8,200 research outputs found
Enactivism, action and normativity: a Wittgensteinian analysis
In this paper, we offer a criticism, inspired by Wittgenstein’s rule-following considerations, of the enactivist account of perception and action. We start by setting up a non-descriptivist naturalism regarding the mind and continue by defining enactivism and exploring its more attractive theoretical features. We then proceed to analyse its proposal to understand normativity non-socially. We argue that such a thesis is ultimately committed to the problematic idea that normative practices can be understood as private and factual. Finally, we offer a characterization of normativity as an essentially social phenomenon and apply our criticisms to other approaches that share commitments with enactivism
Dependence of inner accretion disk stress on parameters: the Schwarzschild case
We explore the parameter dependence of inner disk stress in black hole
accretion by contrasting the results of a number of simulations, all employing
3-d general relativistic MHD in a Schwarzschild spacetime. Five of these
simulations were performed with the intrinsically conservative code HARM3D,
which allows careful regulation of the disk aspect ratio, H/R; our simulations
span a range in H/R from 0.06 to 0.17. We contrast these simulations with two
previously reported simulations in a Schwarzschild spacetime in order to
investigate possible dependence of the inner disk stress on magnetic topology.
In all cases, much care was devoted to technical issues: ensuring adequate
resolution and azimuthal extent, and averaging only over those time-periods
when the accretion flow is in approximate inflow equilibrium. We find that the
time-averaged radial-dependence of fluid-frame electromagnetic stress is almost
completely independent of both disk thickness and poloidal magnetic topology.
It rises smoothly inward at all radii (exhibiting no feature associated with
the ISCO) until just outside the event horizon, where the stress plummets to
zero. Reynolds stress can also be significant near the ISCO and in the plunging
region; the magnitude of this stress, however, depends on both disk thickness
and magnetic topology. The two stresses combine to make the net angular
momentum accreted per unit rest-mass 7-15% less than the angular momentum of
the ISCO.Comment: Accepted for publication in ApJ, 52 pages, 38 figures, AASTEX.
High-resolution versions can be found at the following links:
http://ccrg.rit.edu/~scn/papers/schwarzstress.ps,
http://ccrg.rit.edu/~scn/papers/schwarzstress.pd
A Second Order Godunov Method for Multidimensional Relativistic Magnetohydrodynamics
We describe a new Godunov algorithm for relativistic magnetohydrodynamics
(RMHD) that combines a simple, unsplit second order accurate integrator with
the constrained transport (CT) method for enforcing the solenoidal constraint
on the magnetic field. A variety of approximate Riemann solvers are implemented
to compute the fluxes of the conserved variables. The methods are tested with a
comprehensive suite of multidimensional problems. These tests have helped us
develop a hierarchy of correction steps that are applied when the integration
algorithm predicts unphysical states due to errors in the fluxes, or errors in
the inversion between conserved and primitive variables. Although used
exceedingly rarely, these corrections dramatically improve the stability of the
algorithm. We present preliminary results from the application of these
algorithms to two problems in RMHD: the propagation of supersonic magnetized
jets, and the amplification of magnetic field by turbulence driven by the
relativistic Kelvin-Helmholtz instability (KHI). Both of these applications
reveal important differences between the results computed with Riemann solvers
that adopt different approximations for the fluxes. For example, we show that
use of Riemann solvers which include both contact and rotational
discontinuities can increase the strength of the magnetic field within the
cocoon by a factor of ten in simulations of RMHD jets, and can increase the
spectral resolution of three-dimensional RMHD turbulence driven by the KHI by a
factor of 2. This increase in accuracy far outweighs the associated increase in
computational cost. Our RMHD scheme is publicly available as part of the Athena
code.Comment: 75 pages, 28 figures, accepted for publication in ApJS. Version with
high resolution figures available from
http://jila.colorado.edu/~krb3u/Athena_SR/rmhd_method_paper.pd
Explaining cooperative groups via social niche construction
Cooperative behaviours can be defined as those that benefit others at an apparent cost to self. How these kinds of behaviours evolve has been a topic of great interest in evolutionary biology, as the Darwinian paradigm seems to suggest that nature will be “red in tooth and claw” and that we would not expect one organism to evolve to help another. The evolution-of-cooperation literature has therefore generally been about showing how the altruism involved in these cases is only apparent (see Bergstrom 2002 for an excellent review). Consider kin selection, in which interactions are more likely to occur between related individuals. The cost of altruism to the individual is real but, having identified the correct score-keeping level as the genetic one, it turns out that the cooperative act is not costly but profitable. More generally, successful explanations for cooperation rely on the presence of a population structure that clusters cooperators together, such that they enjoy the benefits of each others' actions. However, the question that has been left largely unaddressed is how does this structure itself evolve? If we want to really explain why organisms cooperate, then we need to explain not just their adaptation to their social environment, but how they came to live in that environment. Recent work by Powers (2010) and Powers et al. (in press) has addressed this question. They show that social behaviour can exert indirect selection pressure on population structure-modifying traits, causing individuals to adaptively modify their population structure to support greater cooperation. Moreover, they argue that any component of selection on structure-modifying traits that is due to social behaviour must be in the direction of increased cooperation; that component of selection cannot be in favour of the conditions for greater selfishness. Powers et al. then examine the conditions under which this component of selection on population structure exists. They argue that not only can population structure drive the evolution of cooperation, as in classical models, but that the benefits of greater cooperation can in turn drive the evolution of population structure: a positive feedback process that they refer to as social niche construction (after Odling-Smee et al. 2003). Maynard Smith and Szathmary (1995) note that most of the big unanswered questions in biology are not about how a particular behaviour is selected for at one level of organization but about the emergence of whole new levels of organization, e.g., the transition from single- to multi-celled organisms, or from solitary insects to eusocial colonies. Any satisfactory account of these transitions must explain how the individuals came to live in a population structure that supported high degrees of cooperation, as well as showing that cooperation is individually advantageous given that structure. The social niche construction process identified by Powers et al. can explain some of the major transitions, by showing how a new selective level can begin through evolution of individual characters, such as group size preference or dispersal tendency. The potential emergence of reliable cooperation via the co-evolution of individual cooperative and population-structuring behaviours demonstrates that groups of cooperating agents can create an environment in which they become so “locked in” to their group identity that the group warrants redescription as an individual in its own right. Consider the move from independent protozoans, to an intermediate cooperative stage as in slime moulds, to fully multi-cellular animals. Such creation of population structures that support cooperation parallels negotiation of a social contract. What are the philosophical implications of this perspective for understanding and explaining human social behaviour? On the one hand, it gives respectability and unique explanatory value to group-selectionist accounts. Explaining the origin of within-group cooperation and the origin of the groups themselves become part of the same project, which in turn means that we cannot understand social and cooperative behaviour in humans without understanding human population-structuring traits, e.g., living in family groups, group fission-fusion behaviours, migratory behaviours, etc. What will the explanations we seek look like? de Pinedo and Noble (2008) have argued that the description of evolved behaviour cannot be exclusively in mechanistic terms: we need both explanations that focus on an agent’s interaction with its environment, and explanations that focus on the physical or computational enabling conditions of such an interaction. In a context in which what counts as an agent is taken for granted, de Pinedo and Noble argue that both agent and sub-agent level explanations will be required. The perspective being outlined here forces an expansion of that position and reminds us that agency is not to be taken for granted; that it emerges from a lower level of organization after a history of selection brings simpler entities together in a coherent cooperative whole. The implication is that truly multi-level explanations will be necessary in the area of social behaviour. We explain the origin of the multi-cellular organism as the result of a cooperative merger of single-celled organisms, and we explain the origin of a super-organism such as an ant colony in a similar way. At each transition, the autonomous agents of the previous level become component mechanisms in the next, but no explanatory level can be entirely done away with. A human being is an example of a multi-cellular organism with a highly developed social aspect, occupying an intermediate point between radical individual independence and total group cohesion. To fully explain human behaviour, we need to know about the cellular machinery that enables personal-level agency. But we also need to know how human machinery fits together into families, communities and nations that will, at least partially, have their own emergent goals and purposes: “partially” because we are not yet a super-organism, of course. In conclusion, the perspective we outline suggests a view of the social contract as not at all unique to Hobbesian rational agents who have become tired of an insecure and violent lifestyle. Instead the ongoing negotiation of the social contract amongst ourselves can be seen as echoing earlier, now-successfully-concluded negotiations between the entities that became our genes and then our cells
The Roman Villa: An Investigation of Typology
The Roman villa is a distinct typology emerging from urban precedents which are transformed by site, programs, and constructional methods and materials in order to represent the Roman city. The reconstruction of Pliny\u27s Laurentine villa, as described in his letter to Clusinius Gallus, provides the context for this investigation of typology and for an exploration of Pliny\u27s urban reproposition...
This scenario allows the incorporation and transformation of typological elements as well as spatial comparisons relative to the chronology o Roman structural technology. The language of the villa reflects the actual materials of the Romans while their assembly necessarily reflects the era of this reconstruction
The Public Work: Redefining the Standard for an Urban Society
A common lament of our era is that our cities are undefined, usually in reference to the lack of an easily circumscribed boundary. The once defined, bounded, defensible city is committed to history, undone by modern weaponry, howitzers, bombers, automobiles and their highways, and the interconnectivity of cyberspace
GRMHD prediction of coronal variability in accreting black holes
On the basis of data from an energy-conserving 3D general relativistic MHD
simulation, we predict the statistical character of variability in the coronal
luminosity from accreting black holes. When the inner boundary of the corona is
defined to be the electron scattering photosphere, its location depends only on
the mass accretion rate in Eddington units (\dot{M}). Nearly independent of
viewing angle and \dot{M}, the power spectrum over the range of frequencies
from approximately the orbital frequency at the innermost stable circular orbit
(ISCO) to ~100 times lower is well approximated by a power-law with index -2,
crudely consistent with the observed power spectra of hard X-ray fluctuations
in AGN and the hard states of Galactic binary black holes. The underlying
physical driver for variability in the light curve is variations in the
accretion rate caused by the chaotic character of MHD turbulence, but the power
spectrum of the coronal light output is significantly steeper. Part of this
contrast is due to the fact that the mass accretion rate can be significantly
modulated by radial epicyclic motions that do not result in dissipation, and
therefore do not drive luminosity fluctuations. The other part of this contrast
is due to the inward decrease of the characteristic inflow time, which leads to
decreasing radial coherence length with increasing fluctuation frequency.Comment: Accepted for publication in ApJ, 35 pages, 11 figures (8 color and 3
greyscale), AASTEX. High-resolution versions can be found at the following
links: [PS] http://www.pha.jhu.edu/~scn/papers/grmhd_var.ps [PDF]
http://www.pha.jhu.edu/~scn/papers/grmhd_var.pd
Enhancement of biogas potential of primary sludge by co-digestion with cow manure and brewery sludge
Anaerobic digestion (AD) has long been used to treat different types of organic wastes especially in the developed world. However, organic wastes are still more often considered as a waste instead of a resource in the developing world, which contributes to environmental pollution arising from their disposal. This study has been conducted at Bugolobi Sewage Treatment Plant (BSTP), where two organic wastes, cow manure and brewery sludge were co-digested with primary sludge in different proportions. This study was done in lab-scale reactors at mesophilic temperature and sludge retention time of 20 d. The main objective was to evaluate the biodegradability of primary sludge generated at BSTP, Kampala, Uganda and enhance its ability of biogas production. When the brewery sludge was added to primary STP sludge at all proportions, the biogas production rate increased by a factor of 3. This was significantly (p<0.001) higher than observed gas yield (337 +/- 18 mL/(L.d)) in the control treatment containing (only STP sludge). Co-digesting STP sludge with cow manure did not show different results compared to the control treatment. In conclusion, Bugolobi STP sludge is poorly anaerobically degradable with low biogas production but co-digestion with brewery sludge enhanced the biogas production rate, while co-digestion with cow manure was not beneficial
The Submillimeter Bump in Sgr A* from Relativistic MHD Simulations
Recent high resolution observations of the Galactic center black hole allow
for direct comparison with accretion disk simulations. We compare
two-temperature synchrotron emission models from three dimensional, general
relativistic magnetohydrodynamic simulations to millimeter observations of Sgr
A*. Fits to very long baseline interferometry and spectral index measurements
disfavor the monochromatic face-on black hole shadow models from our previous
work. Inclination angles \le 20 degrees are ruled out to 3 \sigma. We estimate
the inclination and position angles of the black hole, as well as the electron
temperature of the accretion flow and the accretion rate, to be i=50+35-15
degrees, \xi=-23+97-22 degrees, T_e=(5.4 +/- 3.0)x10^10 K and
Mdot=(5+15-2)x10^-9 M_sun / yr respectively, with 90% confidence. The black
hole shadow is unobscured in all best fit models, and may be detected by
observations on baselines between Chile and California, Arizona or Mexico at
1.3mm or .87mm either through direct sampling of the visibility amplitude or
using closure phase information. Millimeter flaring behavior consistent with
the observations is present in all viable models, and is caused by magnetic
turbulence in the inner radii of the accretion flow. The variability at
optically thin frequencies is strongly correlated with that in the accretion
rate. The simulations provide a universal picture of the 1.3mm emission region
as a small region near the midplane in the inner radii of the accretion flow,
which is roughly isothermal and has \nu/\nu_c ~ 1-20, where \nu_c is the
critical frequency for thermal synchrotron emission.Comment: 14 pages, 17 figures, accepted by Ap
Radiative efficiency and thermal spectrum of accretion onto Schwarzschild black holes
Recent general relativistic magneto-hydrodynamic (MHD) simulations of
accretion onto black holes have shown that, contrary to the basic assumptions
of the Novikov-Thorne model, there can be substantial magnetic stress
throughout the plunging region. Additional dissipation and radiation can
therefore be expected. We use data from a particularly well-resolved simulation
of accretion onto a non-spinning black hole to compute both the radiative
efficiency of such a flow and its spectrum if all emitted light is radiated
with a thermal spectrum whose temperature matches the local effective
temperature. This disk is geometrically thin enough (H/r ~= 0.06) that little
heat is retained in the flow. In terms of light reaching infinity (i.e., after
allowance for all relativistic effects and for photon capture by the black
hole), we find that the radiative efficiency is at least ~=6-10% greater than
predicted by the Novikov-Thorne model (complete radiation of all heat might
yield another ~6%). We also find that the spectrum more closely resembles the
Novikov-Thorne prediction for a/M ~= 0.2--0.3 than for the correct value,
a/M=0. As a result, if the spin of a non-spinning black hole is inferred by
model-fitting to a Novikov-Thorne model with known black hole mass, distance,
and inclination, the inferred a/M is too large by ~= 0.2--0.3.Comment: Submitted to ApJ, 26 pages, 12 figures (some in color), AASTE
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