5,053 research outputs found
Non-linear conformally invariant generalization of the Poisson equation to D>2 dimensions
I propound a non-linear generalization of the Poisson equation describing a
"medium" in D dimensions with a "dielectric constant" proportional to the field
strength to the power D-2. It is the only conformally invariant scalar theory
that is second order, and in which the scalar couples to the sources
via a contact term. The symmetry is used to generate
solutions for the field for some non-trivial configurations (e.g. for two
oppositely charged points). Systems comprising N point charges afford further
application of the symmetry. For these I derive e.g. exact expressions for the
following quantities: the general two-point-charge force; the energy function
and the forces in any three-body configuration with zero total charge; the
few-body force for some special configurations; the virial theorem for an
arbitrary, bound, many-particle system relating the time-average kinetic energy
to the particle charges. Possible connections with an underlying conformal
quantum field theory are mentioned.Comment: Revtex, 16 pages. To be published in Phys. Rev.
Dark matter and non-Newtonian gravity from General Relativity coupled to a fluid of strings
An exact solution of Einstein's field equations for a point mass surrounded
by a static, spherically symmetric fluid of strings is presented. The solution
is singular at the origin. Near the string cloud limit there is a
correction to Newton's force law. It is noted that at large distances and small
accelerations, this law coincides with the phenomenological force law invented
by Milgrom in order to explain the flat rotation curves of galaxies without
introducing dark matter. When interpreted in the context of a cosmological
model with a string fluid, the new solution naturally explains why the critical
acceleration of Milgrom is of the same order of magnitude as the Hubble
parameter.Comment: 12 pages, REVTeX, no figure
The Pioneer anomaly and the holographic scenario
In this paper we discuss the recently obtained relation between the
Verlinde's holographic model and the first phenomenological Modified Newtonian
dynamics. This gives also a promising possible explanation to the Pioneer
anomaly.Comment: 5 pages, Accepted for publication in Astrophysics & Space Scienc
Development of a Flame Resistant Silicone Rubber
Flame resistant silicone rubber using aluminum silicates and aromatic bromide
The Bright Side of Dark Matter
We show that it is not possible in the absence of dark matter to construct a
four-dimensional metric that explains galactic observations. In particular, by
working with an effective potential it is shown that a metric which is
constructed to fit flat rotation curves in spiral galaxies leads to the wrong
sign for the bending of light i.e. repulsion instead of attraction. Hence,
without dark matter the motion of particles on galactic scales cannot be
explained in terms of geodesic motion on a four- dimensional metric. This
reveals a new bright side to dark matter: it is indispensable if we wish to
retain the cherished equivalence principle.Comment: 7 pages, latex, no figures. Received an honorable mention in the 1999
Gravity research Foundation Essay Competition. Submitted to Phys. Rev. Let
Near-optimal asymmetric binary matrix partitions
We study the asymmetric binary matrix partition problem that was recently
introduced by Alon et al. (WINE 2013) to model the impact of asymmetric
information on the revenue of the seller in take-it-or-leave-it sales.
Instances of the problem consist of an binary matrix and a
probability distribution over its columns. A partition scheme
consists of a partition for each row of . The partition acts
as a smoothing operator on row that distributes the expected value of each
partition subset proportionally to all its entries. Given a scheme that
induces a smooth matrix , the partition value is the expected maximum
column entry of . The objective is to find a partition scheme such that
the resulting partition value is maximized. We present a -approximation
algorithm for the case where the probability distribution is uniform and a
-approximation algorithm for non-uniform distributions, significantly
improving results of Alon et al. Although our first algorithm is combinatorial
(and very simple), the analysis is based on linear programming and duality
arguments. In our second result we exploit a nice relation of the problem to
submodular welfare maximization.Comment: 17 page
Born reciprocity and the 1/r potential
Many structures in nature are invariant under the transformation
(p,r)->(br,-p/b), where b is some scale factor. Born's reciprocity hypothesis
affirms that this invariance extends to the entire Hamiltonian and equations of
motion. We investigate this idea for atomic physics and galactic motion, where
one is basically dealing with a 1/r potential and the observations are very
accurate, so as to determine the scale . We find that an Hz has essentially no effect on atomic physics but
might possibly offer an explanation for galactic rotation, without invoking
dark matter.Comment: 14 pages, with 4 figures, Latex, requires epsf.tex and iop style
file
Gravitational anomalies signaling the breakdown of classical gravity
Recent observations for three types of astrophysical systems severely
challenge the GR plus dark matter scenario, showing a phenomenology which is
what modified gravity theories predict. Stellar kinematics in the outskirts of
globular clusters show the appearance of MOND type dynamics on crossing the
threshold. Analysis shows a ``Tully-Fisher'' relation in these systems,
a scaling of dispersion velocities with the fourth root of their masses.
Secondly, an anomaly has been found at the unexpected scales of wide binaries
in the solar neighbourhood. Binary orbital velocities cease to fall along
Keplerian expectations, and settle at a constant value, exactly on crossing the
threshold. Finally, the inferred infall velocity of the bullet cluster
is inconsistent with the standard cosmological scenario, where much smaller
limit encounter velocities appear. This stems from the escape velocity limit
present in standard gravity; the ``bullet'' should not hit the ``target'' at
more than the escape velocity of the joint system, as it very clearly did.
These results are consistent with extended gravity, but would require rather
contrived explanations under GR, each. Thus, observations now put us in a
situation where modifications to gravity at low acceleration scales cease to be
a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38,
4
Role for targeted resection in the multidisciplinary treatment of metastatic gastrointestinal stromal tumor
The management of advanced gastrointestinal stromal tumors (GISTs) has evolved in the modern era due to the discovery of c-kit mutations and the development of tyrosine kinase inhibitors (TKIs). Until the advent of TKIs such as imatinib, the median survival reported for patients with advanced GIST was 19 months. Although surgery is the treatment of choice for resectable primary GIST, its role in cases of recurrence and metastasis remains to be unclear. This review outlines the potential beneficial role of repeat surgical resection in the multidisciplinary treatment of advanced GIST in the era of TKIs
Gaseous drag on a gravitational perturber in Modified Newtonian Dynamics and the structure of the wake
We calculate the structure of a wake generated by, and the dynamical friction
force on, a gravitational perturber travelling through a gaseous medium of
uniform density and constant background acceleration g_ext, in the context of
Modified Newtonian Dynamics (MOND). The wake is described as a linear
superposition of two terms. The dominant part displays the same structure as
the wake generated in Newtonian gravity scaled up by a factor
mu^{-1}(g_ext/a_0), where a_{0} is the constant MOND acceleration and mu the
interpolating function. The structure of the second term depends greatly on the
angle between g_{ext} and and the velocity of the perturber. We evaluate the
dynamical drag force numerically and compare our MOND results with the
Newtonian case. We mention the relevance of our calculations to orbit evolution
of globular clusters and satellites in a gaseous proto-galaxy. Potential
differences in the X-ray emission of gravitational galactic wakes in MOND and
in Newtonian gravity with a dark halo are highlighted.Comment: 13 pages, 7 figures, accepted for publication in MNRA
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