971 research outputs found
Non-equilibrium dynamics of bulk-deterministic cellular automata
In this thesis we study simple one-dimensional nonequilibirum many-body systems, namely, reversible cellular automata (RCA). These are discrete time lattice models exhibiting emergent collective excitations---solitons---that move with fixed velocities and that interact via pairwise scattering. In particular, we study the attractively interacting Rule 201 RCA and noninteracting Rule 150 RCA which, together with the extensively studied repulsively interacting Rule 54 RCA constitute arguably the simplest one-dimensional microscopic physical models of strongly interacting and asymptotically freely propagating particles, to investigate interacting nonequilibrium many-body dynamics.
After a brief literature review of the field, we present the first publication-style chapter which considers the Rule 201 RCA. Here, we study the stationary or steady state properties of systems with periodic, deterministic, and stochastic boundary conditions. We demonstrate that, despite the complexities of the model, specifically, a reducible state space and nontrivial topological vacuum, the model exhibits a simple and intuitive quasiparticle interpretation, reminiscent of the simpler Rule 54 RCA. This enables us to obtain exact expressions for the steady states in terms of a highly versatile matrix product state (MPS) representation that takes an instructive generalized Gibbs ensemble form.
In the second publication-style chapter, we study the Rule 150 RCA. Due to its simplicity, originating from the noninteracting dynamics, we are able to obtain many exact results relating to its dynamics. To start, we generalize the MPS ansatz used to study the Rule 201 RCA, and find its exact steady state distribution for identical boundary conditions. We proceed to extend the MPS ansatz further and obtain a class of eigenvectors that form the dominant decay modes of the Markov propagator. Following this, we postulate a conjecture for the complete spectrum, which is in perfect agreement with numerics obtained via exact diagonalization of computationally tractable system sizes, providing access to the full relaxation dynamics. From here, we further utilise the ansatz to investigate the large deviation statistics and obtain exact expressions for its scaled cumulant generating function and rate function, which demonstrate the existence of a dynamical first order phase transition.
The third and final publication-style chapter focuses on the exact dynamical large deviations statistics of the Rule 201 RCA. Specifically, we employ the methods introduced to study the large deviations of the Rule 54 RCA and show that they fail here to provide any insight into the atypical dynamical behaviour of the Rule 201 RCA. We suggest that this is due to the restrictions imposed by the local dynamical rules, which limits the support of the local observables. In spite of this, we explicitly derived an exact analytic expression for the dominant eigenvalue of the tilted Markov propagator, from which several large deviation statistics can be obtained
Non-equilibrium dynamics of bulk-deterministic cellular automata
In this thesis we study simple one-dimensional nonequilibirum many-body systems, namely, reversible cellular automata (RCA). These are discrete time lattice models exhibiting emergent collective excitations---solitons---that move with fixed velocities and that interact via pairwise scattering. In particular, we study the attractively interacting Rule 201 RCA and noninteracting Rule 150 RCA which, together with the extensively studied repulsively interacting Rule 54 RCA constitute arguably the simplest one-dimensional microscopic physical models of strongly interacting and asymptotically freely propagating particles, to investigate interacting nonequilibrium many-body dynamics.
After a brief literature review of the field, we present the first publication-style chapter which considers the Rule 201 RCA. Here, we study the stationary or steady state properties of systems with periodic, deterministic, and stochastic boundary conditions. We demonstrate that, despite the complexities of the model, specifically, a reducible state space and nontrivial topological vacuum, the model exhibits a simple and intuitive quasiparticle interpretation, reminiscent of the simpler Rule 54 RCA. This enables us to obtain exact expressions for the steady states in terms of a highly versatile matrix product state (MPS) representation that takes an instructive generalized Gibbs ensemble form.
In the second publication-style chapter, we study the Rule 150 RCA. Due to its simplicity, originating from the noninteracting dynamics, we are able to obtain many exact results relating to its dynamics. To start, we generalize the MPS ansatz used to study the Rule 201 RCA, and find its exact steady state distribution for identical boundary conditions. We proceed to extend the MPS ansatz further and obtain a class of eigenvectors that form the dominant decay modes of the Markov propagator. Following this, we postulate a conjecture for the complete spectrum, which is in perfect agreement with numerics obtained via exact diagonalization of computationally tractable system sizes, providing access to the full relaxation dynamics. From here, we further utilise the ansatz to investigate the large deviation statistics and obtain exact expressions for its scaled cumulant generating function and rate function, which demonstrate the existence of a dynamical first order phase transition.
The third and final publication-style chapter focuses on the exact dynamical large deviations statistics of the Rule 201 RCA. Specifically, we employ the methods introduced to study the large deviations of the Rule 54 RCA and show that they fail here to provide any insight into the atypical dynamical behaviour of the Rule 201 RCA. We suggest that this is due to the restrictions imposed by the local dynamical rules, which limits the support of the local observables. In spite of this, we explicitly derived an exact analytic expression for the dominant eigenvalue of the tilted Markov propagator, from which several large deviation statistics can be obtained
On the Enhanced Interstellar Scattering Toward B1849+005
(Abridged) This paper reports new Very Large Array (VLA) and Very Long
Baseline Array (VLBA) observations of the extragalactic source B1849+005 at
frequencies between 0.33 and 15 GHz and the re-analysis of archival VLA
observations at 0.33, 1.5, and 4.9 GHz. The structure of this source is complex
but interstellar scattering dominates the structure of the central component at
least to 15 GHz. An analysis of the phase structure functions of the
interferometric visibilities shows the density fluctuations along this line of
sight to be anisotropic (axial ratio = 1.3) with a frequency-independent
position angle, and having an inner scale of roughly a few hundred kilometers.
The anisotropies occur on length scales of order 10^{15} cm (D/5 kpc), which
within the context of certain magnetohydrodynamic turbulence theories indicates
the length scale on which the kinetic and magnetic energy densities are
comparable. A conservative upper limit on the velocity of the scattering
material is 1800 km/s. In the 0.33 GHz field of view, there are a number of
other sources that might also be heavily scattered. Both B1849+005 and PSR
B1849+00 are highly scattered, and they are separated by only 13'. If the lines
of sight are affected by the same ``clump'' of scattering material, it must be
at least 2.3 kpc distant. However, a detailed attempt to account for the
scattering observables toward these sources does not produce a self-consistent
set of parameters for such a clump. A clump of H\alpha emission, possibly
associated with the H II region G33.418-0.004, lies between these two lines of
sight, but it seems unable to account for all of the required excess
scattering.Comment: 23 pages, LaTeX2e AASTeX, 13 figures in 14 PostScript files, accepted
for publication in Ap
Anomalous Radio-Wave Scattering from Interstellar Plasma Structures
This paper considers scattering screens that have arbitrary spatial
variations of scattering strength transverse to the line of sight, including
screens that are spatially well confined, such as disks and filaments. We
calculate the scattered image of a point source and the observed pulse shape of
a scattered impulse. The consequences of screen confinement include: (1) Source
image shapes that are determined by the physical extent of the screen rather
than by the shapes of much-smaller diffracting microirregularities. These
include image elongations and orientations that are frequency dependent. (2)
Variation with frequency of angular broadening that is much weaker than the
trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including
frequency-independent cases; and (3) Similar departure of the pulse broadening
time from the usually expected \nu^{-4} scaling law. We briefly discuss
applications that include scattering of pulses from the Crab pulsar by
filaments in the Crab Nebula; image asymmetries from Galactic scattering of the
sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active
galactic nuclei by intervening galaxies. We also address the consequences for
inferences about the shape of the wavenumber spectrum of electron density
irregularities, which depend on scaling laws for the image size and the pulse
broadening. Future low-frequency (< 100 MHz) array observations will also be
strongly affected by the Galactic structure of scattering material. Our
formalism is derived in the context of radio scattering by plasma density
fluctuations. It is also applicable to optical, UV and X-ray scattering by
grains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by
ApJ, revised version has minor changes to respond to referee comments and
suggestion
Multi-frequency VLBA Observations of the Compact Double B2 2050+36: Constraints on Interstellar Scattering Revisited
We present multi-frequency observations with the Very Long Baseline Array of
the compact double radio source B2 2050+36. Our observations are at 0.33, 0.61,
1.67, 2.3, and 8.4 GHz, with the 0.61 GHz observations forming the third epoch
of observation of this source at that frequency. At 0.61 GHz, the structure of
B2 2050+36 is dominated by two components 56 mas apart. Within the
uncertainties of the various measurements, this separation has remained
unchanged for the past 16 years. Any differential image wander caused by
refractive interstellar scattering is less than 4 mas. Both the lack of
differential image wander and the frequency dependence of the angular diameter
of B2 2050+36 below 1 GHz indicate that the electron density power spectrum
along this line of sight has a spectral index near the Kolmogorov value, with a
value of 4 being highly unlikely. We conclude that diffractive scattering
dominates along this line of sight; results in the literature indicate that
this conclusion also holds true for the line of sight to the pulsar PSR
B2020+28 (8.7 deg. from B2 2050+36). Comparison of our 1.67 GHz observations
with those obtained 21 years previously place a limit on the projected linear
separation velocity of the two components of c.Comment: 19 pages LaTeX2e with AASTeX 5, 3 figures in 7 PostScript files;
accepted for publication in the Ap
Search for Nanosecond Optical Pulses from Nearby Solar‐Type Stars
With "Earth 2000" technology we could generate a directed laser pulse that outshines the broadband visible light of the Sun by 4 orders of magnitude. This is a conservative lower bound for the technical capability of a communicating civilization; optical interstellar communication is thus technically plausible. We have built a pair of systems to detect nanosecond pulsed optical signals from a target list that includes some 13,000 Sun-like stars, and we have made some 16,000 observations totaling nearly 2400 hr during five years of operation. A beam splitter-fed pair of hybrid avalanche photodetectors at the 1.5 m Wyeth Telescope at the Harvard/Smithsonian Oak Ridge Observatory (Agassiz Station) triggers on a coincident pulse pair, initiating measurement of pulse width and intensity at subnanosecond resolution. An identical system at the 0.9 m Cassegrain at Princeton's Fitz-Randolph Observatory performs synchronized observations with 0.1 μs event timing, permitting unambiguous identification of even a solitary pulse. Among the 11,600 artifact-free observations at Harvard, the distribution of 274 observed events shows no pattern of repetition, and is consistent with a model with uniform event rate, independent of target. With one possible exception (HIP 107395), no valid event has been seen simultaneously at the two observatories. We describe the search and candidate events and set limits on the prevalence of civilizations transmitting intense optical pulses
Finding Radio Pulsars in and Beyond the Galactic Center
Radio-wave scattering is enhanced dramatically for Galactic center sources in
a region with radius >~ 15 arc min. Using scattering from Sgr A* and other
sources, we show that pulse broadening for pulsars in the Galactic center is
{\em at least} 6.3 \nu^{-4} seconds (\nu = radio frequency in GHz) and is most
likely 50--200 times larger because the relevant scattering screen appears to
be within the Galactic center region itself. Pulsars beyond---but viewed
through---the Galactic center suffer even greater pulse broadening and are
angularly broadened by <~ 2 {\em arc min}. Periodicity searches at radio
frequencies are likely to find only long period pulsars and, then, only if
optimized by using frequencies >~ 7 GHz and by testing for small numbers of
harmonics in the power spectrum. The optimal frequency is where \Delta_{0.1} is the distance of the
scattering region from Sgr A* in units of 0.1 kpc, P is the period (seconds),
and \alpha is the spectral index. A search for compact sources using aperture
synthesis may be far more successful than searches for periodicities because
the angular broadening is not so large as to desensitize the survey. We
estimate that the number of {\em detectable} pulsars in the Galactic center may
range from <= 1 to 100, with the larger values resulting from recent, vigorous
starbursts. Such pulsars provide unique opportunities for probing the ionized
gas, gravitational potential, and stellar population near Sgr A*.Comment: 13 pages, 4 PS figures, LaTeX and requires AASTeX macro aas2pp4,
accepted by ApJ, also available as
http://astrosun.tn.cornell.edu/SPIGOT/papers/pulsar/gc_psr.web
The Microarcsecond Sky and Cosmic Turbulence
Radio waves are imprinted with propagation effects from ionized media through
which they pass. Owing to electron density fluctuations, compact sources
(pulsars, masers, and compact extragalactic sources) can display a wide variety
of scattering effects. These scattering effects, particularly interstellar
scintillation, can be exploited to provide *superresolution*, with achievable
angular resolutions (<~ 1 microarcsecond) far in excess of what can be obtained
by very long baseline interferometry on terrestrial baselines. Scattering
effects also provide a powerful sub-AU probe of the microphysics of the
interstellar medium, potentially to spatial scales smaller than 100 km, as well
as a tracer of the Galactic distribution of energy input into the interstellar
medium through a variety of integrated measures. Coupled with future gamma-ray
observations, SKA observations also may provide a means of detecting fainter
compact gamma-ray sources. Though it is not yet clear that propagation effects
due to the intergalactic medium are significant, the SKA will either detect or
place stringent constraints on intergalactic scattering.Comment: 20 pages, 8 figures in 8 PostScript files, to appear in "Science with
the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy
Reviews (Elsevier: Amsterdam
HCMV-secreted glycoprotein gpUL4 inhibits TRAIL-mediated apoptosis and NK cell activation
Human cytomegalovirus (HCMV) is a paradigm of pathogen immune evasion and sustains lifelong persistent infection in the face of exceptionally powerful host immune responses through the concerted action of multiple immune-evasins. These reduce NK cell activation by inhibiting ligands for activating receptors, expressing ligands for inhibitory receptors, or inhibiting synapse formation. However, these functions only inhibit direct interactions with the infected cell. To determine whether the virus also expresses soluble factors that could modulate NK function at a distance, we systematically screened all 170 HCMV canonical protein-coding genes. This revealed that UL4 encodes a secreted and heavily glycosylated protein (gpUL4) that is expressed with late-phase kinetics and is capable of inhibiting NK cell degranulation. Analyses of gpUL4 binding partners by mass spectrometry identified an interaction with TRAIL. gpUL4 bound TRAIL with picomolar affinity and prevented TRAIL from binding its receptor, thus acting as a TRAIL decoy receptor. TRAIL is found in both soluble and membrane-bound forms, with expression of the membrane-bound form strongly up-regulated on NK cells in response to interferon. gpUL4 inhibited apoptosis induced by soluble TRAIL, while also binding to the NK cell surface in a TRAIL-dependent manner, where it blocked NK cell degranulation and cytokine secretion. gpUL4 therefore acts as an immune-evasin by inhibiting both soluble and membrane-bound TRAIL and is a viral-encoded TRAIL decoy receptor. Interestingly, gpUL4 could also suppress NK responses to heterologous viruses, suggesting that it may act as a systemic virally encoded immunosuppressive agent
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