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".
Late light reveals what space is made of
August 2009 New Scientist article. "Quantum foam" - grainy bumps in the fabric of space-time - might explain why light from a distant galaxy arrived four minutes later than expected, offering clues about the real nature of gravity.
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.
Splitting Time from Space - New Quantum Theory Topples Einstein's Spacetime
December 2009 Scientific American article about a speculative theory of quantum gravity that treats space and time separately. There is a sidebar that claims to report evidence based on computer simulations.
The Holographic Principle
April 2003 Scientific American Sidebar that gives a very brief overview of the holographic principle.
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

Is Space Digital?
Michael Moyer
Scientific American, February 2012, pp. 30-37
An experiment going up outside of Chicago will attempt to measure the intimate connections among information, matter and spacetime. If it works, it could rewrite the rules for 21st-century physics.
Astrophysics in the lab
Ralf Schützhold; Bill Unruh
Physics World, January 2011, pp. 30-34
Condensed-matter analogues of black holes and other cosmic phenomena may allow physicists to test ideas about quantum gravity and the early universe.
Using Causality to Solve the Puzzle of Quantum Spacetime
Jerzy Jurkiewicz, Renate Loll and Jan Ambjorn
Scientific American, June 2008
The Triangular Universe
Mark Alpert
Scientific American, January 2007
The Illusion of Gravity
Juan Maldacena
Scientific American, November 2005
Out of the Darkness
Georgi Dvali
Scientific American, February 2004
Information in the Holographic Universe
Jacob D. Bekenstein
Scientific American, July 2003
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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