18,602 research outputs found
Action and Energy of the Gravitational Field
We present a detailed examination of the variational principle for metric
general relativity as applied to a ``quasilocal'' spacetime region \M (that
is, a region that is both spatially and temporally bounded). Our analysis
relies on the Hamiltonian formulation of general relativity, and thereby
assumes a foliation of \M into spacelike hypersurfaces . We allow for
near complete generality in the choice of foliation. Using a field--theoretic
generalization of Hamilton--Jacobi theory, we define the quasilocal
stress-energy-momentum of the gravitational field by varying the action with
respect to the metric on the boundary \partial\M. The gravitational
stress-energy-momentum is defined for a two--surface spanned by a spacelike
hypersurface in spacetime. We examine the behavior of the gravitational
stress-energy-momentum under boosts of the spanning hypersurface. The boost
relations are derived from the geometrical and invariance properties of the
gravitational action and Hamiltonian. Finally, we present several new examples
of quasilocal energy--momentum, including a novel discussion of quasilocal
energy--momentum in the large-sphere limit towards spatial infinity.Comment: To be published in Annals of Physics. This final version includes two
new sections, one giving examples of quasilocal energy and the other
containing a discussion of energy at spatial infinity. References have been
added to papers by Bose and Dadhich, Anco and Tun
Canonical Quasilocal Energy and Small Spheres
Consider the definition E of quasilocal energy stemming from the
Hamilton-Jacobi method as applied to the canonical form of the gravitational
action. We examine E in the standard "small-sphere limit," first considered by
Horowitz and Schmidt in their examination of Hawking's quasilocal mass. By the
term "small sphere" we mean a cut S(r), level in an affine radius r, of the
lightcone belonging to a generic spacetime point. As a power series in r, we
compute the energy E of the gravitational and matter fields on a spacelike
hypersurface spanning S(r). Much of our analysis concerns conceptual and
technical issues associated with assigning the zero-point of the energy. For
the small-sphere limit, we argue that the correct zero-point is obtained via a
"lightcone reference," which stems from a certain isometric embedding of S(r)
into a genuine lightcone of Minkowski spacetime. Choosing this zero-point, we
find agreement with Hawking's quasilocal mass expression, up to and including
the first non-trivial order in the affine radius. The vacuum limit relates the
quasilocal energy directly to the Bel-Robinson tensor.Comment: revtex, 22 p, uses amssymb option (can be removed
New variables, the gravitational action, and boosted quasilocal stress-energy-momentum
This paper presents a complete set of quasilocal densities which describe the
stress-energy-momentum content of the gravitational field and which are built
with Ashtekar variables. The densities are defined on a two-surface which
bounds a generic spacelike hypersurface of spacetime. The method used
to derive the set of quasilocal densities is a Hamilton-Jacobi analysis of a
suitable covariant action principle for the Ashtekar variables. As such, the
theory presented here is an Ashtekar-variable reformulation of the metric
theory of quasilocal stress-energy-momentum originally due to Brown and York.
This work also investigates how the quasilocal densities behave under
generalized boosts, i. e. switches of the slice spanning . It is
shown that under such boosts the densities behave in a manner which is similar
to the simple boost law for energy-momentum four-vectors in special relativity.
The developed formalism is used to obtain a collection of two-surface or boost
invariants. With these invariants, one may ``build" several different mass
definitions in general relativity, such as the Hawking expression. Also
discussed in detail in this paper is the canonical action principle as applied
to bounded spacetime regions with ``sharp corners."Comment: Revtex, 41 Pages, 4 figures added. Final version has been revised and
improved quite a bit. To appear in Classical and Quantum Gravit
Non-equilibrium fluctuations and mechanochemical couplings of a molecular motor
We investigate theoretically the violations of Einstein and Onsager
relations, and the efficiency for a single processive motor operating far from
equilibrium using an extension of the two-state model introduced by Kafri {\em
et al.} [Biophys. J. {\bf 86}, 3373 (2004)]. With the aid of the Fluctuation
Theorem, we analyze the general features of these violations and this
efficiency and link them to mechanochemical couplings of motors. In particular,
an analysis of the experimental data of kinesin using our framework leads to
interesting predictions that may serve as a guide for future experiments.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Ti and V layers retard interaction between Al films and polycrystalline Si
Fine-grained polycrystalline Si (poly Si) in contact with Al films recrystallizes at temperatures well below the Si-Al eutectic (577 °C). We show that this interaction can be deferred or suppressed by placing a buffer layer of Ti or V between the Al film and the poly Si. During annealing, Ti or V form TiAl3 or Val3 at the buffer-layer–Al-film interface, but do not react with the poly Si so that the integrity of the poly Si is preserved as long as some unreacted Ti or V remains. The reaction between the Ti or V layer and the Al film is transport limited ([proportional]t^1/2) and characterized by the diffusion constants 1.5×10^15 exp(–1.8eV/kT) Å^2/sec or 8.4×10^12 exp(–1.7eV/kT) Å^2/sec, respectively
Heteroepitaxy of deposited amorphous layer by pulsed electron-beam irradiation
We demonstrate that a single short pulse of electron irradiation of appropriate energy is capable of recrystallizing epitaxially an amorphous Ge layer deposited on either or Si single-crystal substrate. The primary defects observed in the case were dislocations, whereas stacking faults were observed in samples
Sequence of phase formation in planar metal-Si reaction couples
A correlation is found between the sequence of phase formation in thin-film metal-Si interactions and the bulk equilibrium phase diagram. After formation of the first silicide phase, which generally follows the rule proposed by Walser and Bené, the next phase formed at the interface between the first phase and the remaining element (Si or metal) is the nearest congruently melting compound richer in the unreacted element. If the compounds between the first phase and the remaining element are all noncongruently melting compounds (such as peritectic or peritectoid phases), the next phase formed is that with the smallest temperature difference between the liquidus curve and the peritectic (or peritectoid) point
Epitaxial growth of deposited amorphous layer by laser annealing
We demonstrate that a single short pulse of laser irradiation of appropriate energy is capable of recrystallizing in open air an amorphous Si layer deposited on a (100) single-crystal substrate into an epitaxial layer. The laser pulse annealing technique is shown to overcome the interfacial oxide obstacle which usually leads to polycrystalline formation in normal thermal annealing
Plant-Parasitic Nematodes Associated with Breadfruit, Artocarpus altilis (Parkinson) Fosberg.
M.S. Thesis. University of Hawaiʻi at Mānoa 2017
- …