14,470 research outputs found
Classical and Quantum Implications of the Causality Structure of Two-Dimensional Spacetimes with Degenerate Metrics
The causality structure of two-dimensional manifolds with degenerate metrics
is analysed in terms of global solutions of the massless wave equation. Certain
novel features emerge. Despite the absence of a traditional Lorentzian Cauchy
surface on manifolds with a Euclidean domain it is possible to uniquely
determine a global solution (if it exists), satisfying well defined matching
conditions at the degeneracy curve, from Cauchy data on certain spacelike
curves in the Lorentzian region. In general, however, no global solution
satisfying such matching conditions will be consistent with this data.
Attention is drawn to a number of obstructions that arise prohibiting the
construction of a bounded operator connecting asymptotic single particle
states. The implications of these results for the existence of a unitary
quantum field theory are discussed.Comment: 27 pages LaTex (6 Figures), Journal of Mathematical Physics
(Accepted
Gauge Symmetry and Gravito-Electromagnetism
A tensor description of perturbative Einsteinian gravity about an arbitrary
background spacetime is developed. By analogy with the covariant laws of
electromagnetism in spacetime, gravito-electromagnetic potentials and fields
are defined to emulate electromagnetic gauge transformations under
substitutions belonging to the gauge symmetry group of perturbative
gravitation. These definitions have the advantage that on a flat background,
with the aid of a covariantly constant timelike vector field, a subset of the
linearised gravitational field equations can be written in a form that is fully
analogous to Maxwell's equations (without awkward factors of 4 and extraneous
tensor fields). It is shown how the remaining equations in the perturbed
gravitational system restrict the time dependence of solutions to these
equations and thereby prohibit the existence of propagating vector fields. The
induced gravito-electromagnetic Lorentz force on a test particle is evaluated
in terms of these fields together with the torque on a small gyroscope. It is
concluded that the analogy of perturbative gravity to Maxwell's description of
electromagnetism can be valuable for (quasi-)stationary gravitational phenomena
but that the analogy has its limitations.Comment: 29 pages no-fig
Solutions for Neutral for Axi-Dilaton Gravity in 4-Dimensions
We examine a 1 parameter class of actions describing the gravitational
interaction between a pair of scalar fields and Einsteinian gravitation. When
the parameter is positive the theory corresponds to an axi-dilatonic sector of
low energy string theory. We exploit an SL(2,R) symmetry of the theory to
construct a family of electromagnetically neutral solutions with non-zero axion
and dilaton charge from solutions of the Brans-Dicke theory. We also comment on
solutions to the theory with negative coupling parameter.Comment: 7 pages Plain Tex (No Figures), Letter to Editor, Classical and
Quantum Gravit
The encapsulation selectivity for anionic fission products imparted by an electride
The nanoporous oxide 12CaO•7Al2O3 (C12A7) can capture large concentrations of extra-framework species inside its nanopores, while maintaining its thermodynamical stability. Here we use atomistic simulation to predict the efficacy of C12A7 to encapsulate volatile fission products, in its stoichiometric and much more effective electride forms. In the stoichiometric form, while Xe, Kr and Cs are not captured, Br, I and Te exhibit strong encapsulation energies while Rb is only weakly encapsulated from atoms. The high electronegativities of Br, I and Te stabilize their encapsulation as anions. The electride form of C12A7 shows a significant enhancement in the encapsulation of Br, I and Te with all three stable as anions from their atom and dimer reference states. Successive encapsulation of multiple Br, I and Te as single anions in adjacent cages is also energetically favourable. Conversely, Xe, Kr, Rb and Cs are unbound. Encapsulation of homonuclear dimers (Br2, I2 and Te2) and heteronuclear dimers (CsBr and CsI) in a single cage is also unfavourable. Thus, C12A7 offers the desirable prospect of species selectivity
Encapsulation of heavy metals by a nanoporous complex oxide 12CaO · 7Al<sub>2</sub> O<sub>3</sub>
The nanoporous oxide 12CaO ⋅ 7Al2O3 (C12A7) offers the possibility of capturing large concentrations of environmentally damaging extra-framework species in its nanopores. Using density functional theory with a dispersion correction, we predict the structures and energetics of some heavy metals (Cr, Ni, Cu, Zn, Cd, Hg, and Pb) trapped by the stoichiometric and electride form of C12A7. In the stoichiometric form, while Zn, Cd, Hg, and Pb are encapsulated weakly, Cr, Ni, and Cu exhibit strong encapsulation energies. The electride form of C12A7 shows a significant enhancement in the encapsulation of Cr, Ni, Cu, and Pb. Successive encapsulation of multiple Cr, Ni, Cu, and Pb as single species in adjacent cages of C12A7 is also energetically favorable
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