197 research outputs found
Model Flames in the Boussinesq Limit: The Effects of Feedback
We have studied the fully nonlinear behavior of pre-mixed flames in a
gravitationally stratified medium, subject to the Boussinesq approximation. Key
results include the establishment of criterion for when such flames propagate
as simple planar flames; elucidation of scaling laws for the effective flame
speed; and a study of the stability properties of these flames. The simplicity
of some of our scalings results suggests that analytical work may further
advance our understandings of buoyant flames.Comment: 11 pages, 14 figures, RevTex, gzipped tar fil
Calculation of the Electron Self Energy for Low Nuclear Charge
We present a nonperturbative numerical evaluation of the one-photon electron
self energy for hydrogenlike ions with low nuclear charge numbers Z=1 to 5. Our
calculation for the 1S state has a numerical uncertainty of 0.8 Hz for hydrogen
and 13 Hz for singly-ionized helium. Resummation and convergence acceleration
techniques that reduce the computer time by about three orders of magnitude
were employed in the calculation. The numerical results are compared to results
based on known terms in the expansion of the self energy in powers of (Z
alpha).Comment: 10 pages, RevTeX, 2 figure
The Cold Big-Bang Cosmology as a Counter-example to Several Anthropic Arguments
A general Friedmann big-bang cosmology can be specified by fixing a
half-dozen cosmological parameters such as the photon-to-baryon ratio Eta, the
cosmological constant Lambda, the curvature scale R, and the amplitude Q of
(assumed scale-invariant) primordial density fluctuations. There is currently
no established theory as to why these parameters take the particular values we
deduce from observations. This has led to proposed `anthropic' explanations for
the observed value of each parameter, as the only value capable of generating a
universe that can host intelligent life. In this paper, I explicitly show that
the requirement that the universe generates sun-like stars with planets does
not fix these parameters, by developing a class of cosmologies (based on the
classical `cold big-bang' model) in which some or all of the cosmological
parameters differ by orders of magnitude from the values they assume in the
standard hot big-bang cosmology, without precluding in any obvious way the
existence of intelligent life. I also give a careful discussion of the
structure and context of anthropic arguments in cosmology, and point out some
implications of the cold big-bang model's existence for anthropic arguments
concerning specific parameters.Comment: 13 PRD-style pages, 2 postscript figures. Reference 26 corrected.
Accepted to Phys. Rev.
Electron Self Energy for the K and L Shell at Low Nuclear Charge
A nonperturbative numerical evaluation of the one-photon electron self energy
for the K- and L-shell states of hydrogenlike ions with nuclear charge numbers
Z=1 to 5 is described. Our calculation for the 1S state has a numerical
uncertainty of 0.8 Hz in atomic hydrogen, and for the L-shell states (2S and
2P) the numerical uncertainty is 1.0 Hz. The method of evaluation for the
ground state and for the excited states is described in detail. The numerical
results are compared to results based on known terms in the expansion of the
self energy in powers of (Z alpha).Comment: 21 pages, RevTeX, 5 Tables, 6 figure
Cosmological N-Body Simulations
In this review we discuss Cosmological N-Body codes with a special emphasis
on Particle Mesh codes. We present the mathematical model for each component of
N-Body codes. We compare alternative methods for computing each quantity by
calculating errors for each of the components. We suggest an optimum set of
components that can be combined reduce overall errors in N-Body codes.Comment: This article, published in 1997 is somewhat inaccessible; the present
posting is the original versio
Arbitrary Choice of Basic Variables in Density Functional Theory. II. Illustrative Applications
Our recent theory (Ref. 1) enables us to choose arbitrary quantities as the
basic variables of the density functional theory. In this paper we apply it to
several cases. In the case where the occupation matrix of localized orbitals is
chosen as a basic variable, we can obtain the single-particle equation which is
equivalent to that of the LDA+U method. The theory also leads to the
Hartree-Fock-Kohn-Sham equation by letting the exchange energy be a basic
variable. Furthermore, if the quantity associated with the density of states
near the Fermi level is chosen as a basic variable, the resulting
single-particle equation includes the additional potential which could mainly
modify the energy-band structures near the Fermi level.Comment: 27 page
Can spacetime curvature induced corrections to Lamb shift be observable?
The Lamb shift results from the coupling of an atom to vacuum fluctuations of
quantum fields, so corrections are expected to arise when the spacetime is
curved since the vacuum fluctuations are modified by the presence of spacetime
curvature. Here, we calculate the curvature-induced correction to the Lamb
shift outside a spherically symmetric object and demonstrate that this
correction can be remarkably significant outside a compact massive
astrophysical body. For instance, for a neutron star or a stellar mass black
hole, the correction is 25% at a radial distance of ,
16% at and as large as 1.6% even at , where is
the mass of the object, the Newtonian constant, and the speed of light.
In principle, we can look at the spectra from a distant compact super-massive
body to find such corrections. Therefore, our results suggest a possible way of
detecting fundamental quantum effects in astronomical observations.Comment: 13 pages, 3 figures, slight title change, clarifications and more
discussions added, version to be published in JHE
One-loop self-energy correction to the 1s and 2s hyperfine splitting in H-like systems
The one-loop self-energy correction to the hyperfine splitting of the 1s and
2s levels in H-like low-Z atoms is evaluated to all orders in Z\alpha. The
results are compared to perturbative calculations. The residual higher-order
contribution is evaluated. Implications to the specific difference of the
hyperfine structure intervals 8\Delta \nu_2 - \Delta \nu_1 in He^+ are
investigated.Comment: 17 pages, RevTeX, 3 figure
The dynamical Green's function and an exact optical potential for electron-molecule scattering including nuclear dynamics
We derive a rigorous optical potential for electron-molecule scattering
including the effects of nuclear dynamics by extending the common many-body
Green's function approach to optical potentials beyond the fixed-nuclei limit
for molecular targets. Our formalism treats the projectile electron and the
nuclear motion of the target molecule on the same footing whereby the dynamical
optical potential rigorously accounts for the complex many-body nature of the
scattering target. One central result of the present work is that the common
fixed-nuclei optical potential is a valid adiabatic approximation to the
dynamical optical potential even when projectile and nuclear motion are
(nonadiabatically) coupled as long as the scattering energy is well below the
electronic excitation thresholds of the target. For extremely low projectile
velocities, however, when the cross sections are most sensitive to the
scattering potential, we expect the influences of the nuclear dynamics on the
optical potential to become relevant. For these cases, a systematic way to
improve the adiabatic approximation to the dynamical optical potential is
presented that yields non-local operators with respect to the nuclear
coordinates.Comment: 22 pages, no figures, accepted for publ., Phys. Rev.
Interdependent Infrastructure as Linked Social, Ecological, and Technological Systems (SETSs) to Address Lockâin and Enhance Resilience
Traditional infrastructure adaptation to extreme weather events (and now climate change) has typically been technoâcentric and heavily grounded in robustnessâthe capacity to prevent or minimize disruptions via a riskâbased approach that emphasizes control, armoring, and strengthening (e.g., raising the height of levees). However, climate and nonclimate challenges facing infrastructure are not purely technological. Ecological and social systems also warrant consideration to manage issues of overconfidence, inflexibility, interdependence, and resource utilizationâamong others. As a result, technoâcentric adaptation strategies can result in unwanted tradeoffs, unintended consequences, and underaddressed vulnerabilities. Technoâcentric strategies that lockâin today\u27s infrastructure systems to vulnerable future design, management, and regulatory practices may be particularly problematic by exacerbating these ecological and social issues rather than ameliorating them. Given these challenges, we develop a conceptual model and infrastructure adaptation case studies to argue the following: (1) infrastructure systems are not simply technological and should be understood as complex and interconnected social, ecological, and technological systems (SETSs); (2) infrastructure challenges, like lockâin, stem from SETS interactions that are often overlooked and underappreciated; (3) framing infrastructure with a SETS lens can help identify and prevent maladaptive issues like lockâin; and (4) a SETS lens can also highlight effective infrastructure adaptation strategies that may not traditionally be considered. Ultimately, we find that treating infrastructure as SETS shows promise for increasing the adaptive capacity of infrastructure systems by highlighting how lockâin and vulnerabilities evolve and how multidisciplinary strategies can be deployed to address these challenges by broadening the options for adaptation
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