Open Questions: Quantum Theory
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See also: Quantum information and computing 
Quantum cosmology 
Quantum gravity 
Quantum field theory 
Quantum effects technology
Anyone who is not shocked by quantum theory has not understood it.
Niels Bohr
I can safely say that nobody understands quantum mechanics.
... Do not keep saying to yourself, if you can possibly avoid it,
'But how can it be like that?' because you will get 'down the drain',
into a blind alley from which nobody has yet escaped. Nobody knows
how it can be like that.
Richard Feynman

Introduction
The Copenhagen interpretation
The "measurement problem"
Bohm's theory
Bell's theorem
Coherence and decoherence
Consistent histories
The "many worlds" interpretation
Quantum chaos

Introduction
Quantum mechanics works. Nobody doubts that. Whenever we are able to
derive predictions from the equations of quantum mechanics, they can
be verified to the limits of experimental accuracy.
We just don't understand why quantum mechanics works.
Site indexes

Open Directory Project: Quantum Mechanics
 Categorized and annotated links. A version of this
list is at
Google, with entries sorted in "page rank" order.

The Net Advance of Physics: Quantum Mechanics
 An index of tutorial and research articles
located at the
physics preprint archive.

Best of Physics Web: Atomic, Molecular and Quantum Physics
 Directory of best feature articles, news stories, and
external links on quantum physics at the
Physics Web site.

Galaxy: Quantum Mechanics
 Categorized site directory. Entries usually include
descriptive annotations.
Sites with general resources

Qwiki
 "Qwiki is a quantum physics wiki devoted to the collective
creation of content that is technical and useful to practicing
scientists. ... The site is nominally centered around quantum
physics, but all scientists are invited to contribute,
including  but not limited to  computer scientists,
control theorists, electrical engineers, and mathematicians.
More specifically, this site is designed for people who post
content to the Arxiv and
quantph."

New Scientist's Guide to the Quantum World
 Good collection of news and tutorial articles on quantum physics
and quantum computing.

Quantum Quotations
 Stimulating quotations, mostly by physicists, on quantum
mechanics. Collected by Jess Brewer.
Surveys, overviews, tutorials

Quantum mechanics
 Article from
Wikipedia.
See also
Interpretation of quantum mechanics,
Bell's theorem,
EPR paradox,
Quantum entanglement,
Mathematical formulation of quantum mechanics,
Quantum chaos.

On the Interpretation and Philosophical Foundation of Quantum
Mechanics
 Very good essay by
Anton Zeilinger on various interpretations of quantum mechanics.

The Copenhagen Interpretation of Quantum Mechanics
 Good expository article by
Ben Best. Contains a
number of useful embedded external links.

Quantum theory poses reality's deepest mystery
 Brief May 2008 comments by John Wheeler on quantum theory.

Quantum mysteries
 A description of experiments that seem to demonstrate faster
than light communication and other quantum strangeness, by
John Gribbin.

Does God play dice?
 December 2005 article from
Physics World.
"Four theorists  Gerard 't Hooft, Edward Witten, Fay Dowker
and Paul Davies outline their views on the current status of
quantum theory and the way forward."

Probing the limits of the quantum world
 March 2005 article from
Physics World, by
Markus Arndt, Klaus Hornberger, and Anton Zeilinger.
"Molecules with over 100 atoms can be made to interfere, according
to recent experiments that study the transition from the quantum
to the classical world."

New life for Schrödinger's cat
 August 2000 article from
Physics World, by
Tony Leggett. "The observation of quantum superpositions of distinct
macroscopic states by groups at Stony Brook and Delft represents a
milestone in experimental quantum physics. Both teams have reported
spectroscopic evidence for currents of microamps flowing through a
superconducting ring in opposite directions at the same time."

Schrodinger's cat comes into view
 July 2000 new article from
Physics World about
the first demonstration of a macroscopic Schrodinger cat state.

Measuring decoherence in real time
 January 2000 news article from
Physics World about
measurement of quantum decoherence in a mesoscopic system for the
first time.

Quantum theory: weird and wonderful
 December 1999 article from
Physics World, by
Tony Leggett. "Quantum mechanics is the most accurate theory we have to
describe the world, but there is still much about it that we do not
fully understand."

Waveparticle duality seen in carbon60 molecules
 October 1999 news article from
Physics World about
production of an interference pattern in a doubleslit experiment
involving carbon60 molecules.

First for singlephoton measurements
 July 1999 news article from
Physics Web
about experimental techniques that allow repeated observation of
a single photon without destroying it.

John Bell and the most profound discovery of science
 December 1998 article from
Physics World, by
Andrew Whitaker. Some of the theory behind Bell's inequality
is explained, along with biographical details on John Bell.

Intro to Quantum Mechanics
 Singlepage overview of quantum mechanics, by Todd Stedl.

Some Basic Ideas about Quantum Mechanics
 Single page that introduces some of the ideas of quantum
mechanics, by Stephen Jenkins.

Frequently Asked Questions: Quantum Mechanics
 Technical answers to frequently asked questions, based on a
list originally developed by Jim Carolan.

Physics Beyond the Limits of Uncertainty Relations
 Excellent tutorial pages on "subquantum physics" by
Michio Durdevich. There are additional, somewhat more technical
papers on mathematical quantum theory at the author's
home page.

Quantum Dèjá Vu
 Brief October 1999 article from Scientific American,
subtitled "In an exquisite 'quantum nondemolition' experiment,
physicists see a single photon and then see it again."

Schroedinger's Cation
 June 1996 Scientific American Explore article, subtitled
"Physicists prove that an atom can be in two different places at once."

The Transactional Interpretation of Quantum Mechanics
 Technical exposition, originally published in 1986,
of a new alternative interpretation of
quantum mechanics, by John G. Cramer.
 Fungibility and the quantum multiverse
David Deutsch
Physics World, May 2011, pp. 3438
 Physicists have long debated whether the most fundamental
phenomena of quantum physics is quantization, interference, or
entanglement. But it may be something else entirely: fungibility.

A Quantum Threat to Special Relativity
David Z Albert; Rivka Galchen
Scientific American, February 2009

 Chaos on the Quantum Scale
Mason A. Porter; Richard L. Liboff
American Scientist, NovemberDecember 2001, pp. 532537
 During the past century both quantum theory and chaos theory
have emphasized the unpredictability of nature in many phenomena.
Difficult as it may seem, techniques have been developed for
modeling chaos on the quantum scale. Such models will be crucial
for understanding and developing quantum scale technology.
[Abstract and references]
 100 Years of Quantum Mysteries
Max Tegmark; John Archibald Wheeler
Scientific American, February 2001, pp. 6875
 In December 1900 Max Planck announced a formula describing
blackbody radiation. The 100 years since then have seen the
rapid development of quantum mechanics, but a number of "mysteries"
and conceptual issues remain.
 Consistent Histories and Quantum Measurements
Robert B. Griffiths, Roland Omnès
Physics Today, August 1999, pp. 2631
 The understanding of quantum mechanics in terms of
measurements (the socalled Copenhangen interpretation) has
proven problematical for various reasons  for instance the
notion that properties don't exist until measured and the
paradox of Schrödinger's cat. New approaches such as
"consistent histories" look promising.
 Entanglement, Decoherence and the Quantum/Classical Boundary
Serge Haroche
Physics Today, July 1998, pp. 3642
 New experiments attempt to resolve the apparent paradox of
"Schrödinger's cat" without harm to the feline.
 Quantum Theory without Observers
Sheldon Goldstein
Physics Today, March 1998, pp. 4246 and April 1998, pp. 3842
 Alternatives to Bohr's Copenhagen interpretation of quantum
mechanics may ultimately provide a simpler, less paradoxical
theory. The alternatives include "decoherent histories",
"spontaneous localization", Bohm's deterministic theory, and
Bell's strict nonlocality results.
 Bringing Schrödinger's Cat to Life
Philip Yam
Scientific American, June 1997, pp. 124129
 Recent experiments illuminate quantum theory at the
border between the microscopic and macroscopic worlds.
 Does Nature Violate Local Realism?
David Branning
American Scientist, MarchApril 1997, pp. 160167
 The standard interpretation of quantum mechanics is incompatible
with the prequantum notion of "local realism". Experimental results
continue to indicate that local realism is violated, but
loopholes remain.
 Review of Roland Omnès, The Interpretation of Quantum
Mechanics
William Faris
Notices of the AMS, November 1996, pp. 13281339
 The book under review is an attempt to survey different approaches
to the interpretation of quantum mechanics. Omnès' own
position is similar to the traditional Copenhagen interpretation,
augmented by newer ideas such as decoherence and consistent
histories. It is not clear, at least to the reviewer, that a
satisfactory solution of the interpretation problem has yet been
achieved.
[Article in PDF format]
 The Duality in Matter and Light
BertholdGeorg Englert; Marlan O. Scully; Herbert Walther
Scientific American, December 1994, pp. 8682
 According to the complementarity principle of quantum mechanics,
objects in the microworld can exhibit properties of both waves and
particles. New experiments indicate this principle is even more
fundamental than previously supposed.
 Bohm's Alternative to Quantum Mechanics
David Z. Albert
Scientific American, May 1994, pp. 5867
 The quantum theory developed by David Bohm in the 1950s is an
alternative to the Copenhagen interpretation. It is a fully worked out
theory that is deterministic and denies the existence of
superpositions, but it does possess the curious property of
nonlocality.
 Decoherence and the Transition from Quantum to Classical
Wojciech H. Zurek
Physics Today, October 1991, pp. 3644
 The concept of "decoherence" may help clarify our interpretation
of quantum theory. It is applicable not only in defining the branches
in the "many worlds" interpretation of quantum mechanics, but also
in delineating the border between quantum and classical domains in
Bohr's interpretation.
 The Reality of the Quantum World
Abner Shimony
Scientific American, January 1988, pp. 4653
 A series of laboratory experiments have confirmed that the
quantum world really is as strange as the theory suggests. For
example, there are nonlocal effects in which widely separated
entities can be "entangled" so that their behavior is correlated
even though communication between the entities is impossible.
 Tony Hey, Patrick Walters  The New Quantum Universe
Cambridge University Press, 2003
 This is very much a book for the general reader  full of
pictures, anecdotes, and pop culture. But it does cover most
of the basics of quantum physics  waves and particles, the
double slit experiment, the uncertainty principle,
Schrödinger's equation, the Pauli exclusion principle, the
EPR paradox, and all that good stuff. There is also coverage of
aspects of quantum technology, such as BoseEinstein condensates,
superconductivity, quantum information, and quantum computers.
It also ventures into related areas of highenergy physics 
astrophysics, particle physics, and the standard model.
 Amir D. Aczel  Entanglement: The Unlikely Story of How
Scientists, Mathematicians, and Philosophers Proved Einstein's
Spookiest Theory
Four Walls Eight Windows, 2001
 The subject is the socalled paradox of Einstein, Podolsky,
and Rosen. The irony is that rather than discovering a logical
inconsistency in quantum mechanics, Einstein and the others
motivated the confirmation of aspects of quantum mechanics
they regarded as absurd: entanglement and nonlocality. Aczel
has written a pretty good account of the subject for a general
audience, touching on Bell's theorem and the experiments of
Alain Aspect and others which verified the genuinely
"spooky" implications of quantum mechanics.
 Daniel F. Styer  The Strange World of Quantum Mechanics
Cambridge University Press, 2000
 In a nontechnical, mostly nonmathematical, yet generally
precise manner this short book presents an introduction to quantum
mechanics. Topics covereed include the SternGerlach experiment,
measurement, the EinsteinPodolskyRosen paradox, and quantum
cryptography.
[Book home page]
 Sam Treiman  The Odd Quantum
Princeton University Press, 1999
 If you're ready to try to understand what quantum mechanics
is really about and aren't afraid of a little mathematics, this
book is a mustread. As a mathematical theory, quantum mechanics
is not at all mysterious, and this book will introduce you to
that aspect. If what you want is the philosophy, it's not here 
there are numerous alternatives for that.
 Roland Omnès  Understanding Quantum Mechanics
Princeton University Press, 1999
 This is one of the best books to appear which explains the
philosophical foundations of quantum mechanics without demanding
a detailed understanding of the mathematical theory. Omnès
himself is responsible for some of the recent philosophical
clarifications. Here, he starts from the starts from the
standard Copenhagen interpretation, addresses the key "problem of
measurement", and then goes into the consistenthistories
interpretation. Quantum mechanics is still not an easy subject to
understand, but this book is a big help.
 Jeffrey Bub  Interpreting the Quantum World
Cambridge University Press, 1997
 The author's work, which requires some mathematical
sophistication, is a modern study of the interpretation of
quantum mechanics with a view to resolving the "measurement
problem". Its concluding chapter on "the new orthodoxy"
shows how ideas like, "decoherence", the "many worlds" interpretation,
and "consistent histories" fit together in our current understanding
of quantum mechanics.
 David Deutsch  The Fabric of Reality: The Science of Parallel
Universes  and its Implications
Penguin Books, 1997
 One interpretation of quantum mechanics is called the
"many worlds" theory. This interpretation helps illuminate a
variety of notions such as virtual reality, quantum computation,
and the possibility of time travel.
 David Lindley  Where Does the Weirdness Go?
Basic Books, 1996
 Nonmathematical survey of the phemomena on which quantum
theory is based, and the "measurement problem" in particular.
Attempts to demystify some of the seeming paradoxiality of the
theory.
 Victor J. Stenger  The Unconscious Quantum: Metaphysics
in Modern Physics and Cosmology
Prometheus Books, 1995
 Stenger performs a welcome and most useful service by
explaining quantum theory and at the same time debunking
popular attemps to turn the science into a kind of mysticism.
Important topics such as the EPR experiment, Bell's theorem,
hiddeen variables, and nonlocality are thoroughly covered.
 David Wick  The Infamous Boundary: Seven Decades of Heresy
in Quantum Physics
Birkhäuser Boston, 1995
 A nonmathematical history of the experiments and interpretations
of quantum mechanics. The stress is on the controversies themselves
rather than on a definitive resolution of the issues.
 Euan Squires  The Mystery of the Quantum World
Institute of Physics Publishing, 1994
 Squires offers a fairly concise but sophisticated introduction
to the various interpreations of quantum mechanics and the
philosophy of the theory. Topics include measurement, "external
reality", the possible relevance of consciousness, the "many worlds"
interpretation, hidden variables, nonlocality, the EPR thought
experiment, "pilot waves" and Bohm's interpretation, and
Bell's theorem. Attempts to interject theological considerations into
the discussion constitute a minor blemish.
 David Z. Albert  Quantum Mechanics and Experience
Harvard University Press, 1992
 An introduction to the foundations of quantum mechanics with
relatively mild mathematical prerequisites. Includes an extensive
account of Bohm's quantum theory, as developed by John Bell.
 Jim Baggott  The Meaning of Quantum Theory
Oxford University Press, 1992
 Survey of the fundamentals of quantum mechanics, different
interpretations of the theory, the EPR experiment, Bell's theorem.
 F. David Peat  Einstein's Moon: Bell's Theorem and the Curious
Quest for Quantum Reality
Contemporary Books, 1990
 Introduction to interpretations of quantum mechanics. Focuses
on the EinsteinPodolskyRosen "paradox" and how Bell's theorem
has helped resolve it.
 James T. Cushing, Ernan McMullin (eds.)  Philosophical
Consequences of Quantum Theory: Reflections on Bell's Theorem
University of Notre Dame Press, 1989
 Collection of theoretical and philosophical essays on the
meaning and interpretation of Bell's theorem.
 J. S. Bell  Speakable and Unspeakable in Quantum Mechanics
Cambridge University Press, 1987
 A collection of papers on quantum philosophy, by the discoverer
of Bell's Theorem. Covers many topics and uses real mathematics.
 P. C. W. Davies, J. R. Brown (eds.)  The Ghost in the Atom
Cambridge University Press, 1986
 An overview of the philosophy of quantum mechanics, with
interviews of leading experts in quantum physics.
 Alastair Rae  Quantum Physics: Illusion or Reality?
Cambridge University Press, 1986
 Introduction to the conceptual issues of quantum mechanics.
Covers such topics as the possibility of hidden variables, the
Copenhagen interpretation, and the many worlds interpretation.
 Nick Herbert  Quantum Reality: Beyond the New Physics
Anchor Books, 1985
 A breezy but thorough popularization of quantum theory,
reality, and nonlocality. Despite
its age, it remains in print on account of the quality of the
treatment of most of the key topics up to and including Bell's
theorem.
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