30,496 research outputs found
Causality in Time-Neutral Cosmologies
Gell-Mann and Hartle (GMH) have recently considered time-neutral cosmological
models in which the initial and final conditions are independently specified,
and several authors have investigated experimental tests of such models.
We point out here that GMH time-neutral models can allow superluminal
signalling, in the sense that it can be possible for observers in those
cosmologies, by detecting and exploiting regularities in the final state, to
construct devices which send and receive signals between space-like separated
points. In suitable cosmologies, any single superluminal message can be
transmitted with probability arbitrarily close to one by the use of redundant
signals. However, the outcome probabilities of quantum measurements generally
depend on precisely which past {\it and future} measurements take place. As the
transmission of any signal relies on quantum measurements, its transmission
probability is similarly context-dependent. As a result, the standard
superluminal signalling paradoxes do not apply. Despite their unusual features,
the models are internally consistent.
These results illustrate an interesting conceptual point. The standard view
of Minkowski causality is not an absolutely indispensable part of the
mathematical formalism of relativistic quantum theory. It is contingent on the
empirical observation that naturally occurring ensembles can be naturally
pre-selected but not post-selected.Comment: 5 pages, RevTeX. Published version -- minor typos correcte
Research and development at ORNL/CESAR towards cooperating robotic systems for hazardous environments
One of the frontiers in intelligent machine research is the understanding of how constructive cooperation among multiple autonomous agents can be effected. The effort at the Center for Engineering Systems Advanced Research (CESAR) at the Oak Ridge National Laboratory (ORNL) focuses on two problem areas: (1) cooperation by multiple mobile robots in dynamic, incompletely known environments; and (2) cooperating robotic manipulators. Particular emphasis is placed on experimental evaluation of research and developments using the CESAR robot system testbeds, including three mobile robots, and a seven-axis, kinematically redundant mobile manipulator. This paper summarizes initial results of research addressing the decoupling of position and force control for two manipulators holding a common object, and the path planning for multiple robots in a common workspace
Two-parameter generalization of the logarithm and exponential functions and Boltzmann-Gibbs-Shannon entropy
The -sum () and the
-product
() emerge naturally within nonextensive statistical
mechanics. We show here how they lead to two-parameter (namely, and
) generalizations of the logarithmic and exponential functions (noted
respectively and ), as well as of the
Boltzmann-Gibbs-Shannon entropy
(noted ). The remarkable properties of the
-generalized logarithmic function make the entropic form
to satisfy,
for large regions of , important properties such as {\it
expansibility}, {\it concavity} and {\it Lesche-stability}, but not necessarily
{\it composability}.Comment: 9 pages, 4 figure
On the extent and role of the small proteome in the parasitic eukaryote Trypanosoma brucei
Background: Although technical advances in genomics and proteomics research have yielded a better understanding of the coding capacity of a genome, one major challenge remaining is the identification of all expressed proteins, especially those less than 100 amino acids in length. Such information can be particularly relevant to human pathogens, such as Trypanosoma brucei, the causative agent of African trypanosomiasis, since it will provide further insight into the parasite biology and life cycle. Results: Starting with 993 T. brucei transcripts, previously shown by RNA-Sequencing not to coincide with annotated coding sequences (CDS), homology searches revealed that 173 predicted short open reading frames in these transcripts are conserved across kinetoplastids with 13 also conserved in representative eukaryotes. Mining mass spectrometry data sets revealed 42 transcripts encoding at least one matching peptide. RNAi-induced down-regulation of these 42 transcripts revealed seven to be essential in insect-form trypanosomes with two also required for the bloodstream life cycle stage. To validate the specificity of the RNAi results, each lethal phenotype was rescued by co-expressing an RNAi-resistant construct of each corresponding CDS. These previously non-annotated essential small proteins localized to a variety of cell compartments, including the cell surface, mitochondria, nucleus and cytoplasm, inferring the diverse biological roles they are likely to play in T. brucei. We also provide evidence that one of these small proteins is required for replicating the kinetoplast (mitochondrial) DNA. Conclusions: Our studies highlight the presence and significance of small proteins in a protist and expose potential new targets to block the survival of trypanosomes in the insect vector and/or the mammalian host
Higher Dimensional Taub-NUTs and Taub-Bolts in Einstein-Maxwell Gravity
We present a class of higher dimensional solutions to Einstein-Maxwell
equations in d-dimensions. These solutions are asymptotically locally flat,
de-Sitter, or anti-de Sitter space-times. The solutions we obtained depend on
two extra parameters other than the mass and the nut charge. These two
parameters are the electric charge, q and the electric potential at infinity,
V, which has a non-trivial contribution. We Analyze the conditions one can
impose to obtain Taub-Nut or Taub-Bolt space-times, including the
four-dimensional case. We found that in the nut case these conditions coincide
with that coming from the regularity of the one-form potential at the horizon.
Furthermore, the mass parameter for the higher dimensional solutions depends on
the nut charge and the electric charge or the potential at infinity.Comment: 11 pages, LaTe
N-body Gravity and the Schroedinger Equation
We consider the problem of the motion of bodies in a self-gravitating
system in two spacetime dimensions. We point out that this system can be mapped
onto the quantum-mechanical problem of an N-body generalization of the problem
of the H molecular ion in one dimension. The canonical gravitational
N-body formalism can be extended to include electromagnetic charges. We derive
a general algorithm for solving this problem, and show how it reduces to known
results for the 2-body and 3-body systems.Comment: 15 pages, Latex, references added, typos corrected, final version
that appears in CQ
Probing minimal supergravity in the type-I seesaw mechanism with lepton flavour violation at the CERN LHC
The most general supersymmetric seesaw mechanism has too many parameters to
be predictive and thus can not be excluded by any measurements of lepton
flavour violating (LFV) processes. We focus on the simplest version of the
type-I seesaw mechanism assuming minimal supergravity boundary conditions. We
compute branching ratios for the LFV scalar tau decays, , as well as loop-induced LFV decays at low energy, such as
and , exploring their sensitivity to the
unknown seesaw parameters. We find some simple, extreme scenarios for the
unknown right-handed parameters, where ratios of LFV branching ratios correlate
with neutrino oscillation parameters. If the overall mass scale of the left
neutrinos and the value of the reactor angle were known, the study of LFV
allows, in principle, to extract information about the so far unknown
right-handed neutrino parameters.Comment: 29 pages, 27 figures; added explanatory comments, corrected typos,
final version for publicatio
Quasiclassical Coarse Graining and Thermodynamic Entropy
Our everyday descriptions of the universe are highly coarse-grained,
following only a tiny fraction of the variables necessary for a perfectly
fine-grained description. Coarse graining in classical physics is made natural
by our limited powers of observation and computation. But in the modern quantum
mechanics of closed systems, some measure of coarse graining is inescapable
because there are no non-trivial, probabilistic, fine-grained descriptions.
This essay explores the consequences of that fact. Quantum theory allows for
various coarse-grained descriptions some of which are mutually incompatible.
For most purposes, however, we are interested in the small subset of
``quasiclassical descriptions'' defined by ranges of values of averages over
small volumes of densities of conserved quantities such as energy and momentum
and approximately conserved quantities such as baryon number. The
near-conservation of these quasiclassical quantities results in approximate
decoherence, predictability, and local equilibrium, leading to closed sets of
equations of motion. In any description, information is sacrificed through the
coarse graining that yields decoherence and gives rise to probabilities for
histories. In quasiclassical descriptions, further information is sacrificed in
exhibiting the emergent regularities summarized by classical equations of
motion. An appropriate entropy measures the loss of information. For a
``quasiclassical realm'' this is connected with the usual thermodynamic entropy
as obtained from statistical mechanics. It was low for the initial state of our
universe and has been increasing since.Comment: 17 pages, 0 figures, revtex4, Dedicated to Rafael Sorkin on his 60th
birthday, minor correction
- …