9,694 research outputs found
Causal Fermions in Discrete Spacetime
In this paper, we consider fermionic systems in discrete spacetime evolving
with a strict notion of causality, meaning they evolve unitarily and with a
bounded propagation speed. First, we show that the evolution of these systems
has a natural decomposition into a product of local unitaries, which also holds
if we include bosons. Next, we show that causal evolution of fermions in
discrete spacetime can also be viewed as the causal evolution of a lattice of
qubits, meaning these systems can be viewed as quantum cellular automata.
Following this, we discuss some examples of causal fermionic models in discrete
spacetime that become interesting physical systems in the continuum limit:
Dirac fermions in one and three spatial dimensions, Dirac fields and briefly
the Thirring model. Finally, we show that the dynamics of causal fermions in
discrete spacetime can be efficiently simulated on a quantum computer.Comment: 16 pages, 1 figur
Discrete Spacetime and Relativistic Quantum Particles
We study a single quantum particle in discrete spacetime evolving in a causal
way. We see that in the continuum limit any massless particle with a two
dimensional internal degree of freedom obeys the Weyl equation, provided that
we perform a simple relabeling of the coordinate axes or demand rotational
symmetry in the continuum limit. It is surprising that this occurs regardless
of the specific details of the evolution: it would be natural to assume that
discrete evolutions giving rise to relativistic dynamics in the continuum limit
would be very special cases. We also see that the same is not true for
particles with larger internal degrees of freedom, by looking at an example
with a three dimensional internal degree of freedom that is not relativistic in
the continuum limit. In the process we give a formula for the Hamiltonian
arising from the continuum limit of massless and massive particles in discrete
spacetime.Comment: 6 page
NLTE 1.5D Modeling of Red Giant Stars
Spectra for 2D stars in the 1.5D approximation are created from synthetic
spectra of 1D non-local thermodynamic equilibrium (NLTE) spherical model
atmospheres produced by the PHOENIX code. The 1.5D stars have the spatially
averaged Rayleigh-Jeans flux of a K3-4 III star, while varying the temperature
difference between the two 1D component models (),
and the relative surface area covered. Synthetic observable quantities from the
1.5D stars are fitted with quantities from NLTE and local thermodynamic
equilibrium (LTE) 1D models to assess the errors in inferred
values from assuming horizontal homogeneity and LTE. Five different quantities
are fit to determine the of the 1.5D stars: UBVRI
photometric colors, absolute surface flux SEDs, relative SEDs, continuum
normalized spectra, and TiO band profiles. In all cases except the TiO band
profiles, the inferred value increases with increasing
. In all cases, the inferred value
from fitting 1D LTE quantities is higher than from fitting 1D NLTE quantities
and is approximately constant as a function of
within each case. The difference between LTE and NLTE for the TiO bands is
caused indirectly by the NLTE temperature structure of the upper atmosphere, as
the bands are computed in LTE. We conclude that the difference between
values derived from NLTE and LTE modelling is relatively
insensitive to the degree of the horizontal inhomogeneity of the star being
modeled, and largely depends on the observable quantity being fit.Comment: 46 pages, 14 figures, 7 tables, accepted for publication in ApJ on
April 5, 201
Combining Semi-Analytic Models of Galaxy Formation with Simulations of Galaxy Clusters: the Need for AGN Heating
We present hydrodynamical N-body simulations of clusters of galaxies with
feedback taken from semi-analytic models of galaxy formation. The advantage of
this technique is that the source of feedback in our simulations is a
population of galaxies that closely resembles that found in the real universe.
We demonstrate that, to achieve the high entropy levels found in clusters,
active galactic nuclei must inject a large fraction of their energy into the
intergalactic/intracluster media throughout the growth period of the central
black hole. These simulations reinforce the argument of Bower et al. (2008),
who arrived at the same conclusion on the basis of purely semi-analytic
reasoning.Comment: 4 pages, 1 figure. To appear in the proceedings of "The Monster's
Fiery Breath", Eds. Sebastian Heinz and Eric Wilcots (AIP conference series
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