3,604 research outputs found
Subsystems of a finite quantum system and Bell-like inequalities
YesThe set of subsystems Sigma(m) of a finite quantum system Sigma(n) with variables in Z(n) together with logical connectives, is a Heyting algebra. The probabilities tau(m vertical bar rho(n)) Tr vertical bar B(m)rho(n)] (where B(m) is the projector to Sigma(m)) are compatible with associativity of the join in the Heyting algebra, only if the variables belong to the same chain. Consequently, contextuality in the present formalism, has the chains as contexts. Various Bell-like inequalities are discussed. They are violated, and this proves that quantum mechanics is a contextual theory
Observing the spin of a free electron
Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions
Accessing Nuclear Structure for Field Emission, in Lens, Scanning Electron Microscopy (FEISEM)
Scanning electron microscopy (SEM) has had a shorter time course in biology than conventional transmission electron microscopy (TEM) but has nevertheless produced a wealth of images that have significantly complemented our perception of biological structure and function from TEM information. By its nature, SEM is a surface imaging technology, and its impact at the subcellular level has been restricted by the considerably reduced resolution in conventional SEM in comparison to TEM. This restriction has been removed by the recent advent of high-brightness sources used in lensfield emission instruments (FEISEM) which have produced resolution of around 1 nanometre, which is not usually a limiting figure for biological material.
This communication reviews our findings in the use of FEISEM in the imaging of nuclear surfaces, then associated structures, such as nuclear pore complexes, and the relationships of these structures with cytoplasmic and nucleoplasmic elements. High resolution SEM allows the structurally orientated cell biologist to visualise, directly and in three dimensions, subcellular structure and its modulation with a view to understanding its functional significance. Clearly, intracellular surfaces require separation from surrounding structural elements in vivo to allow surface imaging, and we review a combination of biochemical and mechanical isolation methods for nuclear surfaces
Relativistic precession and spin dynamics of an elliptic Rydberg wave packet
Time evolution of wave packets built from the eigenstates of the Dirac
equation for a hydrogenic system is considered. We investigate the space and
spin motion of wave packets which, in the non-relativistic limit, are
stationary states with a probability density distributed uniformly along the
classical, elliptical orbit (elliptic WP). We show that the precession of such
a WP, due to relativistic corrections to the energy eigenvalues, is strongly
correlated with the spin motion. We show also that the motion is universal for
all hydrogenic systems with an arbitrary value of the atomic number Z.Comment: Latex2e, uses IOP style files (included), 10 pages, 5 jpg figures, 1
postscript figure. Relation between precession time and radiative liftime
added (eq.(12)). Accepted for publication in J. Phys.
Sampling a Littoral Fish Assemblage: Comparison of Small-Mesh Fyke Netting and Boat Electrofishing
We compared small-mesh (4-mm) fyke netting and boat electrofishing for sampling a littoral fish assemblage in Muskegon Lake, Michigan. We hypothesized that fyke netting selects for small-bodied fishes and electrofishing selects for large-bodied fishes. Three sites were sampled during May (2004 and 2005), July (2005 only), and September (2004 and 2005). We found that the species composition of captured fish differed considerably between fyke netting and electrofishing based on nonmetric multidimensional scaling (NMDS). Species strongly associated with fyke netting (based on NMDS and relative abundance) included the brook silverside Labidesthes sicculus, banded killifish Fundulus diaphanus, round goby Neogobius melanostomus, mimic shiner Notropis volucellus, and bluntnose minnow Pimephales notatus, whereas species associated with electrofishing included the Chinook salmon Oncorhynchus tshawytscha, catostomids (Moxostoma spp. and Catostomus spp.), freshwater drum Aplodinotus grunniens, walleye Sander vitreus, gizzard shad Dorosoma cepedianum, and common carp Cyprinus carpio. The total length of fish captured by electrofishing was 12.8 cm (95% confidence interval ¼ 5.5– 17.2 cm) greater than that of fish captured by fyke netting. Size selectivity of the gears contributed to differences in species composition of the fish captured, supporting our initial hypothesis. Thus, small-mesh fyke nets and boat electrofishers provided complementary information on a littoral fish assemblage. Our results support use of multiple gear types in monitoring and research surveys of fish assemblages.
Copyright by the American Fisheries Society 2007, Originally published in the North American Journal of Fisheries Management 27: 825-831, 2007
Theoretical Study of Fluid Membranes of Spherical Topology with Internal Degrees of Freedom
A theoretical study of vesicles of topological genus zero is presented. The
bilayer membranes forming the vesicles have various degrees of intrinsic
(tangent-plane) orientational order, ranging from smectic to hexatic,
frustrated by curvature and topology. The field-theoretical model for these
`-atic' surfaces has been studied before in the low temperature (mean-field)
limit. Work presented here includes the effects of thermal fluctuations. Using
the lowest Landau level approximation, the coupling between order and shape is
cast in a simple form, facilitating insights into the behaviour of vesicles.
The order parameter contains vortices, whose effective interaction potential is
found, and renormalized by membrane fluctuations. The shape of the phase space
has a counter-intuitive influence on this potential. A criterion is established
whereby a vesicle of finite rigidity may be burst by its own in-plane order,
and an analogy is drawn with flux exclusion from a type-I superconductor.Comment: 34 pages + 4 Postscript figures. Uses RevTe
Light Hadron Masses from Lattice QCD
This article reviews lattice QCD results for the light hadron spectrum. We
give an overview of different formulations of lattice QCD, with discussions on
the fermion doubling problem and improvement programs. We summarize recent
developments in algorithms and analysis techniques, that render calculations
with light, dynamical quarks feasible on present day computer resources.
Finally, we summarize spectrum results for ground state hadrons and resonances
using various actions.Comment: 53 pages, 24 figures, one table; Rev.Mod.Phys. (published version);
v2: corrected typ
Radioactive decays at limits of nuclear stability
The last decades brought an impressive progress in synthesizing and studying
properties of nuclides located very far from the beta stability line. Among the
most fundamental properties of such exotic nuclides, usually established first,
is the half-life, possible radioactive decay modes, and their relative
probabilities. When approaching limits of nuclear stability, new decay modes
set in. First, beta decays become accompanied by emission of nucleons from
highly excited states of daughter nuclei. Second, when the nucleon separation
energy becomes negative, nucleons start to be emitted from the ground state.
Here, we present a review of the decay modes occurring close to the limits of
stability. The experimental methods used to produce, identify and detect new
species and their radiation are discussed. The current theoretical
understanding of these decay processes is overviewed. The theoretical
description of the most recently discovered and most complex radioactive
process - the two-proton radioactivity - is discussed in more detail.Comment: Review, 68 pages, 39 figure
Synaptic Scaling Balances Learning in a Spiking Model of Neocortex
Learning in the brain requires complementary mechanisms: potentiation and
activity-dependent homeostatic scaling. We introduce synaptic scaling to a
biologically-realistic spiking model of neocortex which can learn changes in
oscillatory rhythms using STDP, and show that scaling is necessary to balance
both positive and negative changes in input from potentiation and atrophy. We
discuss some of the issues that arise when considering synaptic scaling in such
a model, and show that scaling regulates activity whilst allowing learning to
remain unaltered.Comment: 10 page
Majorana: from atomic and molecular, to nuclear physics
In the centennial of Ettore Majorana's birth (1906-1938?), we re-examine some
aspects of his fundamental scientific production in atomic and molecular
physics, including a not well known short communication. There, Majorana
critically discusses Fermi's solution of the celebrated Thomas-Fermi equation
for electron screening in atoms and positive ions. We argue that some of
Majorana's seminal contributions in molecular physics already prelude to the
idea of exchange interactions (or Heisenberg-Majorana forces) in his later
workson theoretical nuclear physics. In all his papers, he tended to emphasize
the symmetries at the basis of a physical problem, as well as the limitations,
rather than the advantages, of the approximations of the method employed.Comment: to appear in Found. Phy
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