity, e-print available as (2003)
- More advanced introductory/expository works:
- Carlo Rovelli, Quantum Gravity, Cambridge University Press (2004);
- Thomas Thiemann, Introduction to modern canonical quantum general relativity, e-print available as
- Thomas Thiemann, Introduction to Modern Canonical Quantum General Relativity, Cambridge University Press (2007)
- Abhay Ashtekar, New Perspectives in Canonical Gravity, Bibliopolis (1988).
- Abhay Ashtekar, Lectures on Non-Perturbative Canonical Gravity, World Scientific (1991)
- Rodolfo Gambini and Jorge Pullin, Loops, Knots, Gauge Theories and Quantum Gravity, Cambridge University Press (1996)
- Hermann Nicolai, Kasper Peeters, Marija Zamaklar, Loop quantum gravity: an outside view, e-print available as
- H. Nicolai and K. Peeters, Loop and Spin Foam Quantum Gravity: A Brief Guide for Beginners, e-print available as
- Conference proceedings:
- Fundamental research papers:
- Abhay Ashtekar, New variables for classical and quantum gravity, Phys. Rev. Lett., 57, 2244-2247, 1986
- Abhay Ashtekar, New Hamiltonian formulation of general relativity, Phys. Rev. D36, 1587-1602, 1987
- Roger Penrose, Angular momentum: an approach to combinatorial space-time in Quantum Theory and Beyond, ed. Ted Bastin, Cambridge University Press, 1971
- Carlo Rovelli and Lee Smolin, Knot theory and quantum gravity, Phys. Rev. Lett., 61 (1988) 1155
- Carlo Rovelli and Lee Smolin, Loop space representation of quantum general relativity, Nuclear Physics B331 (1990) 80-152
- Carlo Rovelli and Lee Smolin, Discreteness of area and volume in quantum gravity, Nucl. Phys., B442 (1995) 593-622, e-print available as
External links
- by Lee Smolin
- Wired magazine, News:
- September 2006, The Economist, article
- Gamma-ray Large Area Space Telescope: http://glast.gsfc.nasa.gov/
- Article from by Z.K. Silagadze.
- - According to a model based on "loop quantum gravity" theory, a parent universe that existed before ours may have left an imprint (New Scientist, 10 April 2008)
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