92 research outputs found
Testing and Structured Design
This paper describes part of an integrated circuit testing project carried out at Caltech between 1979 and 1982. The central theme and result of the project is a language or
notation for describing tests for complex integrated circuits. The evolution of this test language has been guided by many considerations, including (1) its implementation in a working, interactive test system called FIFI, (2) its fit to ideas about the architecture of
high-performance test instruments, and (3) its expressivity for a design-for-testability strategy for chip designs structured in the general style presented by Mead and Conway
[1]
Leaf color and vine size are related to yield in a phylloxera-infested vineyard
The uneven spread of phylloxera infestation and associated vine symptoms in vineyards usually complicates yield estimates and vineyard replacement decisions. In a Cabernet Sauvignon vineyard with AXR#1 rootstock the current season's and following season's yields of 40 vine plots correlated (r ≥ 0.77, p ≤ 0.05) with early to midseason leaf and canopy spectra measured in the field, laboratory and remotely with aircraft-borne sensors
Static and dynamic traversable wormhole geometries satisfying the Ford-Roman constraints
It was shown by Ford and Roman in 1996 that quantum field theory severely
constrains wormhole geometries on a macroscopic scale. The first part of this
paper discusses a wide class of wormhole solutions that meet these constraints.
The type of shape function used is essentially generic. The constraints are
then discussed in conjunction with various redshift functions. Violations of
the weak energy condition and traversability criteria are also considered. The
second part of the paper analyzes analogous time-dependent (dynamic) wormholes
with the aid of differential forms. It is shown that a violation of the weak
energy condition is not likely to be avoidable even temporarily.Comment: 16 pages AMSTe
Magnetic strings in anti-de Sitter General Relativity
We obtain spacetimes generated by static and spinning magnetic string sources
in Einstein Relativity with negative cosmological constant. Since the spacetime
is asymptotically a cylindrical anti-de Sitter spacetime, we will be able to
calculate the mass, momentum, and electric charge of the solutions. We find two
families of solutions, one with longitudinal magnetic field and the other with
angular magnetic field. The source for the longitudinal magnetic field can be
interpreted as composed by a system of two symmetric and superposed
electrically charged lines with one of the electrically charged lines being at
rest and the other spinning. The angular magnetic field solution can be
similarly interpreted as composed by charged lines but now one is at rest and
the other has a velocity along the axis. This solution cannot be extended down
to the origin.Comment: Latex, 26 page
Axially symmetric rotating traversable wormholes
This paper generalizes the static and spherically symmetric traversable
wormhole geometry to a rotating axially symmetric one with a time-dependent
angular velocity by means of an exact solution. It was found that the violation
of the weak energy condition, although unavoidable, is considerably less severe
than in the static spherically symmetric case. The radial tidal constraint is
more easily met due to the rotation. Similar improvements are seen in one of
the lateral tidal constraints. The magnitude of the angular velocity may have
little effect on the weak energy condition violation for an axially symmetric
wormhole. For a spherically symmetric one, however, the violation becomes less
severe with increasing angular velocity. The time rate of change of the angular
velocity, on the other hand, was found to have no effect at all. Finally, the
angular velocity must depend only on the radial coordinate, confirming an
earlier result.Comment: 17 pages, AMSTe
Gravastar energy conditions revisited
We consider the gravastar model where the vacuum phase transition between the
de Sitter interior and the Schwarzschild or Schwarzschild-de Sitter exterior
geometries takes place at a single spherical delta-shell. We derive sharp
analytic bounds on the surface compactness (2m/r) that follow from the
requirement that the dominant energy condition (DEC) holds at the shell. In the
case of Schwarzschild exterior, the highest surface compactness is achieved
with the stiff shell in the limit of vanishing (dark) energy density in the
interior. In the case of Schwarzschild-de Sitter exterior, in addition to the
gravastar configurations with the shell under surface pressure, gravastar
configurations with vanishing shell pressure (dust shells), as well as
configurations with the shell under surface tension, are allowed by the DEC.
Respective bounds on the surface compactness are derived for all cases. We also
consider the speed of sound on the shell as derived from the requirement that
the shell is stable against the radial perturbations. The causality requirement
(sound speed not exceeding that of light) further restricts the space of
allowed gravastar configurations.Comment: LaTeX/IOP-style, 16 pages, 2 figures, changes wrt v1: motivation for
eq. (6) clarified, several referecnes added (to appear in Class. Quantum
Grav.
Microlensing by natural wormholes: theory and simulations
We provide an in depth study of the theoretical peculiarities that arise in
effective negative mass lensing, both for the case of a point mass lens and
source, and for extended source situations. We describe novel observational
signatures arising in the case of a source lensed by a negative mass. We show
that a negative mass lens produces total or partial eclipse of the source in
the umbra region and also show that the usual Shapiro time delay is replaced
with an equivalent time gain. We describe these features both theoretically, as
well as through numerical simulations. We provide negative mass microlensing
simulations for various intensity profiles and discuss the differences between
them. The light curves for microlensing events are presented and contrasted
with those due to lensing produced by normal matter. Presence or absence of
these features in the observed microlensing events can shed light on the
existence of natural wormholes in the Universe.Comment: 16 pages, 24 postscript figures (3 coloured), revtex style, submitted
to Phys. Rev.
Morris-Thorne wormholes with a cosmological constant
First, the ideas introduced in the wormhole research field since the work of
Morris and Thorne are briefly reviewed, namely, the issues of energy
conditions, wormhole construction, stability, time machines and astrophysical
signatures. Then, spherically symmetric and static traversable Morris-Thorne
wormholes in the presence of a generic cosmological constant are analyzed. A
matching of an interior solution to the unique exterior vacuum solution is done
using directly the Einstein equations. The structure as well as several
physical properties and characteristics of traversable wormholes due to the
effects of the cosmological term are studied. Interesting equations appear in
the process of matching. For instance, one finds that for asymptotically flat
and anti-de Sitter spacetimes the surface tangential pressure of the
thin-shell, at the boundary of the interior and exterior solutions, is always
strictly positive, whereas for de Sitter spacetime it can take either sign as
one could expect, being negative (tension) for relatively high cosmological
constant and high wormhole radius, positive for relatively high mass and small
wormhole radius, and zero in-between. Finally, some specific solutions with
generic cosmological constant, based on the Morris-Thorne solutions, are
provided.Comment: latex, 49 pages, 8 figures. Expanded version of the paper published
in Physical Review
Phase-space and Black Hole Entropy of Higher Genus Horizons in Loop Quantum Gravity
In the context of loop quantum gravity, we construct the phase-space of
isolated horizons with genus greater than 0. Within the loop quantum gravity
framework, these horizons are described by genus g surfaces with N punctures
and the dimension of the corresponding phase-space is calculated including the
genus cycles as degrees of freedom. From this, the black hole entropy can be
calculated by counting the microstates which correspond to a black hole of
fixed area. We find that the leading term agrees with the A/4 law and that the
sub-leading contribution is modified by the genus cycles.Comment: 22 pages, 9 figures. References updated. Minor changes to match
version to appear in Class. Quant. Gra
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