371 research outputs found
What is the Temperature Dependence of the Casimir Effect?
There has been recent criticism of our approach to the Casimir force between
real metallic surfaces at finite temperature, saying it is in conflict with the
third law of thermodynamics and in contradiction with experiment. We show that
these claims are unwarranted, and that our approach has strong theoretical
support, while the experimental situation is still unclear.Comment: 6 pages, REVTeX, final revision includes two new references and
related discussio
On the Temperature Dependence of the Casimir Effect
The temperature dependence of the Casimir force between a real metallic plate
and a metallic sphere is analyzed on the basis of optical data concerning the
dispersion relation of metals such as gold and copper. Realistic permittivities
imply, together with basic thermodynamic considerations, that the transverse
electric zero mode does not contribute. This results in observable differences
with the conventional prediction, which does not take this physical requirement
into account. The results are shown to be consistent with the third law of
thermodynamics, as well as being consistent with current experiments. However,
the predicted temperature dependence should be detectable in future
experiments. The inadequacies of approaches based on {\it ad hoc} assumptions,
such as the plasma dispersion relation and the use of surface impedance without
transverse momentum dependence, are discussed.Comment: 14 pages, 3 eps figures, revtex4. New version includes clarifications
and new reference. Accepted for publication in Phys. Rev.
On the origin of the distribution of binary-star periods
Pre-main sequence and main-sequence binary systems are observed to have
periods, P, ranging from one day to 10^(10) days and eccentricities, e, ranging
from 0 to 1. We pose the problem if stellar-dynamical interactions in very
young and compact star clusters may broaden an initially narrow period
distribution to the observed width. N-body computations of extremely compact
clusters containing 100 and 1000 stars initially in equilibrium and in cold
collapse are preformed. In all cases the assumed initial period distribution is
uniform in the narrow range 4.5 < log10(P) < 5.5 (P in days) which straddles
the maximum in the observed period distribution of late-type Galactic-field
dwarf systems. None of the models lead to the necessary broadening of the
period distribution, despite our adopted extreme conditions that favour
binary--binary interactions. Stellar-dynamical interactions in embedded
clusters thus cannot, under any circumstances, widen the period distribution
sufficiently. The wide range of orbital periods of very young and old binary
systems is therefore a result of cloud fragmentation and immediate subsequent
magneto-hydrodynamical processes operating within the multiple proto-stellar
system.Comment: 11 pages, 4 figures, ApJ, in pres
Phase transitions in simplified models with long-range interactions
We study the origin of phase transitions in some simplified models with long
range interactions. For the ring model, we show that a possible new phase
transition predicted in a recent paper by Nardini and Casetti from an energy
landscape analysis does not occur. Instead of such phase transitions we observe
a sharp, although without any non-analiticity, change from a core-halo to an
only core configuration in the spatial distribution functions for low energies.
By introducing a new class of solvable simplified models without any critical
points in the potential energy, we show that a similar behaviour to the ring
model is obtained, with a first order phase transition from an almost
homogeneous high energy phase to a clustered phase, and the same core-halo to
core configuration transition at lower energies. We discuss the origin of these
features of the simplified models, and show that the first order phase
transition comes from the maximization of the entropy of the system as a
function of energy an an order parameter, as previously discussed by Kastner,
which seems to be the main mechanism causing phase transitions in long-range
interacting systems
Online Games
When we agreed to edit the theme on online
games for this Encyclopedia our first question
was, âWhat is meant by online games?â Scholars
of games distinguish between nondigital
games (such as board games) and digital games,
rather than between online and offline games.
With networked consoles and smartphones it is
becoming harder and harder to find players in
the wealthy industrialized countries who play
âofflineâ digital games. Most games developers
now include some element of online activity in
their game and the question is:What is the degree
to which the gameplay experience occurs online?
Is online gameplay more a multiplayer than an
individual experience? If we move beyond the
technological meaning of âbeing onlineâ we
should, as Newman (2002) argued, be concerned
with varying degrees of participation during
gameplay
The Promiscuous Nature of Stars in Clusters
The recent availability of special purpose computers designed for calculating
gravitational interactions of N-bodies at extremely high speed has provided the
means to model globular clusters on a star-by-star basis for the first time. By
endeavouring to make the N-body codes that operate on these machines as
realistic as possible, the addition of stellar evolution being one example,
much is being learnt about the interaction between the star cluster itself and
the stars it contains. A fascinating aspect of this research is the ability to
follow the orbits of individual stars in detail and to document the formation
of observed exotic systems. This has revealed that many stars within a star
cluster lead wildly promiscuous lives, interacting, often intimately and in
rapid succession, with a variety of neighbours.Comment: 15 pages, 1 figure, to appear in the Astrophysical Journa
A pilgrimage to gravity on GPUs
In this short review we present the developments over the last 5 decades that
have led to the use of Graphics Processing Units (GPUs) for astrophysical
simulations. Since the introduction of NVIDIA's Compute Unified Device
Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body
simulations and is so popular these days that almost all papers about high
precision N-body simulations use methods that are accelerated by GPUs. With the
GPU hardware becoming more advanced and being used for more advanced algorithms
like gravitational tree-codes we see a bright future for GPU like hardware in
computational astrophysics.Comment: To appear in: European Physical Journal "Special Topics" : "Computer
Simulations on Graphics Processing Units" . 18 pages, 8 figure
Casimir energy and entropy between dissipative mirrors
We discuss the Casimir effect between two identical, parallel slabs,
emphasizing the role of dissipation and temperature. Starting from quite
general assumptions, we analyze the behavior of the Casimir entropy in the
limit T->0 and link it to the behavior of the slab's reflection coefficients at
low frequencies. We also derive a formula in terms of a sum over modes, valid
for dissipative slabs that can be interpreted in terms of a damped quantum
oscillator.Comment: 8 pages, 1 figur
Casimir Force on a Micrometer Sphere in a Dip: Proposal of an Experiment
The attractive Casimir force acting on a micrometer-sphere suspended in a
spherical dip, close to the wall, is discussed. This setup is in principle
directly accessible to experiment. The sphere and the substrate are assumed to
be made of the same perfectly conducting material.Comment: 11 pages, 1 figure; to appear in J. Phys. A: Math. Ge
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