7,722 research outputs found
Communication through an extra dimension
If our visible universe is considered a trapped shell in a five-dimensional
hyper-universe, all matter in it may be connected by superluminal signals
traveling through the fifth dimension. Events in the shell are still causal,
however, the propagation of signals proceeds at different velocities depending
on the fifth coordinate.Comment: Latex format, 12 pages. References modifie
Finite Temperature Dynamics of the Spin 1/2 Bond Alternating Heisenberg Antiferromagnetic Chain
We present results for the dynamic structure factor of the S=1/2 bond
alternating Heisenberg chain over a large range of frequencies and
temperatures. Data are obtained from a numerical evaluation of thermal averages
based on the calculation of all eigenvalues and eigenfunctions for chains of up
to 20 spins. Interpretation is guided by the exact temperature dependence in
the noninteracting dimer limit which remains qualitatively valid up to an
interdimer exchange . The temperature induced central peak
around zero frequency is clearly identified and aspects of the crossover to
spin diffusion in its variation from low to high temperatures are discussed.
The one-magnon peak acquires an asymmetric shape with increasing temperature.
The two-magnon peak is dominated by the S=1 bound state which remains well
defined up to temperatures of the order of J. The variation with temperature
and wavevector of the integrated intensity for one and two magnon scattering
and of the central peak are discussed.Comment: 8 pages, 8 figure
The effects of particle loading on turbulence structure and modelling
The objective of the present research was to extend the Direct Numerical Simulation (DNS) approach to particle-laden turbulent flows using a simple model of particle/flow interaction. The program addressed the simplest type of flow, homogeneous, isotropic turbulence, and examined interactions between the particles and gas phase turbulence. The specific range of problems examined include those in which the particle is much smaller than the smallest length scales of the turbulence yet heavy enough to slip relative to the flow. The particle mass loading is large enough to have a significant impact on the turbulence, while the volume loading was small enough such that particle-particle interactions could be neglected. Therefore, these simulations are relevant to practical problems involving small, dense particles conveyed by turbulent gas flows at moderate loadings. A sample of the results illustrating modifications of the particle concentration field caused by the turbulence structure is presented and attenuation of turbulence by the particle cloud is also illustrated
The Projection Postulate of Quantum Mechanics on the Lightcone
We discuss an interpretation of the projection postulate that implies
collapse of the wavefunction along the lightcone.Comment: 5 pages, LaTex, SJSU/TP-93-1
EDQNM closure: A homogeneous simulation to support it. A quasi-homogeneous simulation to disprove it
It is known that two-point closures are useful tools for understanding and predicting turbulence. Among the various closures, the Eddy Damped Quasi-Normal Markovian (EDQNM) approach is one of the simplest and, at the same time, most useful. Direct numerical simulations (DNS) can provide information that can be used to test the validity of two-point theories. It is the purpose of the present work to use DNS to validate, or improve upon, EDQNM. A case was selected for which EDQNM is known to give satisfactory results: homogeneous isotropic turbulence. Quantities were then evaluated which may be used to test the assumptions of two-point closure approximations: spectral Lagrangian time scales. The goal was to make a careful and refined study to validate the EDQNM theory. A reference case was built for which EDQNM is likely to give poor results. An attempt to generate a quasi-homogeneous turbulent field containing organized structures, was built by artifically injecting them in the initial conditions. The results of direct simulations using such initial conditions are expected to provide a challenge for EDQNM since this kind of field is simple enough to allow comparisons with two-point theories, but at the same time contains coherent structures which cannot be expected to be accurately accounted for by closures based on expansions about Gaussianity
Lorentz-invariant Bohmian mechanics
A derivation of the Bohm model, and some general comments about it, are given. A modification of the model which is formally local and Lorentz-invariant is introduced, and its properties studied for a simple experiment
Do macroscopic properties dictate microscopic probabilities?
Aharonov and Reznik have recently (in quant-ph/0110093) argued that the form
of the probabilistic predictions of quantum theory can be seen to follow from
properties of macroscopic systems. An error in their argument is identified.Comment: LaTeX, 6 pages, no figure
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