649 research outputs found
Renormalization Group Analysis of a Quivering String Model of Posture Control
Scaling concepts and renormalization group (RG) methods are applied to a
simple linear model of human posture control consisting of a trembling or
quivering string subject to damping and restoring forces. The string is driven
by uncorrelated white Gaussian noise intended to model the corrections of the
physiological control system. We find that adding a weak quadratic nonlinearity
to the posture control model opens up a rich and complicated phase space
(representing the dynamics) with various non-trivial fixed points and basins of
attraction. The transition from diffusive to saturated regimes of the linear
model is understood as a crossover phenomenon, and the robustness of the linear
model with respect to weak non-linearities is confirmed. Correlations in
posture fluctuations are obtained in both the time and space domain. There is
an attractive fixed point identified with falling. The scaling of the
correlations in the front-back displacement, which can be measured in the
laboratory, is predicted for both the large-separation (along the string) and
long-time regimes of posture control.Comment: 20 pages, 13 figures, RevTeX, accepted for publication in PR
No realistic wormholes from ghost-free scalar-tensor phantom dark energy
It is proved that no wormholes can be formed in viable scalar-tensor models
of dark energy admitting its phantom-like () behaviour in cosmology,
even in the presence of electric or magnetic fields, if the non-minimal
coupling function is everywhere positive and the scalar field
itself is not a ghost. Some special static, spherically symmetric wormhole
solutions may exist if is allowed to reach zero or to become
negative, so that the effective gravitational constant becomes negative in some
region making the graviton a ghost. If remains non-negative, such solutions
require severe fine tuning and a very peculiar kind of model. If is
allowed, it is argued (and confirmed by previous investigations) that such
solutions are generically unstable under non-static perturbations, the
instability appearing right near transition surfaces to negative .Comment: 8 pages, late
Classical and semi-classical energy conditions
The standard energy conditions of classical general relativity are (mostly)
linear in the stress-energy tensor, and have clear physical interpretations in
terms of geodesic focussing, but suffer the significant drawback that they are
often violated by semi-classical quantum effects. In contrast, it is possible
to develop non-standard energy conditions that are intrinsically non-linear in
the stress-energy tensor, and which exhibit much better well-controlled
behaviour when semi-classical quantum effects are introduced, at the cost of a
less direct applicability to geodesic focussing. In this article we will first
review the standard energy conditions and their various limitations. (Including
the connection to the Hawking--Ellis type I, II, III, and IV classification of
stress-energy tensors). We shall then turn to the averaged, nonlinear, and
semi-classical energy conditions, and see how much can be done once
semi-classical quantum effects are included.Comment: V1: 25 pages. Draft chapter, on which the related chapter of the book
"Wormholes, Warp Drives and Energy Conditions" (to be published by Springer),
will be based. V2: typos fixed. V3: small typo fixe
Jerk, snap, and the cosmological equation of state
Taylor expanding the cosmological equation of state around the current epoch
is the simplest model one can consider that does not make any a priori
restrictions on the nature of the cosmological fluid. Most popular cosmological
models attempt to be ``predictive'', in the sense that once somea priori
equation of state is chosen the Friedmann equations are used to determine the
evolution of the FRW scale factor a(t). In contrast, a retrodictive approach
might usefully take observational dataconcerning the scale factor, and use the
Friedmann equations to infer an observed cosmological equation of state. In
particular, the value and derivatives of the scale factor determined at the
current epoch place constraints on the value and derivatives of the
cosmological equation of state at the current epoch. Determining the first
three Taylor coefficients of the equation of state at the current epoch
requires a measurement of the deceleration, jerk, and snap -- the second,
third, and fourth derivatives of the scale factor with respect to time.
Higher-order Taylor coefficients in the equation of state are related to
higher-order time derivatives of the scale factor. Since the jerk and snap are
rather difficult to measure, being related to the third and fourth terms in the
Taylor series expansion of the Hubble law, it becomes clear why direct
observational constraints on the cosmological equation of state are so
relatively weak; and are likely to remain weak for the foreseeable future.Comment: V1: 10 pages; uses iopart.cls setstack.sty V2: six additional
references, some clarifying comments and discussion, no physics changes. V3:
significant additions based on community feedback; explicit calculations now
carried out to fourth order in redshift. V4: Discussion of current
observational situation added. This version accepted for publication in
Classical and Quantum Gravity. Now 15 page
Vacuum decay in quantum field theory
We study the contribution to vacuum decay in field theory due to the
interaction between the long and short-wavelength modes of the field. The field
model considered consists of a scalar field of mass with a cubic term in
the potential. The dynamics of the long-wavelength modes becomes diffusive in
this interaction. The diffusive behaviour is described by the reduced Wigner
function that characterizes the state of the long-wavelength modes. This
function is obtained from the whole Wigner function by integration of the
degrees of freedom of the short-wavelength modes. The dynamical equation for
the reduced Wigner function becomes a kind of Fokker-Planck equation which is
solved with suitable boundary conditions enforcing an initial metastable vacuum
state trapped in the potential well. As a result a finite activation rate is
found, even at zero temperature, for the formation of true vacuum bubbles of
size . This effect makes a substantial contribution to the total decay
rate.Comment: 27 pages, RevTeX, 1 figure (uses epsf.sty
Vacuum decay in quantum field theory
We study the contribution to vacuum decay in field theory due to the
interaction between the long and short-wavelength modes of the field. The field
model considered consists of a scalar field of mass with a cubic term in
the potential. The dynamics of the long-wavelength modes becomes diffusive in
this interaction. The diffusive behaviour is described by the reduced Wigner
function that characterizes the state of the long-wavelength modes. This
function is obtained from the whole Wigner function by integration of the
degrees of freedom of the short-wavelength modes. The dynamical equation for
the reduced Wigner function becomes a kind of Fokker-Planck equation which is
solved with suitable boundary conditions enforcing an initial metastable vacuum
state trapped in the potential well. As a result a finite activation rate is
found, even at zero temperature, for the formation of true vacuum bubbles of
size . This effect makes a substantial contribution to the total decay
rate.Comment: 27 pages, RevTeX, 1 figure (uses epsf.sty
Scalar-mediated forward-backward asymmetry
A large forward-backward asymmetry in production, for large
invariant mass of the system, has been recently observed by the CDF
collaboration. Among the scalar mediated mechanisms that can explain such a
large asymmetry, only the t-channel exchange of a color-singlet weak-doublet
scalar is consistent with both differential and integrated cross
section measurements. Constraints from flavor changing processes dictate a very
specific structure for the Yukawa couplings of such a new scalar. No sizable
deviation in the differential or integrated production cross section
is expected at the LHC.Comment: 22 pages, 1 figure and 2 tables. v2: Corrected Eqs.(50,51,74),
adapted Fig.1, Tab.1 and relevant discussions. Extended discussion of top
decay and single to
Rare B decays and Tevatron top-pair asymmetry
The recent Tevatron result on the top quark forward-backward asymmetry, which
deviates from its standard model prediction by 3.4, has prompted many
authors to build new models to account for this anomaly. Among the various
proposals, we find that those mechanisms which produce via - or
-channel can have a strong correlation to the rare B decays. We demonstrate
this link by studying a model with a new charged gauge boson, . In terms of
the current measurements on decays, we conclude that the branching
ratio for is affected most by the new effects.
Furthermore, using the world average branching ratio for the exclusive B decays
at level, we discuss the allowed values for the new parameters.
Finally, we point out that the influence of the new physics effects on the
direct CP asymmetry in B decays is insignificant.Comment: 15 page, 6 figures, typos corrected and references added, final
version to appear journa
Match-action: the role of motion and audio in creating global change blindness in film
An everyday example of change blindness is our difficulty to detect cuts in an edited moving-image. Edit Blindness (Smith & Henderson, 2008) is created by adhering to the continuity editing conventions of Hollywood, e.g. coinciding a cut with a sudden onset of motion (Match-Action). In this study we isolated the roles motion and audio play in limiting awareness of match-action cuts by removing motion before and/or after cuts in existing Hollywood film clips and presenting the clips with or without the original soundtrack whilst participants tried to detect cuts. Removing post-cut motion significantly decreased cut detection time and the probability of missing the cut. By comparison, removing pre-cut motion had no effect suggesting, contrary to the editing literature, that the onset of motion before a cut may not be as critical for creating edit blindness as the motion after a cut. Analysis of eye movements indicated that viewers reoriented less to new content across intact match-action cuts than shots with motion removed. Audio played a surprisingly large part in creating edit blindness with edit blindness mostly disappearing without audio. These results extend film editor intuitions and are discussed in the context of the Attentional Theory of Cinematic Continuity (Smith, 2012a)
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