Open Questions: Quantum Gravity
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See also: Quantum theory 
Black holes 
Quantum cosmology 
Quantum geometry
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, Mtheory, 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 spacetime 
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.

Gravitywave detectors target quantum gravity
 March 1999 news article from
Physics World.
"Largescale interferometers built to detect gravitational waves
from astrophysical sources may also be able to test various
theories of quantum gravity."

Gammaray 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 gammaray 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).
 Is Space Digital?
Michael Moyer
Scientific American, February 2012, pp. 3037
 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
21stcentury physics.
 Astrophysics in the lab
Ralf Schützhold; Bill Unruh
Physics World, January 2011, pp. 3034
 Condensedmatter 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. 1619
 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 microstructure at much larger scales.
 Quantum Gravity
Paul Renteln
American Scientist, NovemberDecember 1991, pp. 508527
 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, NovemberDecember 1989, pp. 3236
 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. 4856
 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. 112129
 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.
 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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Copyright © 2002 by Charles Daney, All Rights Reserved