439 research outputs found
Dendritic reidite from the Chesapeake Bay impact horizon, Ocean Drilling Program Site 1073 (offshore northeastern USA): A fingerprint of distal ejecta?
High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the highpressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta
Tidal Stabilization of Rigidly Rotating, Fully Relativistic Neutron Stars
It is shown analytically that an external tidal gravitational field increases
the secular stability of a fully general relativistic, rigidly rotating neutron
star that is near marginal stability, protecting it against gravitational
collapse. This stabilization is shown to result from the simple fact that the
energy required to raise a tide on such a star, divided by the
square of the tide's quadrupole moment , is a decreasing function of the
star's radius , (where, as changes, the
star's structure is changed in accord with the star's fundamental mode of
radial oscillation). If were positive, the tidal
coupling would destabilize the star. As an application, a rigidly rotating,
marginally secularly stable neutron star in an inspiraling binary system will
be protected against secular collapse, and against dynamical collapse, by tidal
interaction with its companion. The ``local-asymptotic-rest-frame'' tools used
in the analysis are somewhat unusual and may be powerful in other studies of
neutron stars and black holes interacting with an external environment. As a
byproduct of the analysis, in an appendix the influence of tidal interactions
on mass-energy conservation is elucidated.Comment: Revtex, 10 pages, 2 figures; accepted for publication in Physical
Review D. Revisions: Appendix rewritten to clarify how, in Newtonian
gravitation theory, ambiguity in localization of energy makes interaction
energy ambiguous but leaves work done on star by tidal gravity unambiguous.
New footnote 1 and Refs. [11] and [19
The Quantum Propagator for a Nonrelativistic Particle in the Vicinity of a Time Machine
We study the propagator of a non-relativistic, non-interacting particle in
any non-relativistic ``time-machine'' spacetime of the type shown in Fig.~1: an
external, flat spacetime in which two spatial regions, at time and
at time , are connected by two temporal wormholes, one leading from
the past side of to t the future side of and the other from the
past side of to the future side of . We express the propagator
explicitly in terms of those for ordinary, flat spacetime and for the two
wormholes; and from that expression we show that the propagator satisfies
completeness and unitarity in the initial and final ``chronal regions''
(regions without closed timelike curves) and its propagation from the initial
region to the final region is unitary. However, within the time machine it
satisfies neither completeness nor unitarity. We also give an alternative proof
of initial-region-to-final-region unitarity based on a conserved current and
Gauss's theorem. This proof can be carried over without change to most any
non-relativistic time-machine spacetime; it is the non-relativistic version of
a theorem by Friedman, Papastamatiou and Simon, which says that for a free
scalar field, quantum mechanical unitarity follows from the fact that the
classical evolution preserves the Klein-Gordon inner product
DETERMINATION OF THE METAL PARTICLE SIZE OF SUPPORTED Pt, Rh, AND Ir CATALYSTS. A CALIBRATION OF HYDROGEN CHEMISORPTION BY EXAFS
Conversion of conventional gravitational-wave interferometers into QND interferometers by modifying their input and/or output optics
The LIGO-II gravitational-wave interferometers (ca. 2006--2008) are designed
to have sensitivities at about the standard quantum limit (SQL) near 100 Hz.
This paper describes and analyzes possible designs for subsequent, LIGO-III
interferometers that can beat the SQL. These designs are identical to a
conventional broad-band interferometer (without signal recycling), except for
new input and/or output optics. Three designs are analyzed: (i) a
"squeezed-input interferometer" (conceived by Unruh based on earlier work of
Caves) in which squeezed vacuum with frequency-dependent (FD) squeeze angle is
injected into the interferometer's dark port; (ii) a "variational-output"
interferometer (conceived in a different form by Vyatchanin, Matsko and
Zubova), in which homodyne detection with FD homodyne phase is performed on the
output light; and (iii) a "squeezed-variational interferometer" with squeezed
input and FD-homodyne output. It is shown that the FD squeezed-input light can
be produced by sending ordinary squeezed light through two successive
Fabry-Perot filter cavities before injection into the interferometer, and
FD-homodyne detection can be achieved by sending the output light through two
filter cavities before ordinary homodyne detection. With anticipated technology
and with laser powers comparable to that planned for LIGO-II, these
interferometers can beat the amplitude SQL by factors in the range from 3 to 5,
corresponding to event rate increases between ~30 and ~100 over the rate for a
SQL-limited interferometer.Comment: Submitted to Physical Review D; RevTeX manuscript with 16 figures;
prints to 33 pages in Physical Review double column format. Minor revisions
have been made in response to referee repor
Managing juvenile idiopathic arthritis within the context of their life:What we learnt from children and youth living with juvenile idiopathic arthritis and their parents
Introduction: Juvenile idiopathic arthritis (JIA) is the most common rheumatic disease in children and causes short- and long-term disability. Optimal management requires pharmacologic and non-pharmacologic interventions. Few studies have explored the youth and family experience of the management of JIA. This study's objective was to explore the management experience of youth with JIA and their parents. Methods: This qualitative study used semi-structured interviews with youth 12–18 years of age with JIA receiving biological medication and parents of children with JIA on biological medication. Participants were recruited in clinics using convenience sampling. A thematic analysis approach was employed for data analysis. Results: Nine youth and 14 parents participated. Four themes were identified that encompassed an overarching theme of participants managing JIA within the context of their life: aspects of life affected by JIA and its management, lived experience with JIA management, medication decision-making, and involvement in decision-making. Juvenile idiopathic arthritis management is situated within the context of their life but is normally (outside acute events) not central. Conclusion: Two dimensions were added to those in the literature: parents' overall approaches to health and the sense of urgency surrounding decision-making. Our findings reinforce the importance of person- and family-centred care in paediatric rheumatology. That is, identifying what matters most to youth and their parents given their current life circumstances to provide a foundation for discussions of how they want to manage their JIA.</p
Spacetime dynamics of spinning particles - exact electromagnetic analogies
We compare the rigorous equations describing the motion of spinning test
particles in gravitational and electromagnetic fields, and show that if the
Mathisson-Pirani spin condition holds then exact gravito-electromagnetic
analogies emerge. These analogies provide a familiar formalism to treat
gravitational problems, as well as a means for comparing the two interactions.
Fundamental differences are manifest in the symmetries and time projections of
the electromagnetic and gravitational tidal tensors. The physical consequences
of the symmetries of the tidal tensors are explored comparing the following
analogous setups: magnetic dipoles in the field of non-spinning/spinning
charges, and gyroscopes in the Schwarzschild, Kerr, and Kerr-de Sitter
spacetimes. The implications of the time projections of the tidal tensors are
illustrated by the work done on the particle in various frames; in particular,
a reciprocity is found to exist: in a frame comoving with the particle, the
electromagnetic (but not the gravitational) field does work on it, causing a
variation of its proper mass; conversely, for "static observers," a stationary
gravitomagnetic (but not a magnetic) field does work on the particle, and the
associated potential energy is seen to embody the Hawking-Wald spin-spin
interaction energy. The issue of hidden momentum, and its counterintuitive
dynamical implications, is also analyzed. Finally, a number of issues regarding
the electromagnetic interaction and the physical meaning of Dixon's equations
are clarified.Comment: 32+11 pages, 5 figures. Edited and further improved version, with new
Section C.2 unveiling analogies for arbitrary spin conditions, and new Sec.
3.2.3 in the Supplement making connection to the post-Newtonian
approximation; former Sec. III.B.4 and Appendix C moved to the (reshuffled)
Supplement; references updated. The Supplement is provided in ancillary file.
Matches the final published versio
Investigation of the thermal behavior of 4-vinylpyridine–trimethylolpropane trimethacrylate copolymeric microspheres
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