438 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 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
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.
Analytical and Numerical Verification of the Nernst Theorem for Metals
In view of the current discussion on the subject, an effort is made to show
very accurately both analytically and numerically how the Drude dispersion
model gives consistent results for the Casimir free energy at low temperatures.
Specifically, for the free energy near T=0 we find the leading term to be
proportional to T^2 and the next-to-leading term proportional to T^{5/2}. These
terms give rise to zero Casimir entropy as T approaches zero, and is thus in
accordance with Nernst's theorem.Comment: 19 pages latex, 3 figures. v4: Figures updated. This is the final
version, accepted for publication in Physical Review
Star Clusters
This review concentrates almost entirely on globular star clusters. It
emphasises the increasing realisation that few of the traditional problems of
star cluster astronomy can be studied in isolation: the influence of the Galaxy
affects dynamical evolution deep in the core, and the spectrum of stellar
masses; in turn the evolution of the core determines the highest stellar
densities, and the rate of encounters. In this way external tidal effects
indirectly influence the formation and evolution of blue stragglers, binary
pulsars, X-ray sources, etc. More controversially, the stellar density appears
to influence the relative distribution of normal stars. In the opposite sense,
the evolution of individual stars governs much of the early dynamics of a
globular cluster, and the existence of large numbers of primordial binary stars
has changed important details of our picture of the dynamical evolution. New
computational tools which will become available in the next few years will help
dynamical theorists to address these questions.Comment: 10 pages, 3 figures, Te
Reply to "Comment on 'Analytic and Numerical Verification of the Nernst Theorem for Metals'"
In this Reply to the preceding Comment of Klimchitskaya and Mostepanenko (cf.
also quant-ph/0703214), we summarize and maintain our position that the Drude
dispersion relation when inserted in the Lifshitz formula gives a
thermodynamically satisfactory description of the Casimir force, also in the
limiting case when the relaxation frequency goes to zero (perfect crystals).Comment: 4 pages, no figures; to appear in Phys. Rev.
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
N-body models of globular clusters: metallicity, half-light radii and mass-to-light ratios
Size differences of approx. 20% between red (metal-rich) and blue
(metal-poor) sub-populations of globular clusters have been observed,
generating an ongoing debate as to weather these originate from projection
effects or the difference in metallicity. We present direct N-body simulations
of metal-rich and metal-poor stellar populations evolved to study the effects
of metallicity on cluster evolution. The models start with N = 100000 stars and
include primordial binaries. We also take metallicity dependent stellar
evolution and an external tidal field into account. We find no significant
difference for the half-mass radii of those models, indicating that the
clusters are structurally similar. However, utilizing observational tools to
fit half-light (or effective) radii confirms that metallicity effects related
to stellar evolution combined with dynamical effects such as mass segregation
produce an apparent size difference of 17% on average. The metallicity effect
on the overall cluster luminosity also leads to higher mass-to-light ratios for
metal-rich clusters.Comment: 14 pages, 10 figures, accepted for publication in MNRA
Online Games
When we agreed to edit the theme on online
games for this Encyclopedia our first question
was, “What is meant by online games?” 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
“offline” 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 “being online” we
should, as Newman (2002) argued, be concerned
with varying degrees of participation during
gameplay
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