3,352 research outputs found
Almost product manifolds as the low energy geometry of Dirichlet branes
Any candidate theory of quantum gravity must address the breakdown of the
classical smooth manifold picture of space-time at distances comparable to the
Planck length. String theory, in contrast, is formulated on conventional
space-time. However, we show that in the low energy limit, the dynamics of
generally curved Dirichlet p-branes possess an extended local isometry group,
which can be absorbed into the brane geometry as an almost product structure.
The induced kinematics encode two invariant scales, namely a minimal length and
a maximal speed, without breaking general covariance. Quantum gravity effects
on D-branes at low energy are then seen to manifest themselves by the
kinematical effects of a maximal acceleration. Experimental and theoretical
implications of such new kinematics are easily derived. We comment on
consequences for brane world phenomenology.Comment: 12 pages, invited article in European Physical Journal C, reprinted
in Proceedings of the International School on Subnuclear Physics 2003 Erice
(World Scientific
How quantizable matter gravitates: a practitioner's guide
We present the practical step-by-step procedure for constructing canonical
gravitational dynamics and kinematics directly from any previously specified
quantizable classical matter dynamics, and then illustrate the application of
this recipe by way of two completely worked case studies. Following the same
procedure, any phenomenological proposal for fundamental matter dynamics must
be supplemented with a suitable gravity theory providing the coefficients and
kinematical interpretation of the matter equations, before any of the two
theories can be meaningfully compared to experimental data.Comment: 45 pages, no figure
Echo Delay and Overlap with Emitted Orientation Sounds and Doppler-shift Compensation in the Bat, Rhinolophus ferrumequinum
The compensation of Doppler-shifts by the bat, Rhinolophusferrumequinum,
functions only when certain temporal relations between the echo
and the emitted orientation sound are given. Three echo configurations
were used:
a) Original orientation sounds were electronically Doppler-shifted and
played back either cut at the beginning (variable delay) or at the end (variable
duration) of the echo.
b) Artificial constant frequency echoes with variable delay or duration
were clamped to the frequency of the emitted orientation sound at different
Doppler-shifts.
c) The echoes were only partially Doppler-shifted and the Doppler-shifted
component began after variable delays or had variable durations.
With increasing delay or decreasing duration of the Doppler-shifted echo
the compensation amplitude for a sinusoidally modulated + 3 kHz Dopplershift
(modulation rate 0.08 Hz) decreases for all stimulus configurations
(Figs. 1, 2, 3).
The range of the Doppler-shift compensation system is therefore limited
by the delay due to acoustic travel time to about 4 m distance between
bat and target. In this range the overlap duration of the echo with the
emitted orientation sound is always sufficiently long, when compared with
data on the orientation pulse length during target approach from Schnitzler
(1968) (Fig. 5)
Inelastic collisions in an exactly solvable two-mode Bose-Einstein Condensate
Inelastic collisions occur in Bose-Einstein condensates, in some cases,
producing particle loss in the system. Nevertheless, these processes have not
been studied in the case when particles do not escape the trap. We show that
such inelastic processes are relevant in quantum properties of the system such
as the evolution of the relative population, the self trapping effect and the
probability distribution of particles. Moreover, including inelastic terms in
the model of the two-mode condensate allows for an exact analytical solution.
Using this solution, we show that collisions favor the generation of
entanglement between the modes of the condensate as long as the collision rate
does not exceed the natural frequency of the system
Geometry of physical dispersion relations
To serve as a dispersion relation, a cotangent bundle function must satisfy
three simple algebraic properties. These conditions are derived from the
inescapable physical requirements to have predictive matter field dynamics and
an observer-independent notion of positive energy. Possible modifications of
the standard relativistic dispersion relation are thereby severely restricted.
For instance, the dispersion relations associated with popular deformations of
Maxwell theory by Gambini-Pullin or Myers-Pospelov are not admissible.Comment: revised version, new section on applications added, 46 pages, 9
figure
Sectional Curvature Bounds in Gravity: Regularisation of the Schwarzschild Singularity
A general geometrical scheme is presented for the construction of novel
classical gravity theories whose solutions obey two-sided bounds on the
sectional curvatures along certain subvarieties of the Grassmannian of
two-planes. The motivation to study sectional curvature bounds comes from their
equivalence to bounds on the acceleration between nearby geodesics. A universal
minimal length scale is a necessary ingredient of the construction, and an
application of the kinematical framework to static, spherically symmetric
spacetimes shows drastic differences to the Schwarzschild solution of general
relativity by the exclusion of spacelike singularities.Comment: 20 pages, 1 figure, REVTeX4, updated reference
Brans-Dicke geometry
We reveal the non-metric geometry underlying omega-->0 Brans-Dicke theory by
unifying the metric and scalar field into a single geometric structure. Taking
this structure seriously as the geometry to which matter universally couples,
we show that the theory is fully consistent with solar system tests. This is in
striking constrast with the standard metric coupling, which grossly violates
post-Newtonian experimental constraints.Comment: 8 pages, v2 with additional comment and reference
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