785 research outputs found
Quantum energies with worldline numerics
We present new results for Casimir forces between rigid bodies which impose
Dirichlet boundary conditions on a fluctuating scalar field. As a universal
computational tool, we employ worldline numerics which builds on a combination
of the string-inspired worldline approach with Monte-Carlo techniques.
Worldline numerics is not only particularly powerful for inhomogeneous
background configurations such as involved Casimir geometries, it also provides
for an intuitive picture of quantum-fluctuation-induced phenomena. Results for
the Casimir geometries of a sphere above a plate and a new perpendicular-plates
configuration are presented.Comment: 8 pages, 2 figures, Submitted to the Proceedings of the Seventh
Workshop QFEXT'05 (Barcelona, September 5-9, 2005), Refs updated, version to
appear in JPhys
Worldline algorithms for Casimir configurations
We present improved worldline numerical algorithms for high-precision
calculations of Casimir interaction energies induced by scalar-field
fluctuations with Dirichlet boundary conditions for various Casimir geometries.
Significant reduction of numerical cost is gained by exploiting the symmetries
of the worldline ensemble in combination with those of the configurations. This
facilitates high-precision calculations on standard PCs or small clusters. We
illustrate our strategies using the experimentally most relevant sphere-plate
and cylinder-plate configuration. We compute Casimir curvature effects for a
wide parameter range, revealing the tight validity bounds of the commonly used
proximity force approximation (PFA). We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.
Revisiting the parallel-plate configuration, we find a mapping between the
D-dimensional Casimir energy and properties of a random-chain polymer ensemble.Comment: 23 pages, 9 figure
Non-monotonic thermal Casimir force from geometry-temperature interplay
The geometry dependence of Casimir forces is significantly more pronounced in
the presence of thermal fluctuations due to a generic geometry-temperature
interplay. We show that the thermal force for standard sphere-plate or
cylinder-plate geometries develops a non-monotonic behavior already in the
simple case of a fluctuating Dirichlet scalar. In particular, the attractive
thermal force can increase for increasing distances below a critical
temperature. This anomalous behavior is triggered by a reweighting of relevant
fluctuations on the scale of the thermal wavelength. The essence of the
phenomenon becomes transparent within the worldline picture of the Casimir
effect.Comment: 4 pages, 4 figure
Irreducible Scalar Many-Body Casimir Energies: Theorems and Numerical Studies
We define irreducible N-body spectral functions and Casimir energies and
consider a massless scalar quantum field interacting locally by positive
potentials with classical objects. Irreducible N-body spectral functions in
this case are shown to be conditional probabilities of random walks. The
corresponding irreducible contributions to scalar many-body Casimir energies
are finite and positive/negative for an odd/even number of objects. The force
between any two finite objects separable by a plane is always attractive in
this case. Analytical and numerical world-line results for the irreducible
four-body Casimir energy of a scalar with Dirichlet boundary conditions on a
tic-tac-toe pattern of lines are presented. Numerical results for the
irreducible three-body Casimir energy of a massless scalar satisfying Dirichlet
boundary conditions on three intersecting lines forming an isosceles triangle
are also reported. In both cases the symmetric configuration (square and
isosceles triangle) corresponds to the minimal irreducible contribution to the
Casimir energy.Comment: Writeup of talk given at QFEXT11 (Sept.18-24) in Benasque, Spain. 10
pages, 3 figure
Casimir effect for curved geometries: PFA validity limits
We compute Casimir interaction energies for the sphere-plate and
cylinder-plate configuration induced by scalar-field fluctuations with
Dirichlet boundary conditions. Based on a high-precision calculation using
worldline numerics, we quantitatively determine the validity bounds of the
proximity force approximation (PFA) on which the comparison between all
corresponding experiments and theory are based. We observe the quantitative
failure of the PFA on the 1% level for a curvature parameter a/R > 0.00755.
Even qualitatively, the PFA fails to predict reliably the correct sign of
genuine Casimir curvature effects. We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.Comment: 4 pages, 4 figure
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
Strong laser fields as a probe for fundamental physics
Upcoming high-intensity laser systems will be able to probe the
quantum-induced nonlinear regime of electrodynamics. So far unobserved QED
phenomena such as the discovery of a nonlinear response of the quantum vacuum
to macroscopic electromagnetic fields can become accessible. In addition, such
laser systems provide for a flexible tool for investigating fundamental
physics. Primary goals consist in verifying so far unobserved QED phenomena.
Moreover, strong-field experiments can search for new light but weakly
interacting degrees of freedom and are thus complementary to accelerator-driven
experiments. I review recent developments in this field, focusing on photon
experiments in strong electromagnetic fields. The interaction of
particle-physics candidates with photons and external fields can be
parameterized by low-energy effective actions and typically predict
characteristic optical signatures. I perform first estimates of the accessible
new-physics parameter space of high-intensity laser facilities such as POLARIS
and ELI.Comment: 7 pages, Key Lecture at the ELI Workshop and School on "Fundamental
Physics with Ultra-High Fields", 9 September - 2 October 2008 at Frauenworth
Monastery, German
Mode summation approach to Casimir effect between two objects
In this paper, we explore the TGTG formula from the perspective of mode
summation approach. Both scalar fields and electromagnetic fields are
considered. In this approach, one has to first solve the equation of motion to
find a wave basis for each object. The two T's in the TGTG formula are
T-matrices representing the Lippmann-Schwinger T-operators, one for each of the
objects. The two G's in the TGTG formula are the translation matrices, relating
the wave basis of an object to the wave basis of the other object. After
discussing the general theory, we apply the prescription to derive the explicit
formulas for the Casimir energies for the sphere-sphere, sphere-plane,
cylinder-cylinder and cylinder-plane interactions. First the T-matrices for a
plane, a sphere and a cylinder are derived for the following cases: the object
is imposed with general Robin boundary conditions; the object is
semitransparent; and the object is magnetodielectric. Then the operator
approach is used to derive the translation matrices. From these, the explicit
TGTG formula for each of the scenarios can be written down. Besides summarizing
all the TGTG formulas that have been derived so far, we also provide the TGTG
formulas for some scenarios that have not been considered before.Comment: 42 page
Geothermal Casimir Phenomena
We present first worldline analytical and numerical results for the
nontrivial interplay between geometry and temperature dependencies of the
Casimir effect. We show that the temperature dependence of the Casimir force
can be significantly larger for open geometries (e.g., perpendicular plates)
than for closed geometries (e.g., parallel plates). For surface separations in
the experimentally relevant range, the thermal correction for the
perpendicular-plates configuration exhibits a stronger parameter dependence and
exceeds that for parallel plates by an order of magnitude at room temperature.
This effect can be attributed to the fact that the fluctuation spectrum for
closed geometries is gapped, inhibiting the thermal excitation of modes at low
temperatures. By contrast, open geometries support a thermal excitation of the
low-lying modes in the gapless spectrum already at low temperatures.Comment: 8 pages, 3 figures, contribution to QFEXT07 proceedings, v2:
discussion switched from Casimir energy to Casimir force, new analytical
results included, matches JPhysA versio
The Casimir Energy for a Hyperboloid Facing a Plate in the Optical Approximation
We study the Casimir energy of a massless scalar field that obeys Dirichlet
boundary conditions on a hyperboloid facing a plate. We use the optical
approximation including the first six reflections and compare the results with
the predictions of the proximity force approximation and the semi-classical
method. We also consider finite size effects by contrasting the infinite with a
finite plate. We find sizable and qualitative differences between the new
optical method and the more traditional approaches.Comment: v2: 14 pages, 11 eps figures; typo in eq. (21) removed, clarification
added, fig. 10 improved; version published in Phys. Rev.
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