9,752 research outputs found
Numerical studies of identification in nonlinear distributed parameter systems
An abstract approximation framework and convergence theory for the identification of first and second order nonlinear distributed parameter systems developed previously by the authors and reported on in detail elsewhere are summarized and discussed. The theory is based upon results for systems whose dynamics can be described by monotone operators in Hilbert space and an abstract approximation theorem for the resulting nonlinear evolution system. The application of the theory together with numerical evidence demonstrating the feasibility of the general approach are discussed in the context of the identification of a first order quasi-linear parabolic model for one dimensional heat conduction/mass transport and the identification of a nonlinear dissipation mechanism (i.e., damping) in a second order one dimensional wave equation. Computational and implementational considerations, in particular, with regard to supercomputing, are addressed
Field Theory as Free Fall
It is shown that the classical field equations pertaining to gravity coupled
to other bosonic fields are equivalent to a single geodesic equation,
describing the free fall of a point particle in superspace. Some implications
for quantum gravity are discussed.Comment: 18 pages, plain late
Recommended from our members
Home accidents amongst elderly people: A locality study in Scotland
Aim
The aim of this locality study was to collect information on reported and unreported accidents amongst elderly people living in one locality in Scotland.
Method
Postal Survey- A postal questionnaire was sent to 3,757 men and women aged 65+ years living in one locality. The questionnaire asked respondents to indicate how many accidents they had experienced in the past twelve months, plus to indicate type and location. Information was gathered on living arrangements, ethnicity, gender, age and deprivation. Respondents were asked if they would be willing to take part in an interview study.
Interview Study - One hundred elders who had had at least one accident in the previous twelve months were interviewed.
Results
Postal Survey - Over a third of the respondents in the postal survey reported having had an accident in the previous twelve months. Bumps and drops and falls were the most common type of accident. Most accidents happened in the kitchen. Women reported more falls than men and those living alone reported more accidents than those living with others. Age was associated with the prevalence of accidents, but the association was somewhat curvilinear, with accidents decreasing with age and then increasing again.
Interview Study â Interviewees found it hard to differentiate one accident from another. Considerable reluctance to visit the GP after an accident was noted, with many not attending even for serious accidents. Almost forty percent were âveryâ distressed after their accident, and a quarter reported a loss of confidence. However, most did not worry about accidents. Few thought that their age, health or medications were a cause of their accidents
Quantum Moduli Spaces of String Theories
Generically, string models with supersymmetry are not expected to have
moduli beyond perturbation theory; stringy non-perturbative effects as well as
low energy field-theoretic phenomena such as gluino condensation will lift any
flat directions. In this note, we describe models where some subspace of the
moduli space survives non-perturbatively. Discrete symmetries forbid any
inherently stringy effects, and dynamical considerations control the
field-theoretic effects. The surviving subspace is a space of high symmetry;
the system is attracted to this subspace by a potential which we compute.
Models of this type may be useful for considerations of duality and raise
troubling cosmological questions about string theory. Our considerations also
suggest a mechanism for fixing the expectation value of the dilaton.Comment: 26 pages; uses harvmac. Footnote re fixing dilaton adde
Comments on information loss and remnants
The information loss and remnant proposals for resolving the black hole
information paradox are reconsidered. It is argued that in typical cases
information loss implies energy loss, and thus can be thought of in terms of
coupling to a spectrum of ``fictitious'' remnants. This suggests proposals for
information loss that do not imply planckian energy fluctuations in the low
energy world. However, if consistency of gravity prevents energy
non-conservation, these remnants must then be considered to be real. In either
case, the catastrophe corresponding to infinite pair production remains a
potential problem. Using Reissner-Nordstrom black holes as a paradigm for a
theory of remnants, it is argued that couplings in such a theory may give
finite production despite an infinite spectrum. Evidence for this is found in
analyzing the instanton for Schwinger production; fluctuations from the
infinite number of states lead to a divergent stress tensor, spoiling the
instanton calculation. Therefore naive arguements for infinite production fail.Comment: 30 pages (harvmac l mode) UCSBTH-93-35 (minor reference and typo
corrections
Spherically symmetric false vacuum: no-go theorems and global structure
We enumerate all possible types of spacetime causal structures that can
appear in static, spherically symmetric configurations of a self-gravitating,
real, nonlinear, minimally coupled scalar field \phi in general relativity,
with an arbitrary potential V(\phi), not necessarily positive-definite. It is
shown that a variable scalar field adds nothing to the list of possible
structures with a constant \phi field, namely, Minkowski (or AdS),
Schwarzschild, de Sitter and Schwarzschild - de Sitter. It follows, in
particular, that, whatever is V(\phi), this theory does not admit regular black
holes with flat or AdS asymptotics. It is concluded that the only possible
globally regular, asymptotically flat solutions are solitons with a regular
center, without horizons and with at least partly negative potentials V(\phi).
Extension of the results to more general field models is discussed.Comment: Latex2e, 4 pages, 1 bezier figur
Vacuum Bubble in an Inhomogeneous Cosmology
We study the propagation of bubbles of new vacuum in a radially inhomogeneous
Lemaitre-Tolman-Bondi background that includes a cosmological constant. This
exemplifies the classical evolution of a tunneling bubble through a metastable
state with curvature inhomogeneities, and will be relevant in the context of
the Landscape. We demand that the matter profile in the LTB background satisfy
the weak energy condition. For sample profiles that satisfy this restriction,
we find that the evolution of the bubble (in terms of the physically relevant
coordinates intrinsic to the shell) is largely unaffected by the prsence of
local inhomogeneities. Our setup should also be a useful toy model for
capturing the effects of ambient inhomogeneities on an inflating region.Comment: 31 pages, 21(!) figures, v2: minor changes, figures re-sized (might
require zoom on some systems), references adde
Spontaneous Flavor and Parity Breaking with Wilson Fermions
We discuss the phase diagram of Wilson fermions in the -- plane for
two-flavor QCD. We argue that, as originally suggested by Aoki, there is a
phase in which flavor and parity are spontaneously broken. Recent numerical
results on the spectrum of the overlap Hamiltonian have been interpreted as
evidence against Aoki's conjecture. We show that they are in fact consistent
with the presence of a flavor-parity broken ``Aoki phase''. We also show how,
as the continuum limit is approached, one can study the lattice theory using
the continuum chiral Lagrangian supplemented by additional terms proportional
to powers of the lattice spacing. We find that there are two possible phase
structures at non-zero lattice spacing: (1) there is an Aoki phase of width
with two massless Goldstone pions; (2) there is no
symmetry breaking, and all three pions have an equal non-vanishing mass of
order . Present numerical evidence suggests that the former option is
realized for Wilson fermions. Our analysis then predicts the form of the pion
masses and the flavor-parity breaking condensate within the Aoki phase. Our
analysis also applies for non-perturbatively improved Wilson fermions.Comment: 22 pages, LaTeX, 5 figures (added several references and a comment
Black Hole Remnants and the Information Puzzle
Magnetically charged dilatonic black holes have a perturbatively infinite
ground state degeneracy associated with an infinite volume throat region of the
geometry. A simple argument based on causality is given that these states do
not have a description as ordinary massive particles in a low-energy effective
field theory. Pair production of magnetic black holes in a weak magnetic field
is estimated in a weakly-coupled semiclassical expansion about an instanton and
found to be finite, despite the infinite degeneracy of states. This suggests
that these states may store the information apparently lost in black hole
scattering processes.Comment: 16 pages, revision has 5 figures uuencode
Group and individual variability in speech production networks during delayed auditory feedback
Altering reafferent sensory information can have a profound effect on motor output. Introducing a short delay [delayed auditory feedback (DAF)] during speech production results in modulations of voice and loudness, and produces a range of speech dysfluencies. The ability of speakers to resist the effects of delayed feedback is variable yet it is unclear what neural processes underlie differences in susceptibility to DAF. Here, susceptibility to DAF is investigated by looking at the neural basis of within and between subject changes in speech fluency under 50 and 200âms delay conditions. Using functional magnetic resonance imaging, networks involved in producing speech under two levels of DAF were identified, lying largely within networks active during normal speech production. Independent of condition, fluency ratings were associated with midbrain activity corresponding to periaqueductal grey matter. Across subject variability in ability to produce normal sounding speech under a 200âms delay was associated with activity in ventral sensorimotor cortices, whereas ability to produce normal sounding speech under a 50âms delay was associated with left inferior frontal gyrus activity. These data indicate whilst overlapping cortical mechanisms are engaged for speaking under different delay conditions, susceptibility to different temporal delays in speech feedback may involve different process
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