140 research outputs found
Closed Strings in Misner Space: Stringy Fuzziness with a Twist
Misner space, also known as the Lorentzian orbifold , is the
simplest tree-level solution of string theory with a cosmological singularity.
We compute tree-level scattering amplitudes involving twisted states, using
operator and current algebra techniques. We find that, due to zero-point
quantum fluctuations of the excited modes, twisted strings with a large winding
number are fuzzy on a scale , which can be much larger than
the string scale. Wave functions are smeared by an operator reminiscent of the Moyal-product of non-commutative
geometry, which, since is real, modulates the amplitude rather
than the phase of the wave function, and is purely gravitational in its origin.
We compute the scattering amplitude of two twisted states and one tachyon or
graviton, and find a finite result. The scattering amplitude of two twisted and
two untwisted states is found to diverge, due to the propagation of
intermediate winding strings with vanishing boost momentum. The scattering
amplitude of three twisted fields is computed by analytic continuation from
three-point amplitudes of states with non-zero in the Nappi-Witten plane
wave, and the non-locality of the three-point vertex is found to diverge for
certain kinematical configurations. Our results for the three-point amplitudes
allow in principle to compute, to leading order, the back-reaction on the
metric due to a condensation of coherent winding strings.Comment: 29 pages, Latex2e, uses JHEP3.cls; v3: minor corrections, final
version to appear in JCA
The Medicare and Medicaid Anti-Kickback Statute: Safe Harbors Eradicate Ambiguity
This Note will briefly explore the history of the Medicare and Medicaid programs including the introduction of the PPS. Next, the Note will detail the legislative history surrounding the adoption of the MMAKS and the judicial interpretation applied to its elements. The Note will follow with an analysis of the purpose, goals, and disagreements relating to the MMPPPA\u27s Safe Harbor regulations, resolving their alleged ambiguity against the medical profession. Finally, the Note will advocate support of the recently proposed Health Care Cost Containment and Reform Act of 1992 with emphasis on increasing the budget and size of the staff within the Office of the Inspector General
Quantum evolution across singularities
Attempts to consider evolution across space-time singularities often lead to
quantum systems with time-dependent Hamiltonians developing an isolated
singularity as a function of time. Examples include matrix theory in certain
singular time-dependent backgounds and free quantum fields on the
two-dimensional compactified Milne universe. Due to the presence of the
singularities in the time dependence, the conventional quantum-mechanical
evolution is not well-defined for such systems. We propose a natural way,
mathematically analogous to renormalization in conventional quantum field
theory, to construct unitary quantum evolution across the singularity. We carry
out this procedure explicitly for free fields on the compactified Milne
universe and compare our results with the matching conditions considered in
earlier work (which were based on the covering Minkowski space).Comment: revised with an emphasis on local counterterm subtraction rather than
analyticity; version to be submitted for publicatio
Wavelet transforms in a critical interface model for Barkhausen noise
We discuss the application of wavelet transforms to a critical interface
model, which is known to provide a good description of Barkhausen noise in soft
ferromagnets. The two-dimensional version of the model (one-dimensional
interface) is considered, mainly in the adiabatic limit of very slow driving.
On length scales shorter than a crossover length (which grows with the strength
of surface tension), the effective interface roughness exponent is
, close to the expected value for the universality class of the
quenched Edwards-Wilkinson model. We find that the waiting times between
avalanches are fully uncorrelated, as the wavelet transform of their
autocorrelations scales as white noise. Similarly, detrended size-size
correlations give a white-noise wavelet transform. Consideration of finite
driving rates, still deep within the intermittent regime, shows the wavelet
transform of correlations scaling as for intermediate frequencies.
This behavior is ascribed to intra-avalanche correlations.Comment: RevTeX, 10 pages, 9 .eps figures; Physical Review E, to be publishe
Signature of effective mass in crackling noise asymmetry
Crackling noise is a common feature in many dynamic systems [1-9], the most
familiar instance of which is the sound made by a sheet of paper when crumpled
into a ball. Although seemingly random, this noise contains fundamental
information about the properties of the system in which it occurs. One
potential source of such information lies in the asymmetric shape of noise
pulses emitted by a diverse range of noisy systems [8-12], but the cause of
this asymmetry has lacked explanation [1]. Here we show that the leftward
asymmetry observed in the Barkhausen effect [2] - the noise generated by the
jerky motion of domain walls as they interact with impurities in a soft magnet
- is a direct consequence of a magnetic domain wall's negative effective mass.
As well as providing a means of determining domain wall effective mass from a
magnet's Barkhausen noise our work suggests an inertial explanation for the
origin of avalanche asymmetries in crackling noise phenomena more generally.Comment: 13 pages, 4 figures, to appear in Nature Physic
Finite driving rates in interface models of Barkhausen noise
We consider a single-interface model for the description of Barkhausen noise
in soft ferromagnetic materials. Previously, the model had been used only in
the adiabatic regime of infinitely slow field ramping. We introduce finite
driving rates and analyze the scaling of event sizes and durations for
different regimes of the driving rate. Coexistence of intermittency, with
non-trivial scaling laws, and finite-velocity interface motion is observed for
high enough driving rates. Power spectra show a decay , with
for finite driving rates, revealing the influence of the internal
structure of avalanches.Comment: 7 pages, 6 figures, RevTeX, final version to be published in Phys.
Rev.
Time Dependent Cosmologies and Their Duals
We construct a family of solutions in IIB supergravity theory. These are time
dependent or depend on a light-like coordinate and can be thought of as
deformations of AdS_5 x S^5. Several of the solutions have singularities. The
light-like solutions preserve 8 supersymmetries. We argue that these solutions
are dual to the N=4 gauge theory in a 3+1 dimensional spacetime with a metric
and a gauge coupling that is varying with time or the light-like direction
respectively. This identification allows us to map the question of singularity
resolution to the dual gauge theory.Comment: 13 pages REVTeX and AMSLaTeX. v2: corrected typos and made some
clarifications; reference added; v3: more clarifications, references adde
String spectra near some null cosmological singularities
We construct cosmological spacetimes with null Kasner-like singularities as
purely gravitational solutions with no other background fields turned on. These
can be recast as anisotropic plane-wave spacetimes by coordinate
transformations. We analyse string quantization to find the spectrum of string
modes in these backgrounds. The classical string modes can be solved for
exactly in these time-dependent backgrounds, which enables a detailed study of
the near singularity string spectrum, (time-dependent) oscillator masses and
wavefunctions. We find that for low lying string modes(finite oscillation
number), the classical near-singularity string mode functions are non-divergent
for various families of singularities. Furthermore, for any infinitesimal
regularization of the vicinity of the singularity, we find a tower of string
modes of ultra-high oscillation number which propagate essentially freely in
the background. The resulting picture suggests that string interactions are
non-negligible near the singularity.Comment: Latex, 30pgs; v2. minor clarifications, references adde
Barkhausen noise from zigzag domain walls
We investigate the Barkhausen noise in ferromagnetic thin films with zigzag
domain walls. We use a cellular automaton model that describes the motion of a
zigzag domain wall in an impure ferromagnetic quasi-two dimensional sample with
in-plane uniaxial magnetization at zero temperature, driven by an external
magnetic field. The main ingredients of this model are the dipolar spin-spin
interactions and the anisotropy energy. A power law behavior with a cutoff is
found for the probability distributions of size, duration and correlation
length of the Barkhausen avalanches, and the critical exponents are in
agreement with the available experiments. The link between the size and the
duration of the avalanches is analyzed too, and a power law behavior is found
for the average size of an avalanche as a function of its duration.Comment: 11 pages, 12 figure
Nonstationary dynamics of the Alessandro-Beatrice-Bertotti-Montorsi model
We obtain an exact solution for the motion of a particle driven by a spring
in a Brownian random-force landscape, the Alessandro-Beatrice-Bertotti-Montorsi
(ABBM) model. Many experiments on quasi-static driving of elastic interfaces
(Barkhausen noise in magnets, earthquake statistics, shear dynamics of granular
matter) exhibit the same universal behavior as this model. It also appears as a
limit in the field theory of elastic manifolds. Here we discuss predictions of
the ABBM model for monotonous, but otherwise arbitrary, time-dependent driving.
Our main result is an explicit formula for the generating functional of
particle velocities and positions. We apply this to derive the
particle-velocity distribution following a quench in the driving velocity. We
also obtain the joint avalanche size and duration distribution and the mean
avalanche shape following a jump in the position of the confining spring. Such
non-stationary driving is easy to realize in experiments, and provides a way to
test the ABBM model beyond the stationary, quasi-static regime. We study
extensions to two elastically coupled layers, and to an elastic interface of
internal dimension d, in the Brownian force landscape. The effective action of
the field theory is equal to the action, up to 1-loop corrections obtained
exactly from a functional determinant. This provides a connection to
renormalization-group methods.Comment: 18 pages, 3 figure
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