18,541 research outputs found
Navigation of Spacetime Ships in Unified Gravitational and Electromagnetic Waves
On the basis of a "local" principle of equivalence of general relativity, we
consider a navigation in a kind of "4D-ocean" involving measurements of
conformally invariant physical properties only. Then, applying the Pfaff theory
for PDE to a particular conformally equivariant system of differential
equations, we show the dependency of any kind of function describing "spacetime
waves", with respect to 20 parametrizing functions. These latter, appearing in
a linear differential Spencer sequence and determining gauge fields of
deformations relatively to "ship-metrics" or to "flat spacetime ocean metrics",
may be ascribed to unified electromagnetic and gravitational waves. The present
model is based neither on a classical gauge theory of gravitation or a
gravitation theory with torsion, nor on any Kaluza-Klein or Weyl type
unifications, but rather on a post-Newtonian approach of gravitation in a four
dimensional conformal Cosserat spacetime.Comment: 28 pages. Relative to the second version some changes in the
mathematical results have been corrected without consequences in the physical
model. The conformally flatness of the substratum spacetime which is an
assumption used throughout in the mathematical developements from chapter 2,
has been well precised in the first chapter. Clearer explanations at the very
end of chapter 3 about accelerating frames are given. New references are
indicated and some of them correcte
Relativistic Stereometric Coordinates from Relativistic Localizing Systems and the Projective Geometry of the Spacetime Manifold
Relativistic stereometric coordinates supplied by relativistic auto-locating
positioning systems made up of four satellites supplemented by a fifth one are
defined in addition to the well-known emission and reception coordinates. Such
a constellation of five satellites defines a so-called relativistic localizing
system. The determination of such systems is motivated by the need to not only
locate (within a grid) users utilizing receivers but, more generally, to
localize any spacetime event. The angles measured on the celestial spheres of
the five satellites enter into the definition. Therefore, there are, up to
scalings, intrinsic physical coordinates related to the underlying conformal
structure of spacetime. Moreover, they indicate that spacetime must be endowed
everywhere with a local projective geometry characteristic of a so-called
generalized Cartan space locally modeled on four-dimensional, real projective
space. The particular process of localization providing the relativistic
stereometric coordinates is based, in a way, on an enhanced notion of parallax
in space and time generalizing the usual parallax restricted to space only.Comment: Preprint version of Sec. VIII in the HAL-INRIA document with
reference: hal-00945515, v1. One bibliographic reference (Blagojevic et al.)
more with respect to version
Hadronic and Spin Physics: Review
I will summarize the numerous contributions which were presented in the
session,
Hadronic and Spin Physics, largely dominated by new experimental results.Comment: 9 pages, 4 figures, Invited talk presented at CIPANP 2009, San Diego,
California, USA, May 26-31 2009, To appear in the AIP Conference Proceeding
Spacetime deployments parametrized by gravitational and electromagnetic fields
On the basis of a "Punctual" Equivalence Principle of the general relativity
context, we consider spacetimes with measurements of conformally invariant
physical properties. Then, applying the Pfaff theory for PDE to a particular
conformally equivariant system of differential equations, we make explicit the
dependence of any kind of function describing a "spacetime deployment", on
n(n+1) parametrizing functions, denoting by n the spacetime dimension. These
functions, appearing in a linear differential Spencer sequence and determining
gauge fields of spacetime deformations relatively to a "substrat spacetime",
can be consistently ascribed to unified electromagnetic and gravitational
fields, at any spacetime dimensions n greater or equal to 4.Comment: 26 pages, LaTeX2e, file macro "suppl.sty", correction in the
definition of germs and local ring
Fractal Strings and Multifractal Zeta Functions
For a Borel measure on the unit interval and a sequence of scales that tend
to zero, we define a one-parameter family of zeta functions called multifractal
zeta functions. These functions are a first attempt to associate a zeta
function to certain multifractal measures. However, we primarily show that they
associate a new zeta function, the topological zeta function, to a fractal
string in order to take into account the topology of its fractal boundary. This
expands upon the geometric information garnered by the traditional geometric
zeta function of a fractal string in the theory of complex dimensions. In
particular, one can distinguish between a fractal string whose boundary is the
classical Cantor set, and one whose boundary has a single limit point but has
the same sequence of lengths as the complement of the Cantor set. Later work
will address related, but somewhat different, approaches to multifractals
themselves, via zeta functions, partly motivated by the present paper.Comment: 32 pages, 9 figures. This revised version contains new sections and
figures illustrating the main results of this paper and recent results from
others. Sections 0, 2, and 6 have been significantly rewritte
Neural mechanisms of reactivation-induced updating that enhance and distort memory
We remember a considerable number of personal experiences because we are frequently reminded of them, a process known as memory reactivation. Although memory reactivation helps to stabilize and update memories, reactivation may also introduce distortions if novel information becomes incorporated with memory. Here we used functional magnetic resonance imaging (fMRI) to investigate the neural mechanisms mediating reactivation-induced updating in memory for events experienced during a museum tour. During scanning, participants were shown target photographs to reactivate memories from the museum tour followed by a novel lure photograph from an alternate tour. Later, participants were presented with target and lure photographs and asked to determine whether the photographs showed a stop they visited during the tour. We used a subsequent memory analysis to examine neural recruitment during reactivation that was associated with later true and false memories. We predicted that the quality of reactivation, as determined by online ratings of subjective recollection, would increase subsequent true memories but also facilitate incorporation of the lure photograph, thereby increasing subsequent false memories. The fMRI results revealed that the quality of reactivation modulated subsequent true and false memories via recruitment of left posterior parahippocampal, bilateral retrosplenial, and bilateral posterior inferior parietal cortices. However, the timing of neural recruitment and the way in which memories were reactivated contributed to differences in whether memory reactivation led to distortions or not. These data reveal the neural mechanisms recruited during memory reactivation that modify how memories will be subsequently retrieved, supporting the flexible and dynamic aspects of memory
The effect of the integration interval on the measurement accuracy of RMS values and powers in systems with nonsinusoidal waveforms
In this paper the possibility of errors in the measurement of average values (in particular rms values or active powers) in power systems under nonsinusoidal conditions are discussed. The errors considered are either due to the fact that the measurement time interval is not an exact multiple of the fundamental period of the voltage and current signals, or due to the presence of interharmonics or subharmonics. The errors are calculated and the results are illustrated by means of simple examples
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