An invitation to loop quantum gravity

By Lee Smolin. 2004

2,418 citations recorded by the source corpus.

We describe the basic assumptions and key results of loop quantum gravity, which is a background independent approach to quantum gravity. The emphasis is on the basic physical principles and how one deduces predictions from them, at a level suitable for physicsts in other areas such as string theory, cosmology, particle physics, astrophysics and condensed matter physics. No details are given, but references are provided to guide the interested reader to the literature. The present state of knowledge is summarized in a list of 35 key results on topics including the hamiltonian and path integral quantizations, coupling to matter, extensions to supergravity and higher dimensional theories, as well as applications to black holes, cosmology and Plank scale phenomenology. We describe the near term prospects for observational tests of quantum theories of gravity and the expectations that loop quantum gravity may provide predictions for their outcomes. Finally, we provide answers to frequently asked questions and a list of key open problems.

View Paper: https://www.semanticscholar.org/paper/4703f7ef6658c5aef234bdd5b19fd34f21f522f0

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Citations in the corpus

STE-QUEST—test of the universality of free fall using cold atom interferometry (2013, background): “) predict a violation of Einstein’s Equivalence Principle at some scale [2, 4].”
The asymptotic safety scenario in quantum gravity: an introduction (2006, unknown): “For book-sized expositions see for example [68, 125, 132].”
An Asymptotically Safe Guide to Quantum Gravity and Matter (2018, background): “Right panel: Standard-Model RG flow including running gravitational couplings as in Donà et al. (2014) and is taken from Eichhorn and Held (2018). observe an effective universality of distinct avatars of the Newton coupling, which agree within an estimate of the systematic truncation error.”
The case for background independence (2005, background): “uantum theory[47]. The approaches of Rovelli, however, left open the precise structure that is to tie together the 27For references for this section, see the corresponding discussion and referencesin [7]. 37 network of Hilbert spaces and algebras necessary to describe a whole universe. A templatefor”
Spectral dimension of the universe (2005, background): “An example is that of so-called loop quantum gravity, where the discrete spectra of geometric operators measuring areas and volumes on a kinematical Hilbert space are often taken as evidence for fundamental discreteness in nature [1, 2].”
Quantum nature of the big bang: An analytical and numerical investigation (2006, background): “Loop quantum gravity (LQG) is a background independent, non-perturbative approach to quantum gravity [1, 2, 3].”
Random planar lattices and integrated superBrownian excursion (2002, background): “All these random planar lattices have been considered both in combinatorics (see [6] and references therein) and in mathematical physics (see [3] and references therein; in the physics literature, definitions are usually phrased using “symmetry weights” instead of rooted objects, but this is strictl”
Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference (2006, background): “Despite some 50 years of e ort, in luding thete hni al progress made in re ent years using `loop' variables to solvethe equations (Rovelli 2004), the `problem of time' in anoni al quantumgravity remains unresolved.a8.”
Topological M-theory as unification of form theories of gravity (2004, background): “Here we review a theory known as 2-form gravity [26,27,28,29,30,31,32,33], which also describes the self-dual sector of loop quantum gravity [34].”
Quantum superposition of massive objects and the quantization of gravity (2018, background): “The main aim of these works is to rule out semi-classical gravity as an exact theory [22, 23], which would treat the gravitational field as classical even when the source is in a macroscopic superposition at different locations—in contrast with the”