16,307 research outputs found
Light field appearance manifolds
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (leaves 137-141).Statistical shape and texture appearance models are powerful image representations, but previously had been restricted to 2D or 3D shapes with smooth surfaces and lambertian reflectance. In this thesis we present a novel 4D appearance model using image-based rendering techniques, which can represent complex lighting conditions, structures, and surfaces. We construct a light field manifold capturing the multi-view appearance of an object class and extend previous direct search algorithms to match new light fields or 2D images of an object to a point on this manifold. When matching to a 2D image the reconstructed light field can be used to render unseen views of the object. Our technique differs from previous view-based active appearance models in that model coefficients between views are explicitly linked, and that we do not model any pose variation within the deformable model at a single view. It overcomes the limitations of polygonal based appearance models and uses light fields that are acquired in real-time.by Chris Mario Christoudias.S.M
D-Strings on D-Manifolds
We study the mechanism for appearance of massless solitons in type II string
compactifications. We find that by combining -duality with strong/weak
duality of type IIB in 10 dimensions enhanced gauge symmetries and massless
solitonic hypermultiplets encountered in Calabi-Yau compactifications can be
studied perturbatively using D-strings (the strong/weak dual to type IIB
string) compactified on ``D-manifolds''. In particular the nearly massless
solitonic states of the type IIB compactifications correspond to elementary
states of D-strings. As examples we consider the D-string description of
enhanced gauge symmetries for type IIA string compactification on ALE spaces
with singularities and type IIB on a class of singular Calabi-Yau
threefolds. The class we study includes as a special case the conifold
singularity in which case the perturbative spectrum of the D-string includes
the expected massless hypermultiplet with degeneracy one.Comment: 23 pages, with 3 figures. A compact example is adde
Collision of Shock Waves in Einstein-Maxwell Theory with a Cosmological Constant: A Special Solution
Post-collision space-times of the Cartesian product form M'xM'', where M' and
M'' are two-dimensional manifolds, are known with M' and M'' having constant
curvatures of equal and opposite sign (for the collision of electromagnetic
shock waves) or of the same sign (for the collision of gravitational shock
waves). We construct here a new explicit post-collision solution of the
Einstein-Maxwell vacuum field equations with a cosmological constant for which
M' has constant (nonzero) curvature and M'' has zero curvature.Comment: Latex file, 7 page
Dualities versus Singularities
We show that a subgroup of the modular group of M-theory compactified on a
ten torus, implies the Lorentzian structure of the moduli space, that is
usually associated with naive discussions of quantum cosmology based on the low
energy Einstein action. This structure implies a natural division of the
asymptotic domains of the moduli space into regions which can/cannot be mapped
to Type II string theory or 11D Supergravity (SUGRA) with large radii. We call
these the safe and unsafe domains. The safe domain is the interior of the
future light cone in the moduli space while the unsafe domain contains the
spacelike region and the past light cone. Within the safe domain, apparent
cosmological singularities can be resolved by duality transformations and we
briefly provide a physical picture of how this occurs. The unsafe domains
represent true singularities where all field theoretic description of the
physics breaks down. They violate the holographic principle. We argue that this
structure provides a natural arrow of time for cosmology. All of the Kasner
solutions, of the compactified SUGRA theory interpolate between the past and
future light cones of the moduli space. We describe tentative generalizations
of this analysis to moduli spaces with less SUSY.Comment: JHEP LaTeX, 23 pages, 1 eps-figure. Five references, a paragraph at
the beginning of section 3 and an appendix adde
On the Geometry of the String Landscape and the Swampland
We make a number of conjectures about the geometry of continuous moduli
parameterizing the string landscape. In particular we conjecture that such
moduli are always given by expectation value of scalar fields and that moduli
spaces with finite non-zero diameter belong to the swampland. We also
conjecture that points at infinity in a moduli space correspond to points where
an infinite tower of massless states appear, and that near these regions the
moduli space is negatively curved. We also propose that there is no non-trivial
1-cycle of minimum length in the moduli space. This leads in particular to the
prediction of the existence of a radially massive partner to the axion. These
conjectures put strong constraints on inflaton potentials that can appear in a
consistent quantum theory of gravity. Our conjectures are supported by a number
of highly non-trivial examples from string theory. Moreover it is shown that
these conditions can be violated if gravity is decoupled.Comment: 18 page
A Unique Theory of Gravity and Matter
The author has previously suggested that the ground state for 4-dimensional
quantum gravity can be represented as a condensation of non-linear gravitons
connected by Dirac strings. In this note we suggest that the low-lying
excitations of this state can be described by a quasi-topological action
corresponding to a trilinear coupling of solitonic 8-branes and 7-branes. It is
shown that when the 7-brane excitations are neglected, the effective action can
be interpreted as a theory of conformal gravity in four dimensions. This
suggests that ordinary gravity as well as supersymmetric matter and
phenomenological gauge symmetries arise from the spontaneous breaking of
topological invariance.Comment: PostScript, 12 page
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