3,115 research outputs found
Configuration development study of the X-24C hypersonic research airplane
Bottom line results were made of a three-phase study to determine the feasibility of designing, building, and operating, and maintaining an air-launched high performance aircraft capable of cruising at speeds up to Mach 8 for short durations. The results show that Lockalloy heat-sink structure affords the capability for a 'work-horse' vehicle which can serve as an excellent platform for this research. It was further concluded that the performance of a blended wing body configuration surpassed that of a lifting body design for typical X-24C missions. The cost of a two vehicle program, less engines, B-52 modification and contractor support after delivery, can be kept within $70M (in Jan. 1976 dollars)
A Structure for Quasars
This paper proposes a simple, empirically derived, unifying structure for the
inner regions of quasars. This structure is constructed to explain the broad
absorption line (BAL) regions, the narrow `associated' ultraviolet and X-ray
warm absorbers (NALs); and is also found to explain the broad emission line
regions (BELR), and several scattering features, including a substantial
fraction of the broad X-ray Iron-K emission line, and the bi-conical extended
narrow emission line region (ENLR) structures seen on large kiloparsec scales
in Seyfert images. Small extensions of the model to allow luminosity dependent
changes in the structure may explain the UV and X-ray Baldwin effects and the
greater prevalence of obscuration in low luminosity AGN.Comment: 35 pages, including 8 color figures (figures 4abc are big).
Astrophysical Journal, in press. Expanded version of conference paper
astro-ph/000516
Lack of trust in maternal support is associated with negative interpretations of ambiguous maternal behavior
Attachment theory assumes that children who lack trust in maternal availability for support are more inclined to interpret maternal behavior in congruence with their expectation that mother will remain unavailable for support. To provide the first test of this assumption, early adolescents (9-13 years old) were asked to assess whether ambiguous interactions with mother should be interpreted in a positive or a negative way. In our sample (n = 322), results showed that early adolescents' lack of trust in their mother's availability for support was related to more negative interpretations of maternal behavior. The associations remained significant after controlling for depressive mood. The importance of these findings for our understanding of attachment theory, attachment stability, and clinical practice are discussed
Chronology Protection in Generalized Godel Spacetime
The effective action of a free scalar field propagating in the generalized
Godel spacetime is evaluated by the zeta-function regularization method. From
the result we show that the renormalized stress energy tensor may be divergent
at the chronology horizon. This gives a support to the chronology protection
conjecture.Comment: Latex 6 pages, typos correcte
Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics
We consider two approaches to evading paradoxes in quantum mechanics with
closed timelike curves (CTCs). In a model similar to Politzer's, assuming pure
states and using path integrals, we show that the problems of paradoxes and of
unitarity violation are related; preserving unitarity avoids paradoxes by
modifying the time evolution so that improbable events bewcome certain. Deutsch
has argued, using the density matrix, that paradoxes do not occur in the "many
worlds interpretation". We find that in this approach account must be taken of
the resolution time of the device that detects objects emerging from a wormhole
or other time machine. When this is done one finds that this approach is viable
only if macroscopic objects traversing a wormhole interact with it so strongly
that they are broken into microscopic fragments.Comment: no figure
Quantum Coherence and Closed Timelike Curves
Various calculations of the matrix have shown that it seems to be non
unitary for interacting fields when there are closed timelike curves. It is
argued that this is because there is loss of quantum coherence caused by the
fact that part of the quantum state circulates on the closed timelike curves
and is not measured at infinity. A prescription is given for calculating the
superscattering matrix on space times whose parameters can be
analytically continued to obtain a Euclidean metric. It is illustrated by a
discussion of a spacetime in with two disks in flat space are identified. If
the disks have an imaginary time separation, this corresponds to a heat bath.
An external field interacting with the heat bath will lose quantum coherence.
One can then analytically continue to an almost real separation of the disks.
This will give closed timelike curves but one will still get loss of quantum
coherence.Comment: 13 page
Supersymmetric Rotating Black Holes and Causality Violation
The geodesics of the rotating extreme black hole in five spacetime dimensions
found by Breckenridge, Myers, Peet and Vafa are Liouville integrable and may be
integrated by additively separating the Hamilton-Jacobi equation. This allows
us to obtain the St\"ackel-Killing tensor. We use these facts to give the
maximal analytic extension of the spacetime and discuss some aspects of its
causal structure. In particular, we exhibit a `repulson'-like behaviour
occuring when there are naked closed timelike curves. In this case we find that
the spacetime is geodesically complete (with respect to causal geodesics) and
free of singularities. When a partial Cauchy surface exists, we show, by
solving the Klein-Gordon equation, that the absorption cross-section for
massless waves at small frequencies is given by the area of the hole. At high
frequencies a dependence on the angular quantum numbers of the wave develops.
We comment on some aspects of `inertial time travel' and argue that such time
machines cannot be constructed by spinning up a black hole with no naked closed
timelike curves.Comment: 36 pages,LaTeX,8 figures;added 1 reference and a few comments;
formula (2.6) corrected; a few changes to section
Testing causality violation on spacetimes with closed timelike curves
Generalized quantum mechanics is used to examine a simple two-particle
scattering experiment in which there is a bounded region of closed timelike
curves (CTCs) in the experiment's future. The transitional probability is shown
to depend on the existence and distribution of the CTCs. The effect is
therefore acausal, since the CTCs are in the experiment's causal future. The
effect is due to the non-unitary evolution of the pre- and post-scattering
particles as they pass through the region of CTCs. We use the time-machine
spacetime developed by Politzer [1], in which CTCs are formed due to the
identification of a single spatial region at one time with the same region at
another time. For certain initial data, the total cross-section of a scattering
experiment is shown to deviate from the standard value (the value predicted if
no CTCs existed). It is shown that if the time machines are small, sparsely
distributed, or far away, then the deviation in the total cross-section may be
negligible as compared to the experimental error of even the most accurate
measurements of cross-sections. For a spacetime with CTCs at all points, or one
where microscopic time machines pervade the spacetime in the final moments
before the big crunch, the total cross-section is shown to agree with the
standard result (no CTCs) due to a cancellation effect.Comment: 28 pages, 8 figures, late
Nonlinear Quantum Mechanics at the Planck Scale
I argue that the linearity of quantum mechanics is an emergent feature at the
Planck scale, along with the manifold structure of space-time. In this regime
the usual causality violation objections to nonlinearity do not apply, and
nonlinear effects can be of comparable magnitude to the linear ones and still
be highly suppressed at low energies. This can offer alternative approaches to
quantum gravity and to the evolution of the early universe.Comment: Talk given at the International Quantum Structures 2004 meeting, 16
pages LaTe
- …