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A Tale of Two Globes: Exploring the North/South Divide in Engagement with Open Educational Resources
In this chapter we consider what evidence exists of a divide between the Global North and Global South in terms of engagement with open educational resources (OER), understanding engagement as the production and sharing of educational materials online. We discuss whether identifying educators as contributors or consumers of OER can be empirically grounded, and advocate advancing internet access in developing countries to reach a global balance where sharing is key
Spacetime: Arena or Reality?
For small values of the mass (in relation to the angular momentum and
electric charge), the Kerr-Newman (KN) solution of Einstein equation reduces to
a naked singularity of circular shape. By considering the Hawking and Ellis
extended interpretation of the KN spacetime, as well as Wheeler's idea of
"charge without charge", the non-trivial topological structure of the extended
KN spatial section is found to represent gravitational states with
half-integral angular momentum. As a consequence, it can be consistently
interpreted as a model for the electron-positron system, in which the concepts
of mass, charge and spin emerge from the spacetime geometry. According to this
model, therefore, instead of a simple arena, spacetime must have a concrete
existence, being responsible -- through its highly non-trivial topological
structures -- for the building blocks of (at least some of) the existing matter
in the universe.Comment: Chapter in the book "Relativity and the Dimensionality of the World",
Springer series "Fundamental Theories of Physics", Vol. 153 (2007). Volume
Editor: Vesselin Petko
Kerr-Newman solution as a Dirac particle
For m^2 < a^2 + q^2, with m, a, and q respectively the source mass, angular
momentum per unit mass, and electric charge, the Kerr--Newman (KN) solution of
Einstein's equation reduces to a naked singularity of circular shape, enclosing
a disk across which the metric components fail to be smooth. By considering the
Hawking and Ellis extended interpretation of the KN spacetime, it is shown
first that, similarly to the electron-positron system, this solution presents
four inequivalent classical states. Next, it is shown that due to the
topological structure of the extended KN spacetime it does admit states with
half-integral angular momentum. This last property is corroborated by the fact
that, under a rotation of the space coordinates, those inequivalent states
transform into themselves only after a 4pi rotation. As a consequence, it
becomes possible to naturally represent them in a Lorentz spinor basis. The
state vector representing the whole KN solution is then constructed, and its
evolution is shown to be governed by the Dirac equation. The KN solution can
thus be consistently interpreted as a model for the electron-positron system,
in which the concepts of mass, charge and spin become connected with the
spacetime geometry. Some phenomenological consequences of the model are
explored.Comment: 19 pages, 6 figures. References added, section 2 enhanced, an
appendix and one figure adde
One-loop Parke-Taylor factors for quadratic propagators from massless scattering equations
In this paper we reconsider the Cachazo-He-Yuan construction (CHY) of the so
called scattering amplitudes at one-loop, in order to obtain quadratic
propagators. In theories with colour ordering the key ingredient is the
redefinition of the Parke-Taylor factors. After classifying all the possible
one-loop CHY-integrands we conjecture a new one-loop amplitude for the massless
Bi-adjoint theory. The prescription directly reproduces the quadratic
propagators from of the traditional Feynman approach.Comment: 43 pages, new appendix added, few typos corrected. Accepted for
publication in JHE
Nonrelativistic limit of the abelianized ABJM model and the ADS/CMT correspondence
We consider the nonrelativistic limit of the abelian reduction of the massive
ABJM model proposed in \cite{Mohammed:2012gi}, obtaining a supersymmetric
version of the Jackiw-Pi model. The system exhibits an
Super-Schr\"odinger symmetry with the Jackiw-Pi vortices emerging as BPS
solutions. We find that this -dimensional abelian field theory is dual
to a certain (3+1)-dimensional gravity theory that differs somewhat from
previously considered abelian condensed matter stand-ins for the ABJM model. We
close by commenting on progress in the top-down realization of the AdS/CMT
correspondence in a critical string theory.Comment: 24 pages, no figures; clarifying remarks added, references added,
typos correcte
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