10,216 research outputs found
Ion-by-ion Cooling efficiencies
We present ion-by-ion cooling efficiencies for low-density gas. We use Cloudy
(ver. 08.00) to estimate the cooling efficiencies for each ion of the first 30
elements (H-Zn) individually. We present results for gas temperatures between
1e4 and 1e8K, assuming low densities and optically thin conditions. When
nonequilibrium ionization plays a significant role the ionization states
deviate from those that obtain in collisional ionization equilibrium (CIE), and
the local cooling efficiency at any given temperature depends on specific
non-equilibrium ion fractions. The results presented here allow for an
efficient estimate of the total cooling efficiency for any ionic composition.
We also list the elemental cooling efficiencies assuming CIE conditions. These
can be used to construct CIE cooling efficiencies for non-solar abundance
ratios, or to estimate the cooling due to elements not explicitly included in
any nonequilibrium computation. All the computational results are listed in
convenient online tables.Comment: Submitted to ApJS. Electronic data available at
http://wise-obs.tau.ac.il/~orlyg/ion_by_ion
Infinite time Turing machines and an application to the hierarchy of equivalence relations on the reals
We describe the basic theory of infinite time Turing machines and some recent
developments, including the infinite time degree theory, infinite time
complexity theory, and infinite time computable model theory. We focus
particularly on the application of infinite time Turing machines to the
analysis of the hierarchy of equivalence relations on the reals, in analogy
with the theory arising from Borel reducibility. We define a notion of infinite
time reducibility, which lifts much of the Borel theory into the class
in a satisfying way.Comment: Submitted to the Effective Mathematics of the Uncountable Conference,
200
Chiral transport equation from the quantum Dirac Hamiltonian and the on-shell effective field theory
We derive the relativistic chiral transport equation for massless fermions
and antifermions by performing a semiclassical Foldy-Wouthuysen diagonalization
of the quantum Dirac Hamiltonian. The Berry connection naturally emerges in the
diagonalization process to modify the classical equations of motion of a
fermion in an electromagnetic field. We also see that the fermion and
antifermion dispersion relations are corrected at first order in the Planck
constant by the Berry curvature, as previously derived by Son and Yamamoto for
the particular case of vanishing temperature. Our approach does not require
knowledge of the state of the system, and thus it can also be applied at high
temperature. We provide support for our result by an alternative computation
using an effective field theory for fermions and antifermions: the on-shell
effective field theory. In this formalism, the off-shell fermionic modes are
integrated out to generate an effective Lagrangian for the quasi-on-shell
fermions/antifermions. The dispersion relation at leading order exactly matches
the result from the semiclassical diagonalization. From the transport equation,
we explicitly show how the axial and gauge anomalies are not modified at finite
temperature and density despite the incorporation of the new dispersion
relation into the distribution function.Comment: 9 pages, no figures. v2: Some comments and more details added, typos
fixed and reference list updated. Final version matching the published
articl
A Survey on Continuous Time Computations
We provide an overview of theories of continuous time computation. These
theories allow us to understand both the hardness of questions related to
continuous time dynamical systems and the computational power of continuous
time analog models. We survey the existing models, summarizing results, and
point to relevant references in the literature
The Lost Melody Phenomenon
A typical phenomenon for machine models of transfinite computations is the
existence of so-called lost melodies, i.e. real numbers such that the
characteristic function of the set is computable while itself is
not (a real having the first property is called recognizable). This was first
observed by J. D. Hamkins and A. Lewis for infinite time Turing machine, then
demonstrated by P. Koepke and the author for s. We prove that, for
unresetting infinite time register machines introduced by P. Koepke,
recognizability equals computability, i.e. the lost melody phenomenon does not
occur. Then, we give an overview on our results on the behaviour of
recognizable reals for s. We show that there are no lost melodies for
ordinal Turing machines or ordinal register machines without parameters and
that this is, under the assumption that exists, independent of
. Then, we introduce the notions of resetting and unresetting
-register machines and give some information on the question for which
of these machines there are lost melodies
- …