22,501 research outputs found
Nature of singularity in Einstein-massless scalar theory
We study the static and spherically symmetric exact solution of the
Einstein-massless scalar equations given by Janis, Newman and Winicour. We find
that this solution satisfies the weak energy condition and has strong globally
naked singularity.Comment: 9 pages, LaTex, no figures, misprints corrected, to appear in Int. J.
Mod. Phys.
Naked Singularities as Possible Candidates for Gamma-ray Bursters
Naked singularities appear naturally in dynamically evolving solutions of
Einstein equations involving gravitational collapse of radiation, dust and
perfect fluids, provided the rate of accretion is less than a critical value.
We propose that the gamma-ray bursters (GRBs) are examples of these naked
singularity solutions. For illustration, we show that according to solutions
involving spherically symmetric collapse of pure radiation field, the energy
and the observed duration of a GRB should satisfy,
erg
sec, being the fraction ( to ) of energy
released as gamma rays. All the presently observed GRBs satisfy this condition;
those satisfying the condition close to equality must necessarily be of
cosmological origin with the red-shift factor not exceeding
depending on exact observed flux.Comment: Feb. '92; revised- July 1992; re-revised-August 1992 (submitted
A Model for Quantum Stochastic Absorption in Absorbing Disordered Media
Wave propagation in coherently absorbing disordered media is generally
modeled by adding a complex part to the real part of the potential. In such a
case, it is already understood that the complex potential plays a duel role; it
acts as an absorber as well as a reflector due to the mismatch of the phase of
the real and complex parts of the potential. Although this model gives expected
results for weakly absorbing disordered media, it gives unphysical results for
the strong absorption regime where it causes the system to behave like a
perfect reflector. To overcome this issue, we develop a model here using
stochastic absorption for the modeling of absorption by "fake", or "side",
channels obviating the need for a complex potential. This model of stochastic
absorption eliminates the reflection that is coupled with the absorption in the
complex potential model and absorption is proportional to the magnitude of the
absorbing parameter. Solving the statistics of the reflection coefficient and
its phase for both the models, we argue that stochastic absorption is a
potentially better way of modeling absorbing disordered media.Comment: 5 pages, 4 figure
On the genericity of spacetime singularities
We consider here the genericity aspects of spacetime singularities that occur
in cosmology and in gravitational collapse. The singularity theorems (that
predict the occurrence of singularities in general relativity) allow the
singularities of gravitational collapse to be either visible to external
observers or covered by an event horizon of gravity. It is shown that the
visible singularities that develop as final states of spherical collapse are
generic. Some consequences of this fact are discussed.Comment: 19 pages, To be published in the Raychaudhuri Volume, eds. Naresh
Dadhich, Pankaj Joshi and Probir Ro
Sub-Nanosecond Time of Flight on Commercial Wi-Fi Cards
Time-of-flight, i.e., the time incurred by a signal to travel from
transmitter to receiver, is perhaps the most intuitive way to measure distances
using wireless signals. It is used in major positioning systems such as GPS,
RADAR, and SONAR. However, attempts at using time-of-flight for indoor
localization have failed to deliver acceptable accuracy due to fundamental
limitations in measuring time on Wi-Fi and other RF consumer technologies.
While the research community has developed alternatives for RF-based indoor
localization that do not require time-of-flight, those approaches have their
own limitations that hamper their use in practice. In particular, many existing
approaches need receivers with large antenna arrays while commercial Wi-Fi
nodes have two or three antennas. Other systems require fingerprinting the
environment to create signal maps. More fundamentally, none of these methods
support indoor positioning between a pair of Wi-Fi devices
without~third~party~support.
In this paper, we present a set of algorithms that measure the time-of-flight
to sub-nanosecond accuracy on commercial Wi-Fi cards. We implement these
algorithms and demonstrate a system that achieves accurate device-to-device
localization, i.e. enables a pair of Wi-Fi devices to locate each other without
any support from the infrastructure, not even the location of the access
points.Comment: 14 page
Vertical coordination in high-value commodities
"Rising per capita income, urbanization and globalization are changing the consumption basket in the developing countries towards high-value commodities (like fruits & vegetables, milk, meat, poultry, fish, etc.). This paper explores how smallholders can benefit from the emerging opportunities from a silent demand-driven changes in high-value agriculture in India. The study examines the institutional mechanisms adopted by different firms to integrate small producers of milk, broilers and vegetables in supply chain and their effects on producers' transaction costs and farm profitability. The study finds that the innovative institutional arrangements in the form of contract farming have considerably reduced transaction costs and improved market efficiency to benefit the smallholders. The study does not find any bias against smallholders in contract farming. Also, the study does not find that the relevant firms have exploited their monopsonistic position by paying lower prices to farmers. On the contrary, contract producers were found enjoying benefits of assured procurement of their produce and higher prices. The study lists policy hurdles in scaling up the innovative models of vertical coordination in high-value food commodities" Authors' AbstractHigh value commodities ,Urbanization ,High value agriculture ,Scaling up ,
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