1,514 research outputs found
Effective stress-energy tensors, self-force, and broken symmetry
Deriving the motion of a compact mass or charge can be complicated by the
presence of large self-fields. Simplifications are known to arise when these
fields are split into two parts in the so-called Detweiler-Whiting
decomposition. One component satisfies vacuum field equations, while the other
does not. The force and torque exerted by the (often ignored) inhomogeneous
"S-type" portion is analyzed here for extended scalar charges in curved
spacetimes. If the geometry is sufficiently smooth, it is found to introduce
effective shifts in all multipole moments of the body's stress-energy tensor.
This greatly expands the validity of statements that the homogeneous R field
determines the self-force and self-torque up to renormalization effects. The
forces and torques exerted by the S field directly measure the degree to which
a spacetime fails to admit Killing vectors inside the body. A number of
mathematical results related to the use of generalized Killing fields are
therefore derived, and may be of wider interest. As an example of their
application, the effective shift in the quadrupole moment of a charge's
stress-energy tensor is explicitly computed to lowest nontrivial order.Comment: 22 pages, fixed typos and simplified discussio
Mechanics of extended masses in general relativity
The "external" or "bulk" motion of extended bodies is studied in general
relativity. Compact material objects of essentially arbitrary shape, spin,
internal composition, and velocity are allowed as long as there is no direct
(non-gravitational) contact with other sources of stress-energy. Physically
reasonable linear and angular momenta are proposed for such bodies and exact
equations describing their evolution are derived. Changes in the momenta depend
on a certain "effective metric" that is closely related to a non-perturbative
generalization of the Detweiler-Whiting R-field originally introduced in the
self-force literature. If the effective metric inside a self-gravitating body
can be adequately approximated by an appropriate power series, the
instantaneous gravitational force and torque exerted on it is shown to be
identical to the force and torque exerted on an appropriate test body moving in
the effective metric. This result holds to all multipole orders. The only
instantaneous effect of a body's self-field is to finitely renormalize the
"bare" multipole moments of its stress-energy tensor. The MiSaTaQuWa expression
for the gravitational self-force is recovered as a simple application. A
gravitational self-torque is obtained as well. Lastly, it is shown that the
effective metric in which objects appear to move is approximately a solution to
the vacuum Einstein equation if the physical metric is an approximate solution
to Einstein's equation linearized about a vacuum background.Comment: 39 pages, 2 figures; fixed equation satisfied by the Green function
used to construct the effective metri
Self-forces from generalized Killing fields
A non-perturbative formalism is developed that simplifies the understanding
of self-forces and self-torques acting on extended scalar charges in curved
spacetimes. Laws of motion are locally derived using momenta generated by a set
of generalized Killing fields. Self-interactions that may be interpreted as
arising from the details of a body's internal structure are shown to have very
simple geometric and physical interpretations. Certain modifications to the
usual definition for a center-of-mass are identified that significantly
simplify the motions of charges with strong self-fields. A derivation is also
provided for a generalized form of the Detweiler-Whiting axiom that pointlike
charges should react only to the so-called regular component of their
self-field. Standard results are shown to be recovered for sufficiently small
charge distributions.Comment: 21 page
Electromagnetic self-forces and generalized Killing fields
Building upon previous results in scalar field theory, a formalism is
developed that uses generalized Killing fields to understand the behavior of
extended charges interacting with their own electromagnetic fields. New notions
of effective linear and angular momenta are identified, and their evolution
equations are derived exactly in arbitrary (but fixed) curved spacetimes. A
slightly modified form of the Detweiler-Whiting axiom that a charge's motion
should only be influenced by the so-called "regular" component of its
self-field is shown to follow very easily. It is exact in some interesting
cases, and approximate in most others. Explicit equations describing the
center-of-mass motion, spin angular momentum, and changes in mass of a small
charge are also derived in a particular limit. The chosen approximations --
although standard -- incorporate dipole and spin forces that do not appear in
the traditional Abraham-Lorentz-Dirac or Dewitt-Brehme equations. They have,
however, been previously identified in the test body limit.Comment: 20 pages, minor typos correcte
Acoustic measurement differences on trees and logs from hardwoods in wet and dry condition
peer-reviewedAcoustic velocities measured on standing trees using time-of-flight (TOF) devices have been found to be
between 7% and 36% higher for softwoods than those in logs using resonance techniques based on
longitudinal frequencies. This effect was explained in three different ways: (1) TOF devices on standing
trees measure outerwood containing more mature wood while resonance methods assess the whole crosssection,
(2) the variation in the velocity is due to loading conditions in standing trees, while logs are free
of loads and (3) the acoustic waves are dilatational waves in the case of TOF measurements on standing
trees and one-dimensional longitudinal waves in the case of resonance on logs. This is an important topic
considering the fact that resonance methods are considered more accurate for predicting mechanical
properties and it has been proposed that correction factors should be applied on TOF measurements.
In the present work, four hardwoods from Irish forests were studied and, on average, TOF velocities
measured in the forest above fibre saturation point (FSP) were 19.8% higher than those from resonance
measurements taken on logs immediately after felling. However, this difference reduced to 5.4% when
the measurements were repeated at a moisture content (MC) of about 18% in the laboratory. Therefore,
there is a MC effect on the velocity differences. Furthermore, higher differences were systematically
found in older specimens in wet condition. However, this age effect was small in most cases
A Rigorous Derivation of Electromagnetic Self-force
During the past century, there has been considerable discussion and analysis
of the motion of a point charge, taking into account "self-force" effects due
to the particle's own electromagnetic field. We analyze the issue of "particle
motion" in classical electromagnetism in a rigorous and systematic way by
considering a one-parameter family of solutions to the coupled Maxwell and
matter equations corresponding to having a body whose charge-current density
and stress-energy tensor scale to zero size
in an asymptotically self-similar manner about a worldline as . In this limit, the charge, , and total mass, , of the body go to
zero, and goes to a well defined limit. The Maxwell field
is assumed to be the retarded solution associated with
plus a homogeneous solution (the "external field") that varies
smoothly with . We prove that the worldline must be a
solution to the Lorentz force equations of motion in the external field
. We then obtain self-force, dipole forces, and spin force
as first order perturbative corrections to the center of mass motion of the
body. We believe that this is the first rigorous derivation of the complete
first order correction to Lorentz force motion. We also address the issue of
obtaining a self-consistent perturbative equation of motion associated with our
perturbative result, and argue that the self-force equations of motion that
have previously been written down in conjunction with the "reduction of order"
procedure should provide accurate equations of motion for a sufficiently small
charged body with negligible dipole moments and spin. There is no corresponding
justification for the non-reduced-order equations.Comment: 52 pages, minor correction
Childbirth Supporters’ Experiences in a Built Hospital Birth Environment: Exploring Inhibiting and Facilitating Factors in Negotiating the Supporter Role
© 2016, © The Author(s) 2016. Objective: To explore inhibiting and facilitating design factors influencing childbirth supporters’ experiences. Background: Birthing women benefit from the continuous, cooperative presence of supporters. However, little research has investigated how birth room design facilitates or inhibits supporters’ role navigation. Methods: We conducted an exploratory video ethnographic single case study of childbirth supporters’ experiences, within an Australian hospital birth environment. Video, field notes, and video-cued reflexive interviews with the woman, her midwives, and supporters were thematically analyzed using ethnographic/symbolic interactionist perspectives to frame supporters’ understandings. Results: Findings suggest supporters’ experiences are complex, made more complicated by sparse understanding or accommodation of their needs in the built environment. Supporters’ presence and roles are not facilitated by the physical space; they experience “an unbelonging paradox” of being needed, yet uncertain and “in the way” during “tenuous nest-building” activities. Conclusions: Suggested design guidelines to facilitate supporters’ well-being and their roles in designed hospital birth spaces are provided
Using video in childbirth research: Ethical approval challenges
© The Author(s) 2015. Background: Conducting video-research in birth settings raises challenges for ethics review boards to view birthing women and research-midwives as capable, autonomous decision-makers. Aim: This study aimed to gain an understanding of how the ethical approval process was experienced and to chronicle the perceived risks and benefits. Research design: The Birth Unit Design project was a 2012 Australian ethnographic study that used video recording to investigate the physical design features in the hospital birthing space that might influence both verbal and non-verbal communication and the experiences of childbearing women, midwives and supporters. Participants and research context: Six women, 11 midwives and 11 childbirth supporters were filmed during the women’s labours in hospital birth units and interviewed 6 weeks later. Ethical considerations: The study was approved by an Australian Health Research Ethics Committee after a protracted process of negotiation. Findings: The ethics committee was influenced by a traditional view of research as based on scientific experiments resulting in a poor understanding of video-ethnographic research, a paradigmatic view of the politics and practicalities of modern childbirth processes, a desire to protect institutions from litigation, and what we perceived as a paternalistic approach towards protecting participants, one that was at odds with our aim to facilitate situations in which women could make flexible, autonomous decisions about how they might engage with the research process. Discussion: The perceived need for protection was overly burdensome and against the wishes of the participants themselves; ultimately, this limited the capacity of the study to improve care for women and babies. Conclusion: Recommendations are offered for those involved in ethical approval processes for qualitative research in childbirth settings. The complexity of issues within childbirth settings, as in most modern healthcare settings, should be analysed using a variety of research approaches, beyond efficacy-style randomised controlled trials, to expand and improve practice-based results
Tomato Spotted Wilt Virus NSs Protein Supports Infection and Systemic Movement of a Potyvirus and Is a Symptom Determinant
Plant viruses are inducers and targets of antiviral RNA silencing. To condition susceptibility, most plant viruses encode silencing suppressor proteins that interfere with antiviral RNA silencing. The NSs protein is an RNA silencing suppressor in orthotospoviruses, such as the tomato spotted wilt virus (TSWV). The mechanism of RNA silencing suppression by NSs and its role in virus infection and movement are poorly understood. Here, we cloned and tagged TSWV NSs and expressed it from a GFP-tagged turnip mosaic virus (TuMV-GFP) carrying either a wild-type or suppressor-deficient (AS9) helper component proteinase (HC-Pro). When expressed in cis, NSs restored pathogenicity and promoted systemic infection of suppressor-deficient TuMV-AS9-GFP in Nicotiana benthamiana and Arabidopsis thaliana. Inactivating mutations were introduced in NSs RNA-binding domain one. A genetic analysis with active and suppressor-deficient NSs, in combination with wild-type and mutant plants lacking essential components of the RNA silencing machinery, showed that the NSs insert is stable when expressed from a potyvirus. NSs can functionally replace potyviral HC-Pro, condition virus susceptibility, and promote systemic infection and symptom development by suppressing antiviral RNA silencing through a mechanism that partially overlaps that of potyviral HC-Pro. The results presented provide new insight into the mechanism of silencing suppression by NSs and its effect on virus infection
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