8,614 research outputs found
The Equivalence Principle Revisited
A precise formulation of the strong Equivalence Principle is essential to the
understanding of the relationship between gravitation and quantum mechanics.
The relevant aspects are reviewed in a context including General Relativity,
but allowing for the presence of torsion. For the sake of brevity, a concise
statement is proposed for the Principle: "An ideal observer immersed in a
gravitational field can choose a reference frame in which gravitation goes
unnoticed". This statement is given a clear mathematical meaning through an
accurate discussion of its terms. It holds for ideal observers (time-like
smooth non-intersecting curves), but not for real, spatially extended
observers. Analogous results hold for gauge fields. The difference between
gravitation and the other fundamental interactions comes from their distinct
roles in the equation of force.Comment: RevTeX, 18 pages, no figures, to appear in Foundations of Physic
Gravitation as Anholonomy
A gravitational field can be seen as the anholonomy of the tetrad fields.
This is more explicit in the teleparallel approach, in which the gravitational
field-strength is the torsion of the ensuing Weitzenboeck connection. In a
tetrad frame, that torsion is just the anholonomy of that frame. The infinitely
many tetrad fields taking the Lorentz metric into a given Riemannian metric
differ by point-dependent Lorentz transformations. Inertial frames constitute a
smaller infinity of them, differing by fixed-point Lorentz transformations.
Holonomic tetrads take the Lorentz metric into itself, and correspond to
Minkowski flat spacetime. An accelerated frame is necessarily anholonomic and
sees the electromagnetic field strength with an additional term.Comment: RevTeX4, 10 pages, no figures. To appear in Gen. Rel. Gra
Disorder-induced double resonant Raman process in graphene
An analytical study is presented of the double resonant Raman scattering
process in graphene, responsible for the D and D features in the
Raman spectra. This work yields analytical expressions for the D and
D integrated Raman intensities that explicitly show the dependencies
on laser energy, defect concentration, and electronic lifetime. Good agreement
is obtained between the analytical results and experimental measurements on
samples with increasing defect concentrations and at various laser excitation
energies. The use of Raman spectroscopy to identify the nature of defects is
discussed. Comparison between the models for the edge-induced and the
disorder-induced D band intensity suggests that edges or grain boundaries can
be distinguished from disorder by the different dependence of their Raman
intensity on laser excitation energy. Similarly, the type of disorder can
potentially be identified not only by the intensity ratio
, but also by its laser energy
dependence. Also discussed is a quantitative analysis of quantum interference
effects of the graphene wavefunctions, which determine the most important
phonon wavevectors and scattering processes responsible for the D and
D bands.Comment: 10 pages, 4 figure
Distributed crowd-based annotation of soccer games using mobile devices
Soccer is one of the most loved sports in the world. Millions of people either follow the sport or are actually involved in its practice. Soccer also moves huge financial amounts every year and therefore teams always thrive to be better than the competition. New technologies have become a common place both in the preparation of the games and on the analysis of the games after they are concluded. In this paper, the authors will present a developed system, based on the usage of distributed mobile devices, that will enable the annotation of soccer matches, either in real time or after the matched is concluded (through the observation of other media). The capture of relevant events in the game can be used to better analyse the game and the performance of individual players fostering improvements and better decisions in the future. The application is implemented in the Android platform so that it can be easily installed by typical soccer fans empowering them as match annotators. This crowd of annotators, although not experts, can collectively provide a robust and rich annotation for soccer matches.info:eu-repo/semantics/publishedVersio
Comments on "There is no axiomatic system for the quantum theory"
In a recent paper, Nagata [1] claims to derive inconsistencies from quantum
mechanics. In this paper, we show that the inconsistencies do not come from
quantum mechanics, but from extra assumptions about the reality of observables
Group Theory analysis of phonons in two-dimensional Transition Metal Dichalcogenides
Transition metal dichalcogenides (TMDCs) have emerged as a new two
dimensional materials field since the monolayer and few-layer limits show
different properties when compared to each other and to their respective bulk
materials. For example, in some cases when the bulk material is exfoliated down
to a monolayer, an indirect-to-direct band gap in the visible range is
observed. The number of layers ( even or odd) drives changes in space
group symmetry that are reflected in the optical properties. The understanding
of the space group symmetry as a function of the number of layers is therefore
important for the correct interpretation of the experimental data. Here we
present a thorough group theory study of the symmetry aspects relevant to
optical and spectroscopic analysis, for the most common polytypes of TMDCs,
i.e. , and , as a function of the number of layers. Real space
symmetries, the group of the wave vectors, the relevance of inversion symmetry,
irreducible representations of the vibrational modes, optical selection rules
and Raman tensors are discussed.Comment: 32 pages, 4 figure
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