235 research outputs found
About Superluminal motions and Special Relativity: A Discussion of some recent Experiments, and the solution of the Causal Paradoxes
Some experiments, performed at Berkeley, Cologne, Florence, Vienna, Orsay,
Rennes, etc., led to the claim that something seems to travel with a group
velocity larger than the speed c of light in vacuum. Various other experimental
results seem to point in the same direction: For instance, localized wavelet-
type solutions to Maxwell equations have been found, both theoretically and
experimentally, that travel with superluminal speed. [Even muonic and
electronic neutrinos [it has been proposed] might be "tachyons", since their
square mass appears to be negative]. With regard to the first-mentioned
experiments, it was recently claimed by Guenter Nimtz that those results with
evanescent waves (or tunneling photons) imply superluminal signal and impulse
transmission, and therefore violate Einstein causality. In this note we want to
stress that, on the contrary, all such results do not place relativistic
causality in jeopardy, even if they referred to actual tachyonic motions: In
fact, Special Relativity can cope even with superluminal objects and waves. For
instance, it is possible (at least in microphysics) to solve also the known
causal paradoxes, devised for faster than light motion, although this is not
widely recognized yet. Here we show, in detail and rigorously, how to solve the
oldest causal paradox, originally proposed by Tolman, which is the kernel of
many further tachyon paradoxes (like J.Bell's, F.A.E.Pirani's, J.D.Edmonds' and
others'). The key to the solution is a careful application of tachyon
mechanics, as it unambiguously follows from special relativity. At Last, in one
of the two Appendices, we propose how to evaluate the group-velocity in the
case of evanescent waves. [PACS nos.: 03.30.+p; 03.50.De; 41.20.Jb; 73.40.Gk;
84.40.Az; 42.82.Et ]Comment: LaTeX file: 26 pages, with 5 Figures (and two Appendices). The
original version of this paper appeared in the Journal below
Tractarian Objects and Logical Categories
It has been much debated whether Tractarian objects are what Russell would have called particulars or whether they include also properties and relations. This paper claims that the debate is misguided: there is no logical category such that Wittgenstein intended the reader of the Tractatus to understand his objects either as providing examples of or as not providing examples of that category. This is not to say that Wittgenstein set himself against the very idea of a logical category: quite the contrary. However, where Russell presents his logical variety of particulars and the various types of universal, and Frege presents his of objects and the various types of function, Wittgenstein denies the propriety of such a priori expositions. Wittgenstein envisages a variety of logical types of entity but insists that the nature of these types is something to be discovered only through analysis
Relating imperatives to action
The aim of this chapter is to provide an analysis of the use of logically complex imperatives, in particular, imperatives of the form Do A1 or A2 and Do A, if B. We argue for an analysis of imperatives in terms of classical logic which takes into account the influence of background information on imperatives. We show that by doing so one can avoid some counter-intuitive results which have been associated with analyses of imperatives in terms of classical logic. In particular, I address Hamblin's observations concerning rule-like imperatives and Ross' Paradox. The analysis is carried out within an agent-based logical framework. This analysis explicates what it means for an agent to have a successful policy for action with respect to satisfying his or her commitments, where some of these commitments have been introduced as a result of imperative language use
Physical interpretation of stochastic Schroedinger equations in cavity QED
We propose physical interpretations for stochastic methods which have been
developed recently to describe the evolution of a quantum system interacting
with a reservoir. As opposed to the usual reduced density operator approach,
which refers to ensemble averages, these methods deal with the dynamics of
single realizations, and involve the solution of stochastic Schr\"odinger
equations. These procedures have been shown to be completely equivalent to the
master equation approach when ensemble averages are taken over many
realizations. We show that these techniques are not only convenient
mathematical tools for dissipative systems, but may actually correspond to
concrete physical processes, for any temperature of the reservoir. We consider
a mode of the electromagnetic field in a cavity interacting with a beam of two-
or three-level atoms, the field mode playing the role of a small system and the
atomic beam standing for a reservoir at finite temperature, the interaction
between them being given by the Jaynes-Cummings model. We show that the
evolution of the field states, under continuous monitoring of the state of the
atoms which leave the cavity, can be described in terms of either the Monte
Carlo Wave-Function (quantum jump) method or a stochastic Schr\"odinger
equation, depending on the system configuration. We also show that the Monte
Carlo Wave-Function approach leads, for finite temperatures, to localization
into jumping Fock states, while the diffusion equation method leads to
localization into states with a diffusing average photon number, which for
sufficiently small temperatures are close approximations to mildly squeezed
states.Comment: 12 pages RevTeX 3.0 + 6 figures (GIF format; for higher-resolution
postscript images or hardcopies contact the authors.) Submitted to Phys. Rev.
Meaning and Dialogue Coherence: A Proof-theoretic Investigation
This paper presents a novel proof-theoretic account of dialogue coherence. It focuses on an abstract class of cooperative information-oriented dialogues and describes how their structure can be accounted for in terms of a multi-agent hybrid inference system that combines natural deduction with information transfer and observation. We show how certain dialogue structures arise out of the interplay between the inferential roles of logical connectives (i.e., sentence semantics), a rule for transferring information between agents, and a rule for information flow between agents and their environment. The order of explanation is opposite in direction to that adopted in game-theoretic semantics, where sentence semantics (or a notion of valid inference) is derived from winning dialogue strategies. That approach and the current one may, however, be reconcilable, since we focus on cooperative dialogue, whereas the game-theoretic tradition concentrates on adversarial dialogue
Estrogen-Like Effects of Cadmium in Vivo Do Not Appear to be Mediated via the Classical Estrogen Receptor Transcriptional Pathway
Cadmium is a toxic metal classified as human carcinogen and ubiquitously found in our
environment mainly from anthropogenic activities. Exposure to cadmium has been
associated with increased risk of certain hormone-dependent cancers in humans, and the
metal has been proposed to possess endocrine disruptive properties by mimicking the
physiological actions of estrogens. However, the mechanisms behind these effects are
unclear.
The overall aim of this thesis was to provide mechanistic insights into the
estrogenicity of cadmium that may have implications for the human health. To achieve
this aim, investigations on the estrogen-like effects of cadmium as well as possible
involvement of classical/non-classical estrogen receptor signaling was studied in mice,
and these mechanisms were further scrutinized in cell-based models. Furthermore,
associations of biomarker of cadmium exposure with endogenous circulating sex
hormones were evaluated in a population-based study of women.
Results presented here indicate that exposure to cadmium does not affect the genomic
estrogen response in vivo in mice, suggesting that classical estrogen signaling is not
targeted by cadmium. However, some estrogen-like effects were observed in cadmium
exposed mice, i.e. significant thickening of uterine epithelia, in the absence of uterine
weight increase, and activation of ERK1/2 MAPKs in the liver. This suggests the
existence of alternative signaling pathways modulated by cadmium. In addition,
exposure to a wide dose range of cadmium, dose-dependently increased the expression
of the endogenous genes Mt1, Mt2, p53, c-fos, and Mdm2 in mouse liver, with p53 being
the most sensitive gene. However, phosphorylation of ERK1/2 was already induced at
the lowest exposure level (0.5µg/kg body weight), rendering ERK1/2 a more sensitive
marker of exposure than any change in gene expression. Furthermore, in vivo findings
suggest that cadmium-induced effects are markedly concentration dependent: low-level
exposure activates protein-kinases whereas high-level exposure turns on cellular stress
responses. The data from in vitro studies indicate that cadmium at regular human
exposure levels activates protein-kinase signaling through Raf-MEK-ERK/MAPKs, and
we identified EGFR and GPR30 as the mediating receptors. This cadmium-induced
activation of protein-kinases further leads to a disturbance in Mdm2/p53 balance, with a
significant increase in the Mdm2/p53 ratio in the presence of genotoxic compounds,
which in turn suggest that cadmium may disrupt stress response to genotoxins. In 438
postmenopausal women, a positive association was observed between the concentrations
of cadmium in blood and testosterone in serum, while an inverse association was
observed with estradiol. This may suggest that cadmium affects steroidogenesis.
In conclusion, data presented in this thesis collectively suggests that cadmium-induced
estrogen-like effects do not involve classical estrogen receptor signaling but rather
appear to be mediated via membrane-associated signaling. The activation/
transactivation of GPR30/EGFR-Raf-MEK-ERK/MAPKs and Mdm2 represent a general
mechanism by which cadmium may exert its effects. Since EGFR, ERK and Mdm2 are
all known key players in cancer promotion, cadmium-induced activation of these and
disturbance in the estradiol/testosterone balance in women may have implications for the
promotion/development of hormone-related cancers
Non-Markovian Decay of a Three Level Cascade Atom in a Structured Reservoir
We present a formalism that enables the study of the non-Markovian dynamics
of a three-level ladder system in a single structured reservoir. The
three-level system is strongly coupled to a bath of reservoir modes and two
quantum excitations of the reservoir are expected. We show that the dynamics
only depends on reservoir structure functions, which are products of the mode
density with the coupling constant squared. This result may enable pseudomode
theory to treat multiple excitations of a structured reservoir. The treatment
uses Laplace transforms and an elimination of variables to obtain a formal
solution. This can be evaluated numerically (with the help of a numerical
inverse Laplace transform) and an example is given. We also compare this result
with the case where the two transitions are coupled to two separate structured
reservoirs (where the example case is also analytically solvable)
Bose-Einstein statistics in thermalization and photoluminescence of quantum well excitons
Quasi-equilibrium relaxational thermodynamics is developed to understand
LA-phonon-assisted thermalization of Bose-Einstein distributed excitons in
quantum wells. We study the quantum-statistical effects in the relaxational
dynamics of the effective temperature of excitons . When is less
than the degeneracy temperature , well-developed Bose-Einstein statistics
of quantum well excitons leads to nonexponential and density-dependent
thermalization. At low bath temperatures the thermalization of
quantum-statistically degenerate excitons effectively slows down and . We also analyze the optical decay of Bose-Einstein
distributed excitons in perfect quantum wells and show how nonclassical
statistics influences the effective lifetime . In particular,
of a strongly degenerate gas of excitons is given by ,
where is the intrinsic radiative lifetime of quasi-two-dimensional
excitons. Kinetics of resonant photoluminescence of quantum well excitons
during their thermalization is studied within the thermodynamic approach and
taking into account Bose-Einstein statistics. We find density-dependent
photoluminescence dynamics of statistically degenerate excitons. Numerical
modeling of the thermalization and photoluminescence kinetics of
quasi-two-dimensional excitons are given for GaAs/AlGaAs quantum wells.Comment: 19 pages, 9 figures. Phys. Rev. B (accepted for publication
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