21,854 research outputs found
The implementation and evaluation of an undergraduate virtual reality surveying application
Multi-media applications are increasingly being used to enhance the delivery of on-site and distance learning teaching material. However, production costs are often prohibitive, both in terms of capital investment and development time. Hence it is surprising that authors comment on the failure to adequately evaluate new educational software applications. This paper evaluates an interactive multi-media levelling resource, which comprises text-based guides, video instruction, photo-realistic panoramic scenes and multi-row object movies. Students explore 360 degree images of building sites, using traditional computer input devices, and click on hot spots to gather detailed information about the position of the optical level and staff. Readings are taken directly from the staff and students record backsights and foresights as various change points are introduced. On completion of the levelling exercise, 192 first year undergraduate students completed an evaluation based upon a series of statements drawn from technology-based training literature. The findings suggest that the exercise complements traditional learning approaches, maintains student interest, and reinforces understanding. However, significant differences in student ratings for part-time and full-time cohorts emphasise the importance of designing resources that accommodate the needs of varying student profiles. Suggestions for enhanced interactivity are offered and new areas for development allied to construction technology are identified
Dynamical Formation of Horizons in Recoiling D Branes
A toy calculation of string/D-particle interactions within a world-sheet
approach indicates that quantum recoil effects - reflecting the gravitational
back-reaction on space-time foam due to the propagation of energetic particles
- induces the appearance of a microscopic event horizon, or `bubble', inside
which stable matter can exist. The scattering event causes this horizon to
expand, but we expect quantum effects to cause it to contract again, in a
`bounce' solution. Within such `bubbles', massless matter propagates with an
effective velocity that is less than the velocity of light in vacuo, which may
lead to observable violations of Lorentz symmetry that may be tested
experimentally. The conformal invariance conditions in the interior geometry of
the bubbles select preferentially three for the number of the spatial
dimensions, corresponding to a consistent formulation of the interaction of D3
branes with recoiling D particles, which are allowed to fluctuate independently
only on the D3-brane hypersurface.Comment: 25 pages LaTeX, 4 eps figures include
Dileptons and Photons from Coarse-Grained Microscopic Dynamics and Hydrodynamics Compared to Experimental Data
Radiation of dileptons and photons from high energy nuclear collisions
provides information on the space-time evolution of the hot dense matter
produced therein. We compute this radiation using relativistic hydrodynamics
and a coarse-grained version of the microscopic event generator UrQMD, both of
which provide a good description of the hadron spectra. The currently most
accurate dilepton and photon emission rates from perturbative QCD and from
experimentally-based hadronic calculations are used. Comparisons are made to
data on central Pb-Pb and Pb-Au collisions taken at the CERN SPS at a beam
energy of 158 A GeV. Both hydrodynamics and UrQMD provide very good
descriptions of the photon transverse momentum spectrum measured between 1 and
4 GeV, but slightly underestimate the low mass spectrum of e+e- pairs, even
with greatly broadened rho and omega vector mesons. Predictions are given for
the transverse momentum distribution of dileptons.Comment: 35 pages, 17 figure
Confinement in Gauge Theories from the Condensation of World-Sheet Defects in Liouville String
We present a Liouville-string approach to confinement in four-dimensional
gauge theories, which extends previous approaches to include non-conformal
theories. We consider Liouville field theory on world sheets whose boundaries
are the Wilson loops of gauge theory, which exhibit vortex and spike defects.
We show that world-sheet vortex condensation occurs when the Wilson loop is
embedded in four target space-time dimensions, and show that this corresponds
to the condensation of gauge magnetic monopoles in target space. We also show
that vortex condensation generates a effective string tension corresponding to
the confinement of electric degrees of freedom. The tension is independent of
the string length in a gauge theory whose electric coupling varies
logarithmically with the length scale. The Liouville field is naturally
interpreted as an extra target dimension, with an anti-de-Sitter (AdS)
structure induced by recoil effects on the gauge monopoles, interpreted as D
branes of the effective string theory. Black holes in the bulk AdS space
correspond to world-sheet defects, so that phases of the bulk gravitational
system correspond to the different world-sheet phases, and hence to different
phases of the four-dimensional gauge theory. Deconfinement is associated with a
Berezinskii-Kosterlitz-Thouless transition of vortices on the Wilson-loop world
sheet, corresponding in turn to a phase transition of the black holes in the
bulk AdS space.Comment: 29 pages LATEX, three eps figures incorporate
A supersymmetric D-brane Model of Space-Time Foam
We present a supersymmetric model of space-time foam with two stacks of eight
D8-branes with equal string tensions, separated by a single bulk dimension
containing D0-brane particles that represent quantum fluctuations in the
space-time foam. The ground state configuration with static D-branes has zero
vacuum energy. However, gravitons and other closed-string states propagating
through the bulk may interact with the D0-particles, causing them to recoil and
the vacuum energy to become non zero. This provides a possible origin of dark
energy. Recoil also distorts the background metric felt by energetic massless
string states, which travel at less than the usual (low-energy) velocity of
light. On the other hand, the propagation of chiral matter anchored on the D8
branes is not affected by such space-time foam effects.Comment: 33 pages, latex, five figure
Space-Time Foam may Violate the Principle of Equivalence
The interactions of different particle species with the foamy space-time
fluctuations expected in quantum gravity theories may not be universal, in
which case different types of energetic particles may violate Lorentz
invariance by varying amounts, violating the equivalence principle. We
illustrate this possibility in two different models of space-time foam based on
D-particle fluctuations in either flat Minkowski space or a stack of
intersecting D-branes. Both models suggest that Lorentz invariance could be
violated for energetic particles that do not carry conserved charges, such as
photons, whereas charged particles such electrons would propagate in a
Lorentz-inavariant way. The D-brane model further suggests that gluon
propagation might violate Lorentz invariance, but not neutrinos. We argue that
these conclusions hold at both the tree (lowest-genus) and loop (higher-genus)
levels, and discuss their implications for the phenomenology of quantum
gravity.Comment: 20 pages, 4 figures, the version accepted for publication in the
International Journal of Modern Physics
Prelude to Compressed Baryonic Matter
This is intended to appear as the introduction to "The CBM Physics Book:
compressed baryonic matter in laboratory experiments" (ed. B. Friman, C.
H\"ohne, S. Leupold, J. Knoll, J. Randrup, R. Rapp, P. Senger), to be published
by Springer. At the end there is a new proposal for numerically tractable
models of interacting many-body systems.Comment: 12 pages, to appear in "The CBM Book: compressed baryonic matter in
laboratory experiments
Quantum-Gravitational Diffusion and Stochastic Fluctuations in the Velocity of Light
We argue that quantum-gravitational fluctuations in the space-time background
give the vacuum non-trivial optical properties that include diffusion and
consequent uncertainties in the arrival times of photons, causing stochastic
fluctuations in the velocity of light ``in vacuo''. Our proposal is motivated
within a Liouville string formulation of quantum gravity that also suggests a
frequency-dependent refractive index of the particle vacuum. We construct an
explicit realization by treating photon propagation through quantum excitations
of -brane fluctuations in the space-time foam. These are described by
higher-genus string effects, that lead to stochastic fluctuations in couplings,
and hence in the velocity of light. We discuss the possibilities of
constraining or measuring photon diffusion ``in vacuo'' via -ray
observations of distant astrophysical sources.Comment: 17 pages LATEX, uses axodraw style fil
Compactification and Supersymmetry Breaking in M-theory
Keeping N=1 supersymmetry in 4-dimension and in the leading order, we disuss
the various orbifold compactifications of M-theory suggested by Horava and
Witten on , , , and the compactification by
keeping singlets under symmetry, then the compactification
on . We also discuss the next to leading order K\"ahler potential,
superpotential, and gauge kinetic function in the case. In addition,
we calculate the SUSY breaking soft terms and find out that the universality of
the scalar masses will be violated, but the violation might be very small.Comment: 16 pages, latex, no figure
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