39,303 research outputs found
Time as a guide to cause
How do people learn causal structure? In two studies we investigated
the interplay between temporal order, intervention and covariational cues. In
Study 1 temporal order overrode covariation information, leading to spurious
causal inferences when the temporal cues were misleading. In Study 2 both
temporal order and intervention contributed to accurate causal inference, well
beyond that achievable through covariational data alone. Together the studies
show that people use both temporal order and interventional cues to infer
causal structure, and that these cues dominate the available statistical
information. We endorse a hypothesis-driven account of learning, whereby
people use cues such as temporal order to generate initial models, and then
test these models against the incoming covariational data
Fisheye Consistency: Keeping Data in Synch in a Georeplicated World
Over the last thirty years, numerous consistency conditions for replicated
data have been proposed and implemented. Popular examples of such conditions
include linearizability (or atomicity), sequential consistency, causal
consistency, and eventual consistency. These consistency conditions are usually
defined independently from the computing entities (nodes) that manipulate the
replicated data; i.e., they do not take into account how computing entities
might be linked to one another, or geographically distributed. To address this
lack, as a first contribution, this paper introduces the notion of proximity
graph between computing nodes. If two nodes are connected in this graph, their
operations must satisfy a strong consistency condition, while the operations
invoked by other nodes are allowed to satisfy a weaker condition. The second
contribution is the use of such a graph to provide a generic approach to the
hybridization of data consistency conditions into the same system. We
illustrate this approach on sequential consistency and causal consistency, and
present a model in which all data operations are causally consistent, while
operations by neighboring processes in the proximity graph are sequentially
consistent. The third contribution of the paper is the design and the proof of
a distributed algorithm based on this proximity graph, which combines
sequential consistency and causal consistency (the resulting condition is
called fisheye consistency). In doing so the paper not only extends the domain
of consistency conditions, but provides a generic provably correct solution of
direct relevance to modern georeplicated systems
Peculiarities of massive vectormesons and their zero mass limits
Massive QED, in contrast with its massless counterpart, possesses two
conserved charges; one is a screened (vanishing) Maxwell charge which is
directly associated with the massive vector mesons through the identically
conserved Maxwell current. A somewhat peculiar situation arises for couplings
of Hermitian matter fields to massive vector potentials; in that case the only
current is the screened Maxwell current and the coupling disappears in the
massless limit. In case of selfinteracting massive vector mesons the situation
becomes even more peculiar in that the usually renormalizability guaranteeing
validity of the first order power-counting criterion breaks down in second
order and requires the compensatory presence of additional Hermitian H-fields.
Some aspect of these observation have already been noticed in the BRST gauge
theoretic formulation, but here we use a new setting based on string-local
vector mesons which is required by Hilbert space positivity. The coupling to
H-fields induces Mexican hat like selfinteractions; they are not imposed and
bear no relation with spontaneous symmetry breaking; they are rather
consequences of the foundational causal localization properties realized in a
Hilbert space setting. In case of selfinteracting massive vectormesons their
presence is required in order to maintain the first order power-counting
restriction of renormalizability also in second order. The presentation of the
new Hilbert space setting for vector mesons which replaces gauge theory and
extends on-shell unitarity to its off-shell counterpart is the main motivation
for this work. The new Hilbert space setting also shows that the second order
Lie-algebra structure of selfinteracting vector mesons is a consequence of the
principles of QFT and promises a deeper understanding of the origin of
confinement.Comment: 34 pages Latex, several additional remarks and citations, improved
formulations, same as published versio
Causal Consistency: Beyond Memory
In distributed systems where strong consistency is costly when not
impossible, causal consistency provides a valuable abstraction to represent
program executions as partial orders. In addition to the sequential program
order of each computing entity, causal order also contains the semantic links
between the events that affect the shared objects -- messages emission and
reception in a communication channel , reads and writes on a shared register.
Usual approaches based on semantic links are very difficult to adapt to other
data types such as queues or counters because they require a specific analysis
of causal dependencies for each data type. This paper presents a new approach
to define causal consistency for any abstract data type based on sequential
specifications. It explores, formalizes and studies the differences between
three variations of causal consistency and highlights them in the light of
PRAM, eventual consistency and sequential consistency: weak causal consistency,
that captures the notion of causality preservation when focusing on convergence
; causal convergence that mixes weak causal consistency and convergence; and
causal consistency, that coincides with causal memory when applied to shared
memory.Comment: 21st ACM SIGPLAN Symposium on Principles and Practice of Parallel
Programming, Mar 2016, Barcelone, Spai
An analysis of update ordering in a cluster of replicated servers
This paper analyses update ordering and its impact on the performance of a cluster of replicated servers. We propose a model for update orderings and constraints and develop a number of algorithms for implementing different ordering constraints. A performance study is then carried out to analyse the update ordering model.<br /
The Ambiguity of Simplicity
A system's apparent simplicity depends on whether it is represented
classically or quantally. This is not so surprising, as classical and quantum
physics are descriptive frameworks built on different assumptions that capture,
emphasize, and express different properties and mechanisms. What is surprising
is that, as we demonstrate, simplicity is ambiguous: the relative simplicity
between two systems can change sign when moving between classical and quantum
descriptions. Thus, notions of absolute physical simplicity---minimal structure
or memory---at best form a partial, not a total, order. This suggests that
appeals to principles of physical simplicity, via Ockham's Razor or to the
"elegance" of competing theories, may be fundamentally subjective, perhaps even
beyond the purview of physics itself. It also raises challenging questions in
model selection between classical and quantum descriptions. Fortunately,
experiments are now beginning to probe measures of simplicity, creating the
potential to directly test for ambiguity.Comment: 7 pages, 6 figures, http://csc.ucdavis.edu/~cmg/compmech/pubs/aos.ht
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