18,381 research outputs found
A glance beyond the quantum model
One of the most important problems in Physics is how to reconcile Quantum
Mechanics with General Relativity. Some authors have suggested that this may be
realized at the expense of having to drop the quantum formalism in favor of a
more general theory. However, as the experiments we can perform nowadays are
far away from the range of energies where we may expect to observe non-quantum
effects, it is difficult to theorize at this respect. Here we propose a
fundamental axiom that we believe any reasonable post-quantum theory should
satisfy, namely, that such a theory should recover classical physics in the
macroscopic limit. We use this principle, together with the impossibility of
instantaneous communication, to characterize the set of correlations that can
arise between two distant observers. Although several quantum limits are
recovered, our results suggest that quantum mechanics could be falsified by a
Bell-type experiment if both observers have a sufficient number of detectors
Biodiversity informatics: the challenge of linking data and the role of shared identifiers
A major challenge facing biodiversity informatics is integrating data stored in widely distributed databases. Initial efforts have relied on taxonomic names as the shared identifier linking records in different databases. However, taxonomic names have limitations as identifiers, being neither stable nor globally unique, and the pace of molecular taxonomic and phylogenetic research means that a lot of information in public sequence databases is not linked to formal taxonomic names. This review explores the use of other identifiers, such as specimen codes and GenBank accession numbers, to link otherwise disconnected facts in different databases. The structure of these links can also be exploited using the PageRank algorithm to rank the results of searches on biodiversity databases. The key to rich integration is a commitment to deploy and reuse globally unique, shared identifiers (such as DOIs and LSIDs), and the implementation of services that link those identifiers
Unifying Parsimonious Tree Reconciliation
Evolution is a process that is influenced by various environmental factors,
e.g. the interactions between different species, genes, and biogeographical
properties. Hence, it is interesting to study the combined evolutionary history
of multiple species, their genes, and the environment they live in. A common
approach to address this research problem is to describe each individual
evolution as a phylogenetic tree and construct a tree reconciliation which is
parsimonious with respect to a given event model. Unfortunately, most of the
previous approaches are designed only either for host-parasite systems, for
gene tree/species tree reconciliation, or biogeography. Hence, a method is
desirable, which addresses the general problem of mapping phylogenetic trees
and covering all varieties of coevolving systems, including e.g., predator-prey
and symbiotic relationships. To overcome this gap, we introduce a generalized
cophylogenetic event model considering the combinatorial complete set of local
coevolutionary events. We give a dynamic programming based heuristic for
solving the maximum parsimony reconciliation problem in time O(n^2), for two
phylogenies each with at most n leaves. Furthermore, we present an exact
branch-and-bound algorithm which uses the results from the dynamic programming
heuristic for discarding partial reconciliations. The approach has been
implemented as a Java application which is freely available from
http://pacosy.informatik.uni-leipzig.de/coresym.Comment: Peer-reviewed and presented as part of the 13th Workshop on
Algorithms in Bioinformatics (WABI2013
Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9342058
We report on two new quiescent {\it XMM-Newton} observations (in addition to
the earlier {\it Swift}/XRT and {\it XMM-Newton} coverage) of the cooling
neutron star crust in the low-mass X-ray binary 1RXS J180408.9342058. Its
crust was heated during the 4.5 month accretion outburst of the source.
From our quiescent observations, fitting the spectra with a neutron star
atmosphere model, we found that the crust had cooled from 100 eV to
73 eV from 8 days to 479 days after the end of its outburst.
However, during the most recent observation, taken 860 days after the end
of the outburst, we found that the crust appeared not to have cooled further.
This suggested that the crust had returned to thermal equilibrium with the
neutron star core. We model the quiescent thermal evolution with the
theoretical crustal cooling code NSCool and find that the source requires a
shallow heat source, in addition to the standard deep crustal heating
processes, contributing 0.9 MeV per accreted nucleon during outburst to
explain its observed temperature decay. Our high quality {\it XMM-Newton} data
required an additional hard component to adequately fit the spectra. This
slightly complicates our interpretation of the quiescent data of 1RXS
J180408.9342058. The origin of this component is not fully understood.Comment: Accepted for publication by MNRA
Classical and quantum general relativity: a new paradigm
We argue that recent developments in discretizations of classical and quantum
gravity imply a new paradigm for doing research in these areas. The paradigm
consists in discretizing the theory in such a way that the resulting discrete
theory has no constraints. This solves many of the hard conceptual problems of
quantum gravity. It also appears as a useful tool in some numerical simulations
of interest in classical relativity. We outline some of the salient aspects and
results of this new framework.Comment: 8 pages, one figure, fifth prize of the Gravity Research Foundation
2005 essay competitio
Complex Instantons and Charged Rotating Black Hole Pair Creation
We consider the general process of pair-creation of charged rotating black
holes. We find that instantons which describe this process are necessarily
complex due to regularity requirements. However their associated probabilities
are real, and fully consistent with the interpretation that the entropy of a
charged rotating black hole is the logarithm of the number of its quantum
states.Comment: 11 pages, 1 figure, Latex, text shortened with only minor changes in
content, accepted for Phys Rev Letter
Can black holes and naked singularities be detected in accelerators?
We study the conditions for the existence of black holes that can be produced
in colliders at TeV-scale if the space-time is higher dimensional. On employing
the microcanonical picture, we find that their life-times strongly depend on
the details of the model. If the extra dimensions are compact (ADD model),
microcanonical deviations from thermality are in general significant near the
fundamental TeV mass and tiny black holes decay more slowly than predicted by
the canonical expression, but still fast enough to disappear almost
instantaneously. However, with one warped extra dimension (RS model),
microcanonical corrections are much larger and tiny black holes appear to be
(meta)stable. Further, if the total charge is not zero, we argue that naked
singularities do not occur provided the electromagnetic field is strictly
confined on an infinitely thin brane. However, they might be produced in
colliders if the effective thickness of the brane is of the order of the
fundamental length scale (~1/TeV).Comment: 6 pages, RevTeX 3, 1 figure and 1 table, important changes and
addition
Cosmological Measures without Volume Weighting
Many cosmologists (myself included) have advocated volume weighting for the
cosmological measure problem, weighting spatial hypersurfaces by their volume.
However, this often leads to the Boltzmann brain problem, that almost all
observations would be by momentary Boltzmann brains that arise very briefly as
quantum fluctuations in the late universe when it has expanded to a huge size,
so that our observations (too ordered for Boltzmann brains) would be highly
atypical and unlikely. Here it is suggested that volume weighting may be a
mistake. Volume averaging is advocated as an alternative. One consequence may
be a loss of the argument that eternal inflation gives a nonzero probability
that our universe now has infinite volume.Comment: 15 pages, LaTeX, added references for constant-H hypersurfaces and
also an idea for minimal-flux hypersurface
Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance
We have recently argued that if one introduces a relational time in quantum
mechanics and quantum gravity, the resulting quantum theory is such that pure
states evolve into mixed states. The rate at which states decohere depends on
the energy of the states. There is therefore the question of how this can be
reconciled with Galilean invariance. More generally, since the relational
description is based on objects that are not Dirac observables, the issue of
covariance is of importance in the formalism as a whole. In this note we work
out an explicit example of a totally constrained, generally covariant system of
non-relativistic particles that shows that the formula for the relational
conditional probability is a Galilean scalar and therefore the decoherence rate
is invariant.Comment: 10 pages, RevTe
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