171 research outputs found
Dimensional Consistency Analysis in Complex Algebraic Models
Relations in complex algebraic models include numerous variables and parameter that capture the physical dimensions of the objects represented in models (such as "mass", or "volume" of an object). A model developer must ensure the semantic correctness of the model, which includes consistency across physical dimensions and their units of measure in the model relations. Such dimensional consistency analysis is the subject of the research described in this paper.
We propose a new methodological framework for this type of analysis which comprises:
- a two-level structure for representing knowledge about physical dimensions and units of measure; and
- the dimensional analysis algorithm that uses this structured knowledge for the verification of consistency.
The proposed methodology allows us to resolve issues related to handling complex non-decomposable units of measure and the situation when instances of the same physical dimension are associated with different physical quantities. We illustrate the proposed methodological framework using mathematical relations from a comprehensive environmental model developed at IIASA
Quark-Gluon Plasma/Black Hole duality from Gauge/Gravity Correspondence
The Quark-Gluon Plasma (QGP) is the QCD phase of matter expected to be formed
at small proper-times in the collision of heavy-ions at high energy.
Experimental observations seem to favor a strongly coupled QCD plasma with the
hydrodynamic properties of a quasi-perfect fluid, i.e. rapid thermalization (or
isotropization) and small viscosity. The theoretical investigation of such
properties is not obvious, due to the the strong coupling. The Gauge/Gravity
correspondence provides a stimulating framework to explore the strong coupling
regime of gauge theories using the dual string description. After a brief
introduction to Gauge/Gravity duality, and among various existing studies, we
focus on challenging problems of QGP hydrodynamics, such as viscosity and
thermalization, in terms of gravitational duals of both the static and
relativistically evolving plasma. We show how a Black Hole geometry arises
naturally from the dual properties of a nearly perfect fluid and explore the
lessons and prospects one may draw for actual heavy ion collisions from the
Gauge/Gravity duality approach.Comment: 6 pages, 4 figures, invited talk at the EPS HEP 2007 Conference,
Manchester (UK), and at the ``Deuxiemes rencontres PQG-France'', Etretat
(2007); reference adde
Naturalistic stimuli reveal a dominant role for agentic action in visual representation
Abstract Naturalistic, dynamic movies evoke strong, consistent, and information-rich patterns of activity over a broad expanse of cortex and engage multiple perceptual and cognitive systems in parallel. The use of naturalistic stimuli enables functional brain imaging research to explore cognitive domains that are poorly sampled in highly-controlled experiments. These domains include perception and understanding of agentic action, which plays a larger role in visual representation than was appreciated from experiments using static, controlled stimuli
Running anti-de Sitter radius from QCD-like strings
We consider renormalization effects for a bosonic QCD-like string, whose
partons have propagators instead of Gaussian. Classically this model
resembles (the bosonic part of) the projective light-cone (zero-radius) limit
of a string on an AdS background, where Schwinger parameters give rise to
the fifth dimension. Quantum effects generate dynamics for this dimension,
producing an AdS background with a running radius. The projective
light-cone is the high-energy limit: Holography is enforced dynamically.Comment: 12 page
Neural Responses to Naturalistic Clips of Behaving Animals Under Two Different Task Contexts
The human brain rapidly deploys semantic information during perception to facilitate our interaction with the world. These semantic representations are encoded in the activity of distributed populations of neurons (Haxby et al., 2001; McClelland and Rogers, 2003; Kriegeskorte et al., 2008b) and command widespread cortical real estate (Binder et al., 2009; Huth et al., 2012). The neural representation of a stimulus can be described as a location (i.e., response vector) in a high-dimensional neural representational space (Kriegeskorte and Kievit, 2013; Haxby et al., 2014). This resonates with behavioral and theoretical work describing mental representations of objects and actions as being organized in a multidimensional psychological space (Attneave, 1950; Shepard, 1958, 1987; Edelman, 1998; Gärdenfors and Warglien, 2012). Current applications of this framework to neural representation (e.g., Kriegeskorte et al., 2008b) often implicitly assume that these neural representational spaces are relatively fixed and context-invariant. In contrast, earlier work emphasized the importance of attention and task demands in actively reshaping representational space (Shepard, 1964; Tversky, 1977; Nosofsky, 1986; Kruschke, 1992). A growing body of work in both electrophysiology (e.g., Sigala and Logothetis, 2002; Sigala, 2004; Cohen and Maunsell, 2009; Reynolds and Heeger, 2009) and human neuroimaging (e.g., Hon et al., 2009; Jehee et al., 2011; Brouwer and Heeger, 2013; Çukur et al., 2013; Sprague and Serences, 2013; Harel et al., 2014; Erez and Duncan, 2015; Nastase et al., 2017) has suggested mechanisms by which behavioral goals dynamically alter neural representation
Mode regularization of the susy sphaleron and kink: zero modes and discrete gauge symmetry
To obtain the one-loop corrections to the mass of a kink by mode
regularization, one may take one-half the result for the mass of a widely
separated kink-antikink (or sphaleron) system, where the two bosonic zero modes
count as two degrees of freedom, but the two fermionic zero modes as only one
degree of freedom in the sums over modes. For a single kink, there is one
bosonic zero mode degree of freedom, but it is necessary to average over four
sets of fermionic boundary conditions in order (i) to preserve the fermionic
Z gauge invariance , (ii) to satisfy the basic principle of
mode regularization that the boundary conditions in the trivial and the kink
sector should be the same, (iii) in order that the energy stored at the
boundaries cancels and (iv) to avoid obtaining a finite, uniformly distributed
energy which would violate cluster decomposition. The average number of
fermionic zero-energy degrees of freedom in the presence of the kink is then
indeed 1/2. For boundary conditions leading to only one fermionic zero-energy
solution, the Z gauge invariance identifies two seemingly distinct `vacua'
as the same physical ground state, and the single fermionic zero-energy
solution does not correspond to a degree of freedom. Other boundary conditions
lead to two spatially separated solutions, corresponding to
one (spatially delocalized) degree of freedom. This nonlocality is consistent
with the principle of cluster decomposition for correlators of observables.Comment: 32 pages, 5 figure
Modeling Semantic Encoding in a Common Neural Representational Space
Encoding models for mapping voxelwise semantic tuning are typically estimated separately for each individual, limiting their generalizability. In the current report, we develop a method for estimating semantic encoding models that generalize across individuals. Functional MRI was used to measure brain responses while participants freely viewed a naturalistic audiovisual movie. Word embeddings capturing agent-, action-, object-, and scene-related semantic content were assigned to each imaging volume based on an annotation of the film. We constructed both conventional within-subject semantic encoding models and between-subject models where the model was trained on a subset of participants and validated on a left-out participant. Between-subject models were trained using cortical surface-based anatomical normalization or surface-based whole-cortex hyperalignment. We used hyperalignment to project group data into an individual’s unique anatomical space via a common representational space, thus leveraging a larger volume of data for out-of-sample prediction while preserving the individual’s fine-grained functional–anatomical idiosyncrasies. Our findings demonstrate that anatomical normalization degrades the spatial specificity of between-subject encoding models relative to within-subject models. Hyperalignment, on the other hand, recovers the spatial specificity of semantic tuning lost during anatomical normalization, and yields model performance exceeding that of within-subject models
- …