78,703 research outputs found
Vacuum Spacetime With Multipole Moments: The Minimal Size Conjecture, Black Hole Shadow, and Gravitational Wave Observables
In this work, we explicitly construct the vacuum solution of Einstein's
equations with prescribed multipole moments. By observing the behavior of the
multipole spacetime metric at small distances, we conjecture that for a
sufficiently large multipole moment, there is a minimal size below which no
object in nature can support such a moment. The examples we have investigated
suggest that such minimal size scales as (instead of
), where is the mass and is the th order multipole
moment. With the metric of the "multipole spacetime", we analyze the shape of
black hole shadow for various multipole moments and discuss the prospects of
constraining the moments from shadow observations. In addition, we discuss the
shift of gravitational wave phase with respect to those of the Kerr spacetime,
for a test particle moving around an object with this set of multipole moments.
These phase shifts are required for the program of mapping out the spacetime
multipole moments based on gravitational wave observations of extreme
mass-ratio inspirals.Comment: 18 pages; 8 figure
Moment-Based Order-Independent Transparency
Compositing transparent surfaces rendered in an arbitrary order requires techniques for order-independent transparency. Each surface color needs to be multiplied by the appropriate transmittance to the eye to incorporate occlusion. Building upon moment shadow mapping, we present a moment-based method for compact storage and fast reconstruction of this depth-dependent function per pixel. We work with the logarithm of the transmittance such that the function may be accumulated additively rather than multiplicatively. Then an additive rendering pass for all transparent surfaces yields moments. Moment-based reconstruction algorithms provide approximations to the original function, which are used for compositing in a second additive pass. We utilize existing algorithms with four or six power moments and develop new algorithms using eight power moments or up to four trigonometric moments. The resulting techniques are completely order-independent, work well for participating media as well as transparent surfaces and come in many variants providing different tradeoffs. We also utilize the same approach for the closely related problem of computing shadows for transparent surfaces
Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO has recently
been driven via applied transverse magnetic fields through a continuous quantum
phase transition from spontaneous magnetic order to a quantum paramagnet, and
dramatic changes were observed in the spin dynamics, characteristic of weakly
perturbed 1D Ising quantum criticality. We report here extensive single-crystal
inelastic neutron scattering measurements of the magnetic excitations
throughout the three-dimensional (3D) Brillouin zone in the quantum
paramagnetic phase just above the critical field to characterize the effects of
the finite interchain couplings. In this phase, we observe that excitations
have a sharp, resolution-limited line shape at low energies and over most of
the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the
full bandwidth along the strongly dispersive chain direction and resolve clear
modulations of the dispersions in the plane normal to the chains,
characteristic of frustrated interchain couplings in an antiferromagnetic
isosceles triangular lattice. The dispersions can be well parametrized using a
linear spin-wave model that includes interchain couplings and further neighbor
exchanges. The observed dispersion bandwidth along the chain direction is
smaller than that predicted by a linear spin-wave model using exchange values
determined at zero field, and this effect is attributed to quantum
renormalization of the dispersion beyond the spin-wave approximation in fields
slightly above the critical field, where quantum fluctuations are still
significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and
figure
Liquid-gas coexistence and critical point shifts in size-disperse fluids
Specialized Monte Carlo simulations and the moment free energy (MFE) method
are employed to study liquid-gas phase equilibria in size-disperse fluids. The
investigation is made subject to the constraint of fixed polydispersity, i.e.
the form of the `parent' density distribution of the particle
diameters , is prescribed. This is the experimentally realistic
scenario for e.g. colloidal dispersions. The simulations are used to obtain the
cloud and shadow curve properties of a Lennard-Jones fluid having diameters
distributed according to a Schulz form with a large (40%) degree of
polydispersity. Good qualitative accord is found with the results from a MFE
method study of a corresponding van der Waals model that incorporates
size-dispersity both in the hard core reference and the attractive parts of the
free energy. The results show that polydispersity engenders considerable
broadening of the coexistence region between the cloud curves. The principal
effect of fractionation in this region is a common overall scaling of the
particle sizes and typical inter-particle distances, and we discuss why this
effect is rather specific to systems with Schulz diameter distributions. Next,
by studying a family of such systems with distributions of various widths, we
estimate the dependence of the critical point parameters on . In
contrast to a previous theoretical prediction, size-dispersity is found to
raise the critical temperature above its monodisperse value. Unusually for a
polydisperse system, the critical point is found to lie at or very close to the
extremum of the coexistence region in all cases. We outline an argument showing
that such behaviour will occur whenever size polydispersity affects only the
range, rather than the strength of the inter-particle interactions.Comment: 14 pages, 12 figure
Materiality of Time
Introduced by William Fowler, BFI National Archive and Natalie Brett Pro-Vice Chancellor London College of Communication with a screening of Raban's About Now MMX (2010), 28 minutes.
William Raban reflects on his filmmaking over the last four and a half decades paying particular attention to About Now MMX (2010) which is almost certainly the last of his works to be shot on film. Acknowledged for his contributions to expanded cinema, his films about London and the River Thames, Raban discusses his practice since he was a painting student at Saint Martin’s School of Art (1967-1971)
Phase behavior of weakly polydisperse sticky hard spheres: Perturbation theory for the Percus-Yevick solution
We study the effects of size polydispersity on the gas-liquid phase behaviour
of mixtures of sticky hard spheres. To achieve this, the system of coupled
quadratic equations for the contact values of the partial cavity functions of
the Percus-Yevick solution is solved within a perturbation expansion in the
polydispersity, i.e. the normalized width of the size distribution. This allows
us to make predictions for various thermodynamic quantities which can be tested
against numerical simulations and experiments. In particular, we determine the
leading-order effects of size polydispersity on the cloud curve delimiting the
region of two-phase coexistence and on the associated shadow curve; we also
study the extent of size fractionation between the coexisting phases. Different
choices for the size-dependence of the adhesion strengths are examined
carefully; the Asakura-Oosawa model of a mixture of polydisperse colloids and
small polymers is studied as a specific example.Comment: 43 pages, 12 figures, and 1 tabl
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