500 research outputs found
The Relativistic Generalization of the Gravitational Force for Arbitrary Spacetimes
It has been suggested that re-expressing relativity in terms of forces could
provide fresh insights. The formalism developed for this purpose only applied
to static, or conformally static, space-times. Here we extend it to arbitrary
space-times. It is hoped that this formalism may lead to a workable definition
of mass and energy in relativity.Comment: 16 page
Weyl collineations that are not curvature collineations
Though the Weyl tensor is a linear combination of the curvature tensor, Ricci
tensor and Ricci scalar, it does not have all and only the Lie symmetries of
these tensors since it is possible, in principle, that "asymmetries cancel".
Here we investigate if, when and how the symmetries can be different. It is
found that we can obtain a metric with a finite dimensional Lie algebra of Weyl
symmetries that properly contains the Lie algebra of curvature symmetries.
There is no example found for the converse requirement. It is speculated that
there may be a fundamental reason for this lack of "duality".Comment: 9 page
Similarities Between Classical Timelike Geodesics in a Naked Reissner-Nordstrom Singularity Background and the Behaviour of Electrons in Quantum Theory
It is generally assumed that naked singularities must be physically excluded,
as they could otherwise introduce unpredictable influences in their future null
cones. Considering geodesics for a naked Reissner-Nordstrom singularity, it is
found that the singularity is effectively clothed by its repulsive nature.
Regarding electron as naked singularity, the size of the clothed singularity
(electron) turns out to be classical electro-magnetic radius of the electron,
to an observer falling freely from infinity, initially at rest. The size
shrinks for an observer falling freely from infinity, with a positive initial
velocity. For geodetic parameters corresponding to negative energy there are
trapped geodesics. The similarity of this picture with that arising in the
Quantum Theory is discussed.Comment: 8 pages, 6 figure
On the definition of matter collineations
It is shown that when the stress-energy tensor of a spacetime is diagonal and
is written in the mixed form, its collineations admit infinite dimensional Lie
algebras except possibly in the case when the tensor depends on all the
spacetime coordinates. The result can be extended for more general second rank
tensors
Uniqueness of Flat Spherically Symmetric Spacelike Hypersurfaces Admitted by Spherically Symmetric Static Spactimes
It is known that spherically symmetric static spacetimes admit a foliation by
{\deg}at hypersurfaces. Such foliations have explicitly been constructed for
some spacetimes, using different approaches, but none of them have proved or
even discussed the uniqueness of these foliations. The issue of uniqueness
becomes more important due to suitability of {\deg}at foliations for studying
black hole physics. Here {\deg}at spherically symmetric spacelike hy-
persurfaces are obtained by a direct method. It is found that spherically
symmetric static spacetimes admit {\deg}at spherically symmetric hypersurfaces,
and that these hypersurfaces are unique up to translation under the time- like
Killing vector. This result guarantees the uniqueness of {\deg}at spherically
symmetric foliations for such spacetimes.Comment: 10 page
Asymptotic Behaviour of the Proper Length and Volume of the Schwarzschild Singularity
Though popular presentations give the Schwarzschild singularity as a point it
is known that it is spacelike and not timelike. Thus it has a "length" and is
not a "point". In fact, its length must necessarily be infinite. It has been
proved that the proper length of the Qadir-Wheeler suture model goes to
infinity [1], while its proper volume shrinks to zero, and the asymptotic
behaviour of the length and volume have been calculated. That model consists of
two Friedmann sections connected by a Schwarzschild "suture". The question
arises whether a similar analysis could provide the asymptotic behaviour of the
Schwarzschild black hole near the singularity. It is proved here that, unlike
the behaviour for the suture model, for the Schwarzschild essential singularity
and , where
is the mean extrinsic curvature, or the York time.Comment: 13 pages, 1 figur
Soil Seed Bank of Two Karst Ecosystems in Bogor, Indonesia: Similarity with the Aboveground Vegeta-tion and Its Restoration Potential
Karst ecosystem in Bogor is currently threatened by mining activities. Restoration of the disturbed karst areas is urgently required. Seed banks are considered as an important potential seed sources for restoration. This study was conducted to quantify the composition and species diversity of the aboveground vegetation and the seed bank. The study determined the correspondence between the seed bank with the aboveground vegetation in relation with ecosystem restoration. Twenty 6 m × 6 m vegetation plots were established. A total of sixty soil samples were taken from the study sites. The seed bank was studied using germination experiment. All plant species in the vegetation plots and seedlings growing from all soil samples were identified and the number was counted to determine the composition, index of diversity, and index of similarity. There were 80 species from 41 families found in the seed bank in Mt. Nyungcung, dominated by Clidemia hirta (Melastomataceae), whereas 50 species from 29 families found in Mt. Kapur with Cecropia peltata (Urticaceae) dominated the site. Diversity index of Mt. Nyungcung and Mt. Kapur seed banks were 2.09 ± 0.21 and 1.78 ± 0.47 respectively. The similarity index between seed bank and the aboveground vegetation in the two study sites were 32.86% and 27.66% respectively. Mt. Nyungcung seed bank was more diverse than Mt. Kapur. The similarity between the seed bank with the aboveground vegetation in the two study sites were low. Further assessment is needed to determine the role of the seed bank of Mt. Nyungcung and Mt. Kapur in the restoration of the ecosystems
Survey of deep representation learning for speech emotion recognition
Traditionally, speech emotion recognition (SER) research has relied on manually handcrafted acoustic features using feature engineering. However, the design of handcrafted features for complex SER tasks requires significant manual eort, which impedes generalisability and slows the pace of innovation. This has motivated the adoption of representation learning techniques that can automatically learn an intermediate representation of the input signal without any manual feature engineering. Representation learning has led to improved SER performance and enabled rapid innovation. Its effectiveness has further increased with advances in deep learning (DL), which has facilitated \textit{deep representation learning} where hierarchical representations are automatically learned in a data-driven manner. This paper presents the first comprehensive survey on the important topic of deep representation learning for SER. We highlight various techniques, related challenges and identify important future areas of research. Our survey bridges the gap in the literature since existing surveys either focus on SER with hand-engineered features or representation learning in the general setting without focusing on SER
Securing Machine Learning in the Cloud: A Systematic Review of Cloud Machine Learning Security.
With the advances in machine learning (ML) and deep learning (DL) techniques, and the potency of cloud computing in offering services efficiently and cost-effectively, Machine Learning as a Service (MLaaS) cloud platforms have become popular. In addition, there is increasing adoption of third-party cloud services for outsourcing training of DL models, which requires substantial costly computational resources (e.g., high-performance graphics processing units (GPUs)). Such widespread usage of cloud-hosted ML/DL services opens a wide range of attack surfaces for adversaries to exploit the ML/DL system to achieve malicious goals. In this article, we conduct a systematic evaluation of literature of cloud-hosted ML/DL models along both the important dimensions-attacks and defenses-related to their security. Our systematic review identified a total of 31 related articles out of which 19 focused on attack, six focused on defense, and six focused on both attack and defense. Our evaluation reveals that there is an increasing interest from the research community on the perspective of attacking and defending different attacks on Machine Learning as a Service platforms. In addition, we identify the limitations and pitfalls of the analyzed articles and highlight open research issues that require further investigation
A First Look at CQVID-19 Messages on WhatsApp in Pakistan
The worldwide spread of COVID-19 has prompted extensive online discussions,
creating an `infodemic' on social media platforms such as WhatsApp and Twitter.
However, the information shared on these platforms is prone to be unreliable
and/or misleading. In this paper, we present the first analysis of COVID-19
discourse on public WhatsApp groups from Pakistan. Building on a large scale
annotation of thousands of messages containing text and images, we identify the
main categories of discussion. We focus on COVID-19 messages and understand the
different types of images/text messages being propagated. By exploring user
behavior related to COVID messages, we inspect how misinformation is spread.
Finally, by quantifying the flow of information across WhatsApp and Twitter, we
show how information spreads across platforms and how WhatsApp acts as a source
for much of the information shared on Twitter
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