19,054 research outputs found
Designing an e-tutoring system for large classes: mixed-method research
This study aimed at assessing the perceptions of 167 teachers about the tutoring system
adopted in an online training course involving teachers from 20 Schools of Sesimbra, Setúbal
and Palmela counties. The course, called “Distributed Knowledge with Web 2.0”, was
officially certified as a blended learning modality, with the duration of 50 hours, 41 of which
occurred online in two editions, the first in February and the second in July of 2012, each one
of them involving respectively 82 and 85 teachers, divided in four classes with about 20
trainees each. This blended learning course was designed at producing educational materials
in digital format, and included autonomous and group activities, knowledge sharing and
reflection. A learning environment, supported by the Ning platform, was set up. At the end of
the course, the trainees answered to a pencil and paper survey, in order to evaluate the
adopted online tutoring strategy. Additionally the trainees’ final reports contained evidence of
how the trainees assessed the tutoring model component of the course; both the survey and
the reports were the basis for this research. The results show that the teachers who attended
the two course editions disclosed very positive perceptions about online learning, a modality
they consider adequate to their current professional status and conditions. The trainees also
showed their intention of, in the future, opting for blended training arrangements. Future
developments of this study involve a content analysis of the tutor’s posts, in order to
understand more accurately the tutor’s messages characteristics, in their social and cognitive
dimensions
Boundary Conditions for Kerr-AdS Perturbations
The Teukolsky master equation and its associated spin-weighted spheroidal
harmonic decomposition simplify considerably the study of linear gravitational
perturbations of the Kerr(-AdS) black hole. However, the formulation of the
problem is not complete before we assign the physically relevant boundary
conditions. We find a set of two Robin boundary conditions (BCs) that must be
imposed on the Teukolsky master variables to get perturbations that are
asymptotically global AdS, i.e. that asymptotes to the Einstein Static
Universe. In the context of the AdS/CFT correspondence, these BCs allow a
non-zero expectation value for the CFT stress-energy tensor while keeping fixed
the boundary metric. When the rotation vanishes, we also find the gauge
invariant differential map between the Teukolsky and the Kodama-Ishisbashi
(Regge-Wheeler-Zerilli) formalisms. One of our Robin BCs maps to the scalar
sector and the other to the vector sector of the Kodama-Ishisbashi
decomposition. The Robin BCs on the Teukolsky variables will allow for a
quantitative study of instability timescales and quasinormal mode spectrum of
the Kerr-AdS black hole. As a warm-up for this programme, we use the Teukolsky
formalism to recover the quasinormal mode spectrum of global AdS-Schwarzschild,
complementing previous analysis in the literature.Comment: 33 pages, 6 figure
AdS nonlinear instability: moving beyond spherical symmetry
Anti-de Sitter (AdS) is conjectured to be nonlinear unstable to a weakly
turbulent mechanism that develops a cascade towards high frequencies, leading
to black hole formation [1,2]. We give evidence that the gravitational sector
of perturbations behaves differently from the scalar one studied in [2]. In
contrast with [2], we find that not all gravitational normal modes of AdS can
be nonlinearly extended into periodic horizonless smooth solutions of the
Einstein equation. In particular, we show that even seeds with a single normal
mode can develop secular resonances, unlike the spherically symmetric scalar
field collapse studied in [2]. Moreover, if the seed has two normal modes, more
than one resonance can be generated at third order, unlike the spherical
collapse of [2]. We also show that weak turbulent perturbative theory predicts
the existence of direct and inverse cascades, with the former dominating the
latter for equal energy two-mode seeds.Comment: 7 pages, no figures, 2 table
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