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FOCUS REPORT: QUANTUM MECHANICS
A Summary Group from SCIENCE-WEEK
-------------------------------------------------
ON THE SOKAL HOAX AND PHILOSOPHICAL EXTRAPOLATIONS IN PHYSICS
In the last quarter of this century, many fields outside of
physical science are apparently in the throes of epistemological
crises that are seen as originating in similar crises in physics
during the first quarter of the century. *Complementarity,
uncertainty, relativity, observer interactions -- the perceived
philosophical implications of these ideas have been imported into
the humanities and social sciences where they have rocked
foundations and produced what many critics view as an
intellectual babble. In 1996, theoretical physicist Alan Sokal
concocted an article consisting mostly of the ideations of so-
called "*postmodern" cultural studies of science, the article
concerned with "a transformative hermeneutics of quantum gravity"
and purporting to be an application of theoretical physics to
affirm the thrust of postmodern cultural studies of science in
the humanities and social sciences. The article was accepted and
*published by the journal *Social Text*, and shortly afterward,
in the journal *Lingua Franca*, Sokal revealed that his article
was a complete hoax and designed as a parody of contemporary
postmodern thought. In the academ
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publication SCIENCE-WEEK. If you have not had a previous free
trial subscription to SCIENCE-WEEK, you can subscribe for a
3-issue SW trial without any obligation on your part afterward.
To obtain a trial subscription to SCIENCE-WEEK, simply transmit
FREE TRIAL to <[EMAIL PROTECTED]>. Full SCIENCE-WEEK
subscription details are appended to this file.
-------------------------------------------------------
FOCUS REPORT: QUANTUM MECHANICS
A Summary Group from SCIENCE-WEEK
-------------------------------------------------
ON THE SOKAL HOAX AND PHILOSOPHICAL EXTRAPOLATIONS IN PHYSICS
In the last quarter of this century, many fields outside of
physical science are apparently in the throes of epistemological
crises that are seen as originating in similar crises in physics
during the first quarter of the century. *Complementarity,
uncertainty, relativity, observer interactions -- the perceived
philosophical implications of these ideas have been imported into
the humanities and social sciences where they have rocked
foundations and produced what many critics view as an
intellectual babble. In 1996, theoretical physicist Alan Sokal
concocted an article consisting mostly of the ideations of so-
called "*postmodern" cultural studies of science, the article
concerned with "a transformative hermeneutics of quantum gravity"
and purporting to be an application of theoretical physics to
affirm the thrust of postmodern cultural studies of science in
the humanities and social sciences. The article was accepted and
*published by the journal *Social Text*, and shortly afterward,
in the journal *Lingua Franca*, Sokal revealed that his article
was a complete hoax and designed as a parody of contemporary
postmodern thought. In the academic furor that followed, Sokal's
article was characterized as "an ingenious exposure of the
decline of intellectual standards in contemporary academia," and
"a brilliant parody of the postmodern nonsense rampant among the
cultural studies of science." ... ... Writing in a physics
journal, M. Beller now outlines an argument that theoretical
physicists both past and present have had much responsibility for
what appear to be the nonsensical applications of theoretical
physics to the humanities and social sciences. The author makes
the following points: 1) The philosophical pronouncements
(several of which are quoted at length by Beller) of theoretical
physicists *Niels Bohr, *Max Born, *Werner Heisenberg, *Wolfgang
Pauli, and *Pascual Jordan deserve some of the blame for the
excesses of the postmodern critique of science. 2) Like the
deconstructionist *Jacques Derrida, Bohr was notorious for the
obscurity of his writing. Yet physicists relate to the
obscurities of Derrida and Bohr in fundamentally different ways:
Derrida is treated with contempt and Bohr is treated with awe,
his obscurity attributed to "depth and subtlety". 3) The author
points out that in a widely used compendium of papers in
theoretical physics published in 1983, there is an often cited
reprinted paper by Bohr whose pages are out of order, and yet no
complaints are heard and the mistake, which occurs in both
hardcover and softcover editions, is apparently rarely noticed.
3) The author points out that Bohr intended his philosophy of
complementarity to be an overarching epistemological principle
applicable to physics, biology, psychology, and anthropology.
Pauli argued for application of the quantum concept of reality to
unify science, religion, Jungian archetypes, and extrasensory
perception. Born stated that quantum philosophy would help
humanity cope with the postwar era. Heisenberg expressed the hope
that the results of quantum physics would transform cultural life
by producing a renaissance of ideas. Jordan explored the "formal"
parallels between quantum physics and Freudian psychoanalysis. 4)
Beller points out that the philosophical pronouncements of Bohr
and other founders of quantum physics are not just an
anachronistic curiosity, since contemporary popular writings by
physicists and science writers continue to proclaim the victory
of Bohr's conception of reality, even though the Copenhagen
"orthodox" interpretation of quantum physics -- the abandonment
of causality and the ordinary conception of reality -- is not the
only possible interpretation of quantum physics, and ultimately
it might not even be the surviving one. 5) Beller concludes: "The
opponents of the postmodernist cultural studies of science
conclude confidently from the Sokal affair that 'the emperors
have no clothes.' But who, exactly, are all these naked emperors?
At whom should we be laughing?"
-----------
M. Beller (Hebrew University Jerusalem, IL): The Sokal hoax: at
whom are we laughing?
(Physics Today September 1998)
QY: Mara Beller, Hebrew University, Jerusalem IL.
-----------
Text Notes:
... ... *Complementarity: The idea that a fundamental particle is
neither a wave nor a particle, because these are complementary
modes of description (see below, Report #6).
... ... *postmodern: The term here refers to studies of how
contemporary concepts and methods are determined by historical or
ideological context. So, for example, one set of postmodern
questions concerning science involves the influences of Western
socio-political ideology on the structure and methods of Western
science. The general idea is the consideration of science as a
product of the culture from which it arises. But the term
"postmodern" has a loose usage, with one meaning in literature,
another in art, and a third in the social sciences.
... ... *published: Sokal's paper was published in *Social Text*
(Spring/Summer 1996, p.216), and then exposed immediately by
himself in *Lingua Franca* (May/June 1996, p.62).
... ... *Niels Bohr (1885-1962): Nobel Prize in Physics 1922. He
worked in the fields of atomic structure and nuclear fission, and
he proposed the doctrine of complementarity. As director of the
Institute of Theoretical Physics in Copenhagen from 1920 on, Bohr
was the head of what came to be called the Copenhagen School of
Quantum Mechanics, which produced what came to be called the
"Copenhagen orthodoxy" view of the implications of quantum
mechanics as applied in general to theoretical physics.
... ... *Max Born (1882-1970): Nobel Prize in Physics 1954. Did
fundamental work in quantum theory, particularly work linking the
wave function of the electron to electron distribution
probability. It was Born who apparently coined the term "quantum
mechanics". Born worked with Werner Heisenberg, one of his
students, in the development of the mathematical techniques of
matrix mechanics, an alternative to the Schroedinger wave
equation for calculation of the position and momentum of the
electron in the atom. From Born: "I am now convinced that
theoretical physics is actual philosophy."
... ... *Werner Heisenberg (1901-1976): Nobel Prize in Physics
1932. Developed quantum theory and formulated the uncertainty
principle, which concerns matter, radiation, and their reaction,
and which places absolute limits on the achievable accuracy of
measurement of physical phenomena in the quantum domain.
... ... *Wolfgang Pauli (1900-1958): Nobel Prize in Physics 1945.
Originated the exclusion principle, which states that in a given
system no two fermions (electrons, protons, neutrons, or other
elementary particles of half-integral spin) can be characterized
by the same set of quantum numbers. He also predicted the
existence of neutrinos.
... ... *Pascual Jordan (1902- ): Worked with Born and Heisenberg
in the development of matrix mechanics. Also worked in the
relativistic quantum field theory of electromagnetism (quantum
electrodynamics).
... ... *Jacques Derrida (1930- ): A philosopher whose work spans
literary criticism, psychoanalysis, linguistics, and philosophy,
with an emphasis on the primacy of written text, the
referentiality of language, and the objectivity of conceptual
structures. Founded the school of criticism known as
"deconstruction".
-------------------
Summary & Notes by SCIENCE-WEEK <http://scienceweek.com> 25Sep98
A TUNABLE KONDO EFFECT IN QUANTUM DOTS
Quantum dots are small electrically conducting regions, typically
less than 1 micron in diameter, that contain from one to a few
thousand electrons. Because of the small volume, the electron
energies within the dot are quantized, and the behavior of the
quantum dot is intermediate between that of an atom and that of a
classical macroscopic object. Such intermediate systems are
called "mesoscopic" systems, and in the past several years great
attention has been devoted to the physics of such systems, since
they apparently can provide insights into quantum systems in
general. The electronic states in quantum dots can be probed by
transport when a small *tunnel coupling is allowed between the
dot and nearby source and drain leads. ... ... Cronenwett et al
(3 authors at 2 installations, NL US) report the realization of a
tunable *Kondo effect in small quantum dots, with the capability
of switching a dot from a Kondo system to non-Kondo system as the
number of electrons on the dot is changed from odd to even. The
*Kondo temperature can be tuned by means of a gate voltage as a
single-particle energy state nears the *Fermi energy.
Measurements of the temperature and magnetic field dependence of
a *Coulomb-blockaded dot show good agreement with prediction of
both equilibrium and nonequilibrium Kondo effects.
QY: Sara M. Cronenwatt, Stanford University 415-723-0830.
(Science 24 Jul 98 281:540) (Science-Week 14 Aug 98)
-------------------
Related Background:
... ... *tunnel coupling: This refers to tunneling, a quantum
mechanical phenomenon involving an effective penetration of an
energy barrier resulting from the width of the barrier being less
than the wavelength of the particle.
... ... *Kondo effect: The Kondo effect is a large anomalous
increase in the resistance of certain dilute alloys of magnetic
materials in nonmagnetic hosts as the temperature is lowered. In
general, the Kondo effect occurs when an impurity atom with an
unpaired electron is placed in a metal, producing an interaction
of localized electrons with delocalized electrons.
... ... *Kondo temperature: The temperature at which the Kondo
effect predominates.
... ... *Fermi energy: The average energy of electrons in a
metal.
... ... *Coulomb-blockaded: This refers to an effective blockade
of quantum mechanical tunneling produced by specific energy
barrier constraints.
GAMMA RAY BURSTS: TESTS OF QUANTUM GRAVITY
Quantum field theory is the mathematical fusion of quantum
mechanics with special relativity theory, and the term "quantum
gravity" refers to the fusion of quantum mechanics with general
relativity theory. The essential basis for these fusions is the
so-called "equivalence principle", which identifies the mass
involved in the gravitational force equation with the inertial
mass in the equation that relates any force to the product of
inertial mass and acceleration. (In general relativity, the
equivalence principle states that the observable local effects of
a gravitational field are indistinguishable from those arising
from acceleration of the frame of reference.) There are various
quantum field theories consistent with both quantum mechanics and
special relativity, all postulating that the gravitational force
between two quantum domain particles is generated by the exchange
of an intermediate particle (e.g., a graviton). But a quantum
gravity theory consistent with general relativity has not yet
been achieved, and there are physicists and mathematicians who
say the general form of such a satisfactory theory of quantum
gravity is not yet even clear -- that there is not yet even any
idea of what such a theory should look like.
... ... Amelino-Camelia et al (5 authors at 4 installations, UK
CH GR) suggest that the recent confirmation that at least some
*gamma ray bursts originate at cosmological distances indicates
that radiation from these bursts could be used to probe some of
the fundamental laws of physics, and that in particular, gamma
ray bursts will be sensitive to an energy dispersion predicted by
some approaches to quantum gravity. The essential idea is that
many of the bursts have structure on relatively rapid timescales,
which means that in principle it is possible to look for energy-
dependent dispersion of the radiation, as manifested in the
arrival times of the photons, if several different energy bands
are observed simultaneously. The authors suggest that a simple
estimate indicates that, because of their high energies and
distant origin, observations of these bursts should be sensitive
to a dispersion scale comparable to the Planck energy scale
(approximately 10^(19) *Gev), which is sufficient to test
theories of quantum gravity, and that such observations are
already possible using existing gamma ray detectors.
QY: G. Amelino-Camelia <[EMAIL PROTECTED]>
(Nature 25 Jun 98 393:763) (Science-Week 24 Jul 98)
-------------------
Related Background:
... ... *gamma ray bursts: Gamma ray bursts are intense flashes
of *gamma rays detected at energies up to 10^(6) electronvolts.
They were discovered by US Air Force satellites in 1967 but not
declassified until 1973. The detection of these bursts averages
about 1 per day, and measurements indicate the distribution of
bursts is isotropic, i.e., they are uniformly distributed across
the sky. The current consensus is that gamma ray bursts are
produced by the merger of two *neutron stars, and up to this
point, the bursts that have been noted apparently originate
outside our own galaxy.
... ... *gamma rays: Gamma rays are radiation of high energy,
from about 10^(5) electronvolts to more than 10^(14)
electronvolts -- radiation with the shortest wavelengths and
highest frequencies, the gamma ray region of the electromagnetic
spectrum merging into the adjacent lower energy x-ray region.
... ... *neutron stars: Neutron stars are one of the possible
end-products of stellar evolution. If, following its terminal
stages, the remnant mass of a star is between 1.4 and 2 to 3
solar masses, the star will collapse into a neutron star, a body
with a radius of 10 to 15 kilometers, with a core so dense that
its component protons and electrons have merged into neutrons.
... ... *Gev: Also written as Bev, a billion electronvolts. The
quantity in the report is thus 10^(28) electronvolts. An
electronvolt is defined as the energy acquired by an electron
falling freely through a potential difference of one volt, and is
equal to 1.6022 x 10^(-19) joule.
ENTANGLEMENT, DECOHERENCE, AND THE QUANTUM-CLASSICAL BOUNDARY
Quantum mechanical entanglement is a phenomenon that has caught
the imagination of the public as one of the more bizarre
consequences of fundamental physical theory. Entanglement is
unique to quantum mechanics, and involves a relationship (a
"superposition of states") between the possible quantum states of
two entities such that when the possible states of one entity
collapse to a single state as a result of suddenly imposed
boundary conditions, a similar and related collapse occurs in the
possible states of the entangled entity no matter where or how
far away the entangled entity is located. Entanglement arises
from the wave function equation of quantum mechanics, which has
an array of possible function solutions rather than a single
function solution, with each possible solution describing a set
of possible probabilistic quantum states of the physical system
under consideration. Upon fixation of the appropriate boundary
conditions, the array of possible solutions collapses into a
single solution. For many quantum mechanical physical systems,
the fixation of boundary conditions is a theoretical and
fundamental consequence of some interaction of the physical
system with something outside that system, e.g., an interaction
with the measuring device of an observer. In this context, two
entities that are described by the same array of possible
solutions to the wave function equation are said to be
"coherent", and when events decouple these entities, the
consequence is said to be "decoherence". As a physical
phenomenon, entanglement was discussed many years ago, most
particularly following the publication in 1935 of the often
quoted Einstein-Podolsky-Rosen paper (*Physical Review* 1935
47:777). These discussions have been in the form of "gedanken"
(thought) experiments involving two quantum-mechanical entangled
entities. More recently, however, there have been laboratory
constructions of actual quantum mechanical systems exhibiting
such entanglement phenomena, and the reportage of these
laboratory arrangements by the media have engaged the public
fancy. Essential here is that any purely verbal account of
quantum mechanical phenomena is severely limited by the
constraint that the properties of quantum mechanical systems can
be precisely described only by the equations relevant for those
systems, and all other descriptions usually introduce serious
ambiguities. ... ... Serge Haroche (Ecole Normale Superieure
Paris, FR) reviews quantum mechanical entanglement, decoherence,
and the question of the boundary between the physics of quantum
phenomena and the physics of classical phenomena. Haroche makes
the following points: 1) In quantum mechanics, a particle can be
delocalized (simultaneously occupy various probable positions in
space), can be simultaneously in several energy states, and can
even have several different identities at once. This apparent
"weirdness" behavior is encoded in the wave function of the
particle. 2) Recent decades have witnessed a rash of experiments
designed to test whether nature exhibits implausible nonlocality.
In such experiments, the wave function of a pair of particles
flying apart from each other is entangled into a non-separable
superposition of states. The quantum formalism asserts that
detecting one of the particles has an immediate effect on the
other, even if they are very far apart, even far enough apart to
be out of interaction range. The experiments clearly demonstrate
that the state of one particle is always correlated to the result
of the measurement performed on the other particle, and in just
the strange way predicted by quantum mechanics. 3) An important
question is: Why and how does quantum weirdness disappear
(decoherence) in large systems? In the last 15 years, entirely
solvable models of decoherence have been presented by various
authors (e.g., Leggett, Joos, Omnes, Zeh, Zurek), these models
based on the distinction in large objects between a few relevant
macroscopic observables (e.g., position or momentum) and an
"environment" described by a huge number of variables, such as
positions and velocities of air molecules, number of black-body
radiation photons, etc. The idea of these models, essentially, is
that the environment is "watching" the path followed by the
system (i.e., interacting with the system), and thus effectively
suppressing interference effects and quantum weirdness, and the
result of this process is that for macroscopic systems only
classical physics obtains. 4) In mesoscopic systems, which are
systems between macroscopic and microscopic dimensions,
decoherence may occur slowly enough to be observed. Until
recently, this could only be imagined in a gedanken experiment,
but technological advances have now made such experiments real,
and these experiments have opened this field to practical
investigation.
QY: Serge Haroche, Ecole Normale Superieure Paris, FR.
(Physics Today July 1998) (Science-Week 17 Jul 98)
-------------------
Related Background:
EXPERIMENTAL QUANTUM TELEPORTATION
Quantum teleportation is the transmission and reconstruction over
arbitrary distances of the state of a quantum system, an effect
first suggested by Bennett et al in 1993 (Phys. Rev. Lett.
70:1895). The achievement of the effect depends on the phenomenon
of entanglement, an essential feature of quantum mechanics.
Entanglement is unique to quantum mechanics, and involves a
relationship (a "superposition of states") between the possible
quantum states of two entities such that when the possible states
of one entity collapse to a single state as a result of suddenly
imposed boundary conditions, a similar and related collapse
occurs in the possible states of the entangled entity no matter
where or how far away the entangled entity is located. Polarizat-
ion is essentially a condition in which the properties of photons
are direction dependent, a condition that can be achieved by
passing light through appropriate media. Bouwmeester et al (6
authors, Univ. of Innsbruck, AT) now report an experimental
demonstration of quantum teleportation involving an initial
photon carrying a polarization that is transferred to one of a
pair of entangled photons, with the polarization-acquiring photon
an arbitrary distance from the initial one. The authors suggest
quantum teleportation will be a critical ingredient for quantum
computation networks.
QY: Dik Bouwmeester <[EMAIL PROTECTED]>
(Nature 11 Dec 97) (Science-Week 2 Jan 98)
-------------------
Related Background:
REPORT OF FIRST QUANTUM MECHANICAL ENTANGLEMENT OF ATOMS
... In the past, evidence of quantum mechanical entanglement has
been restricted to elementary particles such as protons,
electrons, and photons. Now E. Hagley et al, using rubidium
atoms prepared in circular Rydberg states (which means the outer
electrons of the atom have been excited to very high energy
states and are far from the nucleus in circular orbits), have
shown quantum mechanical entanglement at the level of atoms.
What is involved is that the experimental apparatus produces two
entangled atoms, one atom in a ground state and the other atom
in an excited state, physically separated so that the
entanglement is non-local, and when a measurement is made on one
atom, let us say the atom in a ground state, the other atom
instantaneously presents itself in the excited state -- the
result of the second atom wave function collapse thus determined
by the result of the first atom wave function collapse. There is
talk that before long quantum mechanical entanglement may be
demonstrated for molecules and perhaps even larger entities.
[Phys. Rev. Lett. 79:1 (1997)]
-------------------
Related Background:
QUANTUM PHOTON ENTANGLEMENT AT A DISTANCE OF SEVEN MILES
Whether or not the quantum mechanical behavior of elementary
particles is called mysterious depends, more or less, on the
attitude one has. If there is a demand that the behavior of these
particles be explainable with the logistic structure of human
language, then some aspects of their behavior seem mysterious
indeed. On the other hand, if there is a willingness to admit
that the logical structure of human language may not at present
be isomorphic with the logical structure of the laws that govern
the behavior of these particles, then it is probably best to put
off notions of mysteries and take the behavior for what it is.
This week there was announced to the popular press, before
publication, the results of a twin-photon experiment in
Switzerland. Nicolas Gisin et al (University of Geneva, CH)
reported that a pair of twin photons split and sent along two
diverging paths, when arriving at terminals seven miles apart,
exhibit the phenomenon of quantum "entanglement". The gist of it
is that the detection of one of the photons effectively causes
the collapse of the spectrum of its wave-function solutions to a
single solution, and this collapse instantaneously causes the
collapse of the possible quantum states of the other photon, in
this case seven miles away. The melodramatic notion (purveyed by
the press) is that information has somehow travelled from one
photon to the other at a speed greater than the speed of light,
with the result that great canons of thought are thereby
destroyed. But perhaps the more prosaic reality is that any
attempt to describe non-classical events with language based on
classical laws and perceptions cannot succeed.
(New York Times 22 Jul 97)
ON QUANTUM COMPUTING WITH MOLECULES
In general, in quantum mechanics, the "superposition principle"
holds that any two quantum mechanical states can be combined in
infinitely many ways to form states that have characteristics
intermediate between those of the two that are combined.
Entanglement is unique to quantum mechanics, and involves a
relationship (a "superposition of states") between the possible
quantum states of two entities such that when the possible states
of one entity collapse to a single state as a result of suddenly
imposed boundary conditions, a similar and related collapse
occurs in the possible states of the entangled entity no matter
where or how far away the entangled entity is located. The idea
of quantum computing received a significant impetus in 1994 when
Peter W. Shor of ATT (US) proposed that quantum entanglement and
superposition could in principle be used to accomplish many
numerical tasks, in particular the factoring of large numbers,
much faster than the best classical calculator. Since the
security of many important encryption systems depends on the
difficulty of factoring large numbers, quantum computing suddenly
became of great practical importance, and Shor's algorithm
provoked computer scientists to learn about quantum mechanics,
and physicists to begin serious considerations of the require-
ments of a quantum computer science. ... ... Gershenfeld and
Chuang (2 installations, US), review the theoretical bases and
current status of quantum computing, in particular their own work
applying nuclear magnetic resonance techniques. The authors point
out the following: 1) In classical computation, the state of a
bit (the fundamental unit of information) is specified by one
number, 0 or 1. An n-bit binary word in a typical computer is
thus described by string of n zeroes and ones. In contrast, in a
quantum computer, the qubit (the fundamental unit of information)
might be represented by an atom in one of two different states, 0
or 1, but unlike classical bits, qubits can exist simultaneously
as 0 or 1, with the probability for each state given by a
numerical coefficient. 2) A quantum computer promises to be
immensely powerful because it can be in multiple states at once
(superposition), and because it can act on all its possible
states simultaneously. Thus, a quantum computer could naturally
perform myriad operations in parallel, using only a single
processing unit. This is the essence of the idea of quantum
computing, although one must understand the expression here is
quite general. 3) The authors have investigated the construction
of a quantum computer based on the nuclear magnetic resonance
behavior of a simple molecular liquid [chloroform, CHCl(sub3)],
with the 2 possible quantum mechanical "spin" states of atoms as
the basic qubit states. Since chloroform is a simple molecule,
the fundamental limitation in this particular system is the small
number of qubits. The authors and other researchers are actively
working to increase the size of the basic molecule in
experimental quantum computing systems, and thus increase the
number of available qubits. 4) The authors conclude: "All along,
ordinary molecules have known how to do a remarkable kind of
computation. People were just not asking them the right
questions."
QY: Neil Gershenfeld, Massachusetts Institute of Technology 617-
253-1000.
(Scientific American June 1998) (Science-Week 12 Jun 98)
[Editor's note: Experimental details of the method and algorithm
used in the above mentioned NMR quantum computing technique were
recently presented by Chuang et al (5 authors 4 installations,
US) in Nature 14 May 1998 393:143]
-------------------
Related Background:
A SILICON-BASED NUCLEAR SPIN QUANTUM COMPUTER
B.E. Kane (University of New South Wales, AU) presents an
analysis of quantum computing and a new scheme for implementing a
quantum mechanical computer. The author proposes: 1) Although the
concept of information underlying all modern computer technology
is essentially classical, "physicists know that nature obeys the
laws of quantum mechanics." The idea of a quantum computer has
been developed theoretically over several decades in order to
understand the capabilities and limitations of machines in which
information is treated quantum mechanically. 3) Logical
operations carried out on the qubits and their measurement to
determine the result of the computation must obey quantum-mech-
anical laws. 4) Quantum computation can in principal only occur
in systems that are almost completely isolated from their
environment and which consequently must dissipate no energy
during the process of computation, conditions that are extra-
ordinarily difficult to fulfill in practice. The author presents
a scheme for implementing a quantum computer on an array of
nuclear spins located on donors in silicon. Logical operations
and measurements can in principle be performed independently and
in parallel on each spin in the array. Specific electronic
devices are described for the manipulation and measurement of
nuclear spins, and the author suggests that the development of a
silicon-based quantum computer can benefit from already existing
highly developed silicon technology.
QY: B.E. Kane ([EMAIL PROTECTED])
(Nature 14 May 98 393:133) (Science-Week 12 Jun 98)
DETAILS OF A PROPOSAL FOR A QUANTUM THEORY WITHOUT OBSERVERS
S. Goldstein, in the second part of a review of the current state
of the development of a quantum theory without observers, makes
the following points: 1) Several current quantum theories without
observers are completely well defined and hence provide a
conclusive refutation of Bohr's claim that such a theory is
impossible. 2) The paradoxes of quantum theory can be resolved in
a surprisingly simple way: by insisting that particles always
have positions and that they move in a manner naturally suggested
by the Schroedinger equation (e.g., the quantum mechanics of
David Bohm as amplified by John Bell). 3) The possibility of a
deterministic reformulation of quantum theory has been regarded
by many physicists as having been conclusively refuted,
particularly by the 1932 refutation of John von Neumann, but the
von Neumann proof is false, and subsequent "refutations" are not
convincing. 4) Bohmian mechanics is by far the simplest and
clearest version of quantum theory. 5) Although none of the
quantum theories without observers is Lorentz invariant, the
author believes such a theory is possible, and that the three
approaches of decoherent histories (which assumes the wave
function is not a complete description of a physical system),
spontaneous localization (which assumes spontaneous and random
collapse of wave functions), and Bohmian mechanics (which assumes
the wave function provides only an incomplete description of a
system and governs the motion of more fundamental variables) have
much to teach us about finding such a theory. QY: Sheldon
Goldstein, Dept. of Mathematics, Rutgers University New Brunswick
908-932-8789.
(Physics Today April 1998 v51:n4:p38) (Science-Week 24 Apr 98)
-------------------
Related Background:
ON QUANTUM THEORY WITHOUT OBSERVERS
One of the fundamental questions of physics is whether pure
states (i.e., states undisturbed by avoidable noise) are states
such that the outcome of every measurement can be exactly
predicted. Classical physics is based on the proposition that the
answer to the question is yes. Orthodox quantum mechanics is a
theory based on the proposition that the answer is no, and that
we can only make precise quantitative statements about probab-
ilities, the limitation due to an essential interaction between
the observer and that which is being measured.
... ... S. Goldstein (Rutgers University New Brunswick, US), in
the first of a two-part review, discusses the idea of quantum
theory without observers, and suggests that despite the claims of
most of the originators of quantum theory, the appeal at a fund-
amental level to observers and measurement, which is so prominent
in orthodox quantum theory, is not needed to account for quantum
phenomena. Referring to the classical Bohr-Einstein debate,
Goldstein says the debate has already been resolved in favor of
Einstein. What Einstein desired and Bohr held impossible -- an
observer-free formulation of quantum mechanics in which the
process of measurement can be analyzed in terms of more fund-
amental concepts, does in fact exist, and there are many such
formulations, several of which have the potential to become a
serious program for the construction of a quantum theory without
observers. QY: Sheldon Goldstein, Rutgers University New
Brunswick 908-932-8789.
(Physics Today March 1998) (Science-Week 20 Mar 98)
RECONSTRUCTING QUANTUM STATES OF ATOM MOTION
In quantum mechanics, the wave function of a system (Schroedinger
wave function, probability amplitude, psi function) is a function
of the coordinates of the particles of the system and of time, a
solution of the Schroedinger wave equation, and a determination
of the average result of every conceivable experiment on the
system. In general, the term "phase space" refers to an n-
dimensional space in which a point (with n coordinates)
represents a particular state of an n-variable system. The
movement of such a phase point in its phase space describes a
phase "trajectory". ... ... Leibfried et al (3 authors at 2
installations, DE US) review recent work concerning the
reconstruction of quantum states of atomic motion by means of
Wigner distributions (Wigner functions). Quantum mechanics allows
only one incomplete glimpse of a wave function, but if systems
can be identically prepared over and over, quantum equivalents of
shadows and mirrors can provide the full picture. In 1932, Eugene
Wigner presented what is now called the Wigner distribution as a
convenient mathematical construct for visualizing quantum
trajectories in phase space. The Wigner distribution retains many
of the features of a probability distribution, except that it can
be negative in some regions of phase space. The authors describe
methods for reconstructing the Wigner distribution of atomic
motion in phase space from sets of repeated measurements. They
suggest that such newly developed measurement techniques may have
abundant future applications in quantum control, quantum
computing, quantum-limited deposition techniques, analysis of
Bose-Einstein condensates of dilute gases, and the study of
quantum decoherence. QY: Dietrich Leibfried, Innsbruck
University, AT
(Physics Today April 1998) (Science-Week 17 Apr 98)
ON RECENT DEVELOPMENTS IN SUPERSTRING THEORY
Bose-Einstein statistics is the statistical mechanics of a system
of indistinguishable particles for which there is no restriction
on the number of particles that may simultaneously exist in the
same quantum energy state. Bosons are particles that obey Bose-
Einstein statistics, and they include photons, pi mesons, all
nuclei having an even number of particles, and all particles with
integer spin. Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
In particle physics, string theory is a theory of elementary
particles based on the idea that the fundamental entities are not
point-like particles but finite lines (strings), or closed loops
formed by strings, the strings one-dimensional curves with zero
thickness and lengths (or loop diameters) of the order of the
Planck length of 10^(-35) meters. The fundamental forces comprise
the gravitational force, the electromagnetic force, the nuclear
strong force, and the nuclear weak force, and the "grand unified
theories" are theories that aim to provide a mathematical frame-
work in which the electromagnetic forces, strong forces, and weak
forces emerge as parts of a single unified force, with the three
forces related by symmetry. Supersymmetry is an aspect of an
extension of the grand unified theories, an attempt to unify all
the four fundamental forces, i.e., linking gravitation to the
electromagnetic force, the strong force, and the weak force
through a supersymmetry scheme, and superstrings are strings in
this scheme that obey supersymmetry. ... ... John H. Schwarz
(California Institute of Technology, US) presents a brief
overview of some of the advances in understanding super-
string theory that have been achieved in the last few years.
String theories that have a symmetry relating bosons and
fermions, called "supersymmetry", are called "superstring"
theories. Major advances in understanding of the physical world
have been achieved during the past century by focusing on
apparent contradictions between well-established theoretical
structures. In each case the reconciliation required a better
theory, often involving radical new concepts and striking exper-
imental predictions. Four major advances of this type were the
discoveries of special relativity, quantum mechanics, general
relativity, and quantum field theory. This was quite an achieve-
ment for one century, but there is one fundamental contradiction
that still needs to be resolved, namely the clash between general
relativity and quantum field theory. Many theoretical physicists
are convinced that superstring theory will provide the answer.
QY: John H. Schwarz, California Inst. of Technology 818-395-6811
(Proc. Natl. Acad. Sci. US 17 Mar 98)
-------------------
Related Background:
ON THE EVOLUTION OF STRING THEORY TO MEMBRANE THEORY
... Membrane theory (M-theory) is a recent extension of string
theory in which the fundamental physical entities are considered
as surfaces in a many-dimensional space (membranes) rather than
as lines or loop elements (open or closed strings). Given all of
the above, some caution is necessary: the translation of a highly
abstract mathematical model of physical reality into non-mathem-
atical language is often an exercise of limited usefulness, and
in this case in particular, we are presenting only the ghost of
the theoretical scheme. String theory was originally invented in
the 1960s as a theory of the strong force, became overshadowed by
the strong force theory of gluons and quarks, then had a revival
in the 1980s -- but with the history more dependent on new work
than on fashion. ... ... M. Duff (Texas A & M Univ., US), who is
active in string theory and membrane theory, in a review of
various aspects of the history and essentials of string theory
and membrane theory, suggests that future historians may judge
the 20th century as "a time when theorists were like children
playing on the seashore, diverting themselves with the smoother
pebbles or prettier shells of superstrings, while the great ocean
of M-theory lay undiscovered before them." QY: Michael J. Duff,
Texas A & M Univ., Dept. Physics 409-847-9451
(Scientific American February 1998)
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