From: Science-Week To: prismx@scienceweek.com ; prismx@scienceweek.com ; prismx@scienceweek.com ; prismx@scienceweek.com Subject: SW Focus Report - Quantum Mechanics Date: Saturday, December 12, 1998 9:24 AM -------------- Enclosure number 1 ---------------- ------------------------------------------------------- SCIENCE-WEEK FREE TRIAL SUBSCRIPTION: This Focus Report is extracted from the full-text Email 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 . 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 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 (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 (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 (kane@newt.phys.unsw.edu.au) (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. 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