15 research outputs found
The quantum vacuum at the foundations of classical electrodynamics
In the classical theory of electromagnetism, the permittivity and the
permeability of free space are constants whose magnitudes do not seem to
possess any deeper physical meaning. By replacing the free space of classical
physics with the quantum notion of the vacuum, we speculate that the values of
the aforementioned constants could arise from the polarization and
magnetization of virtual pairs in vacuum. A classical dispersion model with
parameters determined by quantum and particle physics is employed to estimate
their values. We find the correct orders of magnitude. Additionally, our simple
assumptions yield an independent estimate for the number of charged elementary
particles based on the known values of the permittivity and the permeability,
and for the volume of a virtual pair. Such interpretation would provide an
intriguing connection between the celebrated theory of classical
electromagnetism and the quantum theory in the weak field limit.Comment: Accepted in Applied Physics B: Special Issue for the 50 years of the
laser. Comments are welcome
Spallative ablation of dielectrics by X-ray laser
Short laser pulse in wide range of wavelengths, from infrared to X-ray,
disturbs electron-ion equilibrium and rises pressure in a heated layer. The
case where pulse duration is shorter than acoustic relaxation time
is considered in the paper. It is shown that this short pulse may cause
thermomechanical phenomena such as spallative ablation regardless to
wavelength. While the physics of electron-ion relaxation on wavelength and
various electron spectra of substances: there are spectra with an energy gap in
semiconductors and dielectrics opposed to gapless continuous spectra in metals.
The paper describes entire sequence of thermomechanical processes from
expansion, nucleation, foaming, and nanostructuring to spallation with
particular attention to spallation by X-ray pulse
Supersymmetric string model with 30 kappa--symmetries in an extended D=11 superspace and 30/ 32 BPS states
A supersymmetric string model in the D=11 superspace maximally extended by
antisymmetric tensor bosonic coordinates, , is proposed. It
possesses 30 -symmetries and 32 target space supersymmetries. The usual
preserved supersymmetry--symmetry correspondence suggests that it
describes the excitations of a BPS state preserving all but two
supersymmetries. The model can also be formulated in any superspace, n=32 corresponding to D=11. It may also be treated as a
`higher--spin generalization' of the usual Green--Schwarz superstring. Although
the global symmetry of the model is a generalization of the super--Poincar\'e
group, , it may be
formulated in terms of constrained OSp(2n|1) orthosymplectic supertwistors. We
work out this supertwistor realization and its Hamiltonian dynamics.
We also give the supersymmetric p-brane generalization of the model. In
particular, the supersymmetric membrane model describes
excitations of a 30/32 BPS state, as the supersymmetric
string does, while the supersymmetric 3-brane and 5-brane correspond,
respectively, to 28/32 and 24/32 BPS states.Comment: 23 pages, RevTex4. V2: minor corrections in title and terminology,
some references and comments adde
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When the native is also a non-native: āretrodictingā the complexity of language teacher cognition
Abstract: The impact of native (NS) and non-native speaker (NNS) identities
on second or foreign language teachersā cognition and practices in the classroom
has mainly been investigated in ESL/EFL contexts. Using complexity
theory as a framework, this case study attempts to fill the gap in the literature
by presenting a foreign language teacher in the United States who teaches
French as a NS and German as a NNS teacher at the college level. Specifically,
the study explores the interface between NS/NNS identities, teacher cognition,
practice, and professional identity. The retrodictive qualitative analysis
of semi-structured interviews and classroom observations reveals that the participant
teacherās NS French and NNS German identities influenced her
teacher cognition, specifically in beliefs about teaching grammar. In addition,
previous language learning experiences affect current decision-making processes
in teaching. In terms of teaching practice, the dual NS French and NNS
German identity affects teaching practice in the formal areas of language, target
culture knowledge and awareness, teaching style, and perceptions about
language varieties. Professional identity is construed here as being the mediator
of target second language (L2) cultures in the classroom. The implications
of teacher cognition as a complex system for L2 teaching and teacher education
are discussed