Open Questions: Quantum Gravity

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See also: Quantum theory -- Black holes -- Quantum cosmology -- Quantum geometry

Introduction

Superstrings and M-theory

Loop quantum gravity

Twistor theory of gravity

The holographic principle

Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books

Introduction

Recommended references: Web sites

Site indexes

Open Directory Project: Quantum Gravity: Categorized and annotated links. A version of this list is at Google, with entries sorted in "page rank" order.
Galaxy: Quantum Gravity: Categorized site directory. Entries usually include descriptive annotations. More here.

Sites with general resources

The Quantum Gravity Seminar: Notes from John Baez's quantum gravity seminar, which actually covers a wide range of topics in mathematical physics.
Qgravity.org: quantum gravity, physics and philosophy: Site owned by Lee Smolin to provide resources in quantum gravity, such as information on his books and external links.
Institute for Gravitational Physics & Geometry: Penn State U. research group that works mainly on quantum gravity and gravitational waves. The site includes information on their research and external links.
Simulations of Lorentzian 2d Quantum Gravity: Visualizations of some simulations of a model of 2d Lorentzian quantum gravity, by Jan Ambjørn, K. N. Anagnostopoulos, and R. Loll.
Non Perturbative Quantum Gravity: Loops and Spin Foams: Description of a scientific conference held in May 2004. The Scientific Program gives a good idea of key topics.

Surveys, overviews, tutorials

Quantum gravity: Article from Wikipedia. See also Graviton, Loop quantum gravity, Holographic principle.
Quantum Gravity: Part of the Cambridge Relativity site. Contains material on quantum cosmology, M-theory, and the "holographic principle".
Welcome to quantum gravity: November 2003 article from Physics World, by Matthew Chalmers. "Quantum theory and general relativity will only be unified when theory meets experiment."
Quantum gravity presents the ultimate challenge to theorists: December 1999 article from Physics World, by Paul Davies.
Gravity-wave detectors target quantum gravity: March 1999 news article from Physics World. "Large-scale interferometers built to detect gravitational waves from astrophysical sources may also be able to test various theories of quantum gravity."
Gamma-ray bursts could test quantum gravity: June 1998 news article from Physics Web about the possibility of testing theories of quantum gravity by measuring photons from gamma-ray bursts.
Spacetime Quantum Mechanics: Slide presentation given by James Hartle at the 2001: A Spacetime Odyssey conference.
How far are we from the quantum theory of gravity?: A detailed technical review paper by Lee Smolin that assesses the current status of loop quantum gravity, superstring theory, and other approaches to quantum gravity. Available in PDF and other formats.
Spacetime Warps and the Quantum: A Glimpse of the Future: Slides and audio from a February 1999 lecture by Kip Thorne.
Quantum Gravity and Superstrings: Lecture notes by Steve Lloyd, from a course on Elementary Particle Physics.
Quantum Gravity at the Planck Length: This technical survey paper by Joseph Polchinski gives a string theorist's view of physics and quantum gravity at the Planck scale (as of 1998).

Recommended references: Magazine/journal articles

The Illusion of Gravity
Juan Maldacena
Scientific American, November 2005
Out of the Darkness
Georgi Dvali
Scientific American, February 2004
On Close Inspection
Hans Christian von Baeyer
The Sciences, January/February 2001, pp. 16-19: Any reconciliation of quantum theory and general relativity must allow for the quantization of spacetime itself, implying a smallest possible discrete scale for length and duration. There are several proposals for what spacetime might look like at the smallest dimension, and it may even be possible to observe evidence of this micro-structure at much larger scales.
Quantum Gravity
Paul Renteln
American Scientist, November-December 1991, pp. 508-527: Reconciling the theories of general relativity and quantum mechanics may be the most important and difficult problem in theoretical physics. One key difficulty is that quantum gravity would describe phenomena far outside the range of experimental investigation. There are a number of ideas for approaching the problem. A successful theory could even explain the dimensions of spacetime.
The Quantum Loop
Amitabha Sen; Sharon Butler
The Sciences, November-December 1989, pp. 32-36: A quantum theory of gravity has been difficult to develop, in part, because of the tight relationship between gravity and the geometry of spacetime. Abhay Ashtekar has developed a new way of formulating the eauations of general relativity to make them easire to handle mathematically. Out of this has emerged a picture of spacetime composed of "quantum loops", which may lead to a quantum theory of gravity.
Gravity and Antimatter
Terry Goldman; Richard J. Hughes; Michael Martin Nieto
Scientific American, March 1988, pp. 48-56: Our uncertainty over what a quantum theory of gravity should look like raises the possibility that gravity acts slightly differently on antimatter than it does on matter.
Quantum Gravity
Bryce S. DeWitt
Scientific American, December 1983, pp. 112-129: A quantum theory of gravity would combine three fundamental theories: special relativity, general relativity, and quantum mechanics. Such a theory is needed to understand a variety of phenomena, such as black holes, the big bang, the vacuum, and the structure and dimensionality of spacetime itself.

Recommended references: Books

Lee Smolin -- Three Roads to Quantum Gravity
Basic Books, 2001: The three roads referred to are string theory, loop quantum gravity, and a small collection of other diverse and very speculative ideas. Smolin masterfully presents, in a relatively brief scope, the essentials of the first two approaches through an explanation of our current understanding of quantum theory and gravitation which assumes little prior knowledge of the reader.
[Book home page] [Review]
Crag Callender; Nick Huggett, eds. -- Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity
Cambridge University Press, 2001: Unifying the two fundamental theories of contemporary physics -- general relativity and quantum mechanics -- is the most challenging problem in theoretical physics today. A large part of the problem is philosophical, involving as it does questions about the basic nature of space, time, and matter. The present volume is a collection of fourteen essays by such experts as Edward Witten and Roger Penrose which attempt to clarify and explore many of the issues involved.

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