History of loop quantum gravity
General relativity is the theory of gravitation published by Albert Einstein in 1915. According to it, the force of gravity is a manifestation of the local geometry of spacetime. Mathematically, the theory is modelled after Bernhard Riemann's metric geometry, but the Lorentz group of spacetime symmetries (an essential ingredient of Einstein's own theory of special relativity) replaces the group of rotational symmetries of space. Loop quantum gravity inherits this geometric interpretation of gravity, and posits that a quantum theory of gravity is fundamentally a quantum theory of spacetime.
In the 1920s, the French mathematician Élie Cartan formulated Einstein's theory in the language of bundles and connections, a generalization of Riemann's geometry to which Cartan made important contributions. The so-called Einstein–Cartan theory of gravity not only reformulated but also generalized general relativity, and allowed spacetimes with torsion as well as curvature. In Cartan's geometry of bundles, the concept of parallel transport is more fundamental than that of distance, the centerpiece of Riemannian geometry. A similar conceptual shift occurs between the invariant interval of Einstein's general relativity and the parallel transport of Einstein–Cartan theory.
In the 1960s, physicist Roger Penrose explored the idea of space arising from a quantum combinatorial structure. His investigations resulted in the development of spin networks. Because this was a quantum theory of the rotational group and not the Lorentz group, Penrose went on to develop twistors.
In 1982, Amitabha Sen tried to formulate a Hamiltonian formulation of general relativity based on spinorial variables, where these variables are the left and right spinorial component equivalents of Einstein–Cartan connection of general relativity. Particularly, Sen discovered a new way to write down the two constraints of ADM Hamiltonian formulation of general relativity in terms of these spinorial connections. In his form, the constraints are simply conditions that the Spinorial Weyl curvature is trace free and symmetric. He also discovered the presence of new constraints which he suggested to be interpreted as the equivalent of Gauss constraint of Yang Mills field theories. But Sen's work fell short of giving a full clear systematic theory and particularly failed to clearly discuss the conjugate momenta to the spinorial variables, its physical interpretation, and its relation to the metric (in his work he indicated this as some lambda variable).
In 1986, physicist Abhay Ashtekar completed the project which Amitabha Sen began. He clearly identified the fundamental conjugate variables of spinorial gravity: The configuration variable is as a spinoral connection - a rule for parallel transport (technically, a connection) and the conjugate momentum variable is a coordinate frame (called a vierbein) at each point. So these variable became what we know as Ashtekar variables, a particular flavor of Einstein–Cartan theory with a complex connection.General relativity theory expressed in this way, made possible to pursue quantization of it using well-known techniques from quantum gauge field theory.
The quantization of gravity in the Ashtekar formulation was based on Wilson loops, a technique developed in the 1970s to study the strong-interaction regime of quantum chromodynamics (QCD). It is interesting in this connection that Wilson loops were known to be ill-behaved in the case of standard quantum field theory on (flat) Minkowski space, and so did not provide a nonperturbative quantization of QCD. However, because the Ashtekar formulation was background-independent, it was possible to use Wilson loops as the basis for nonperturbative quantization of gravity.
Sen's initiation and completion by Ashtekar, finally, for the first time, in a setting where the Wheeler–DeWitt equation could be written in terms of a well-defined Hamiltonian operator on a well-defined Hilbert space, and led to construction of the first known exact solution, the so-called Chern–Simons form or Kodama state. The physical interpretation of this state remains obscure.
Around 1990, Carlo Rovelli and Lee Smolin obtained an explicit basis of states of quantum geometry, which turned out to be labeled by Penrose's spin networks. 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. Being closely related to topological quantum field theory and group representation theory, LQG is mostly established at the level of rigor of mathematical physics, as compared to string theory, which is established at the level of rigor of physics.
After the spin network basis was described, progress was made on the analysis of the spectra of various operators resulting in a predicted spectrum for area and volume (see below). Work on the semi-classical limit, the continuum limit, and dynamics was intense after this, but progress slower.
LQG was initially formulated as a quantization of the Hamiltonian ADM formalism, according to which the Einstein equations are a collection of constraints (Gauss, Diffeomorphism and Hamiltonian). The kinematics are encoded in the Gauss and Diffeomorphism constraints, whose solution is the space spanned by the spin network basis. The problem is to define the Hamiltonian constraint as a self-adjoint operator on the kinematical state space. The most promising work in this direction is Thomas Thiemann's Phoenix program.
In the recent years most of the developments in LQG has been done in the covariant formulation of the theory, called Spinfoam Theory. The present version of the covariant dynamics is due to the convergent work of different groups, but it is commonly named after a paper by Jonhatan Engle, Roberto Pereira and Carlo Rovelli in 2008. Spin foams are a framework intended to tackle the problem of dynamics and the continuum limit simultaneously. Heuristically, it would be expected that evolution between spin network states might be described by discrete combinatorial operations on the spin networks, which would then trace a two-dimensional skeleton of spacetime. This approach is related to state-sum models of statistical mechanics and topological quantum field theory such as the Turaeev–Viro model of 3D quantum gravity, and also to the Regge calculus approach to calculate the Feynman path integral of general relativity by discretizing spacetime.
- Topical Reviews
- Carlo Rovelli, Loop Quantum Gravity, Living Reviews in Relativity 1, (1998), 1, online article, 2001 15 August version.
- Thomas Thiemann, Lectures on loop quantum gravity, e-print available as gr-qc/0210094
- Abhay Ashtekar and Jerzy Lewandowski, Background independent quantum gravity: a status report, e-print available as gr-qc/0404018
- Carlo Rovelli and Marcus Gaul, Loop Quantum Gravity and the Meaning of Diffeomorphism Invariance, e-print available as gr-qc/9910079.
- Lee Smolin, The case for background independence, e-print available as hep-th/0507235.
- Popular books:
- Julian Barbour, The End of Time: The Next Revolution in Our Understanding of the Universe
- 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.
- Magazine articles:
- Lee Smolin, "Atoms in Space and Time", Scientific American, January 2004
- Easier introductory, expository or critical works:
- More advanced introductory/expository works:
- Carlo Rovelli, Quantum Gravity, Cambridge University Press (2004); draft available online
- Thomas Thiemann, Introduction to modern canonical quantum general relativity, e-print available as gr-qc/0110034
- 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 hep-th/0501114
- "Loop and Spin Foam Quantum Gravity: A Brief Guide for beginners arXiv:hep-th/0601129 H. Nicolai and K. Peeters.
- Edward Witten, Quantum Background Independence In String Theory, e-print available as hep-th/9306122.
- Conference proceedings:
- John C. Baez (ed.), Knots and Quantum Gravity
- Fundamental research papers:
- Amitabha Sen, Gravity as a spin system, Phys. Lett. B119:89–91, December 1982.
- 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