3,864 research outputs found
Three Great American Disinflations
This paper analyzes the role of transparency and credibility in accounting for the widely divergent macroeconomic effects of three episodes of deliberate monetary contraction: the post-Civil War deflation, the post-WWI deflation, and the Volcker disinflation. Using a dynamic general equilibrium model in which private agents use optimal filtering to infer the central bank's nominal anchor, we demonstrate that the salient features of these three historical episodes can be explained by differences in the design and transparency of monetary policy, even without any time variation in economic structure or model parameters. For a policy regime with relatively high credibility, our analysis highlights the benefits of a gradualist approach (as in the 1870s) rather than a sudden change in policy (as in 1920-21). In contrast, for a policy institution with relatively low credibility (such as the Federal Reserve in late 1980), an aggressive policy stance can play an important signalling role by making the policy shift more evident to private agents.
Tensor-product representations for string-net condensed states
We show that general string-net condensed states have a natural
representation in terms of tensor product states (TPS) . These TPS's are built
from local tensors. They can describe both states with short-range entanglement
(such as the symmetry breaking states) and states with long-range entanglement
(such as string-net condensed states with topological/quantum order). The
tensor product representation provides a kind of 'mean-field' description for
topologically ordered states and could be a powerful way to study quantum phase
transitions between such states. As an attempt in this direction, we show that
the constructed TPS's are fixed-points under a certain wave-function
renormalization group transformation for quantum states.Comment: 11 pages. RevTeX
Quantum ether: photons and electrons from a rotor model
We give an example of a purely bosonic model -- a rotor model on the 3D cubic
lattice -- whose low energy excitations behave like massless U(1) gauge bosons
and massless Dirac fermions. This model can be viewed as a ``quantum ether'': a
medium that gives rise to both photons and electrons. It illustrates a general
mechanism for the emergence of gauge bosons and fermions known as ``string-net
condensation.'' Other, more complex, string-net condensed models can have
excitations that behave like gluons, quarks and other particles in the standard
model. This suggests that photons, electrons and other elementary particles may
have a unified origin: string-net condensation in our vacuum.Comment: 10 pages, 6 figures, RevTeX4. Home page http://dao.mit.edu/~we
Is the electrostatic force between a point charge and a neutral metallic object always attractive?
We give an example of a geometry in which the electrostatic force between a
point charge and a neutral metallic object is repulsive. The example consists
of a point charge centered above a thin metallic hemisphere, positioned concave
up. We show that this geometry has a repulsive regime using both a simple
analytical argument and an exact calculation for an analogous two-dimensional
geometry. Analogues of this geometry-induced repulsion can appear in many other
contexts, including Casimir systems.Comment: 7 pages, 7 figure
A survey of spinning test particle orbits in Kerr spacetime
We investigate the dynamics of the Papapetrou equations in Kerr spacetime.
These equations provide a model for the motion of a relativistic spinning test
particle orbiting a rotating (Kerr) black hole. We perform a thorough parameter
space search for signs of chaotic dynamics by calculating the Lyapunov
exponents for a large variety of initial conditions. We find that the
Papapetrou equations admit many chaotic solutions, with the strongest chaos
occurring in the case of eccentric orbits with pericenters close to the limit
of stability against plunge into a maximally spinning Kerr black hole. Despite
the presence of these chaotic solutions, we show that physically realistic
solutions to the Papapetrou equations are not chaotic; in all cases, the
chaotic solutions either do not correspond to realistic astrophysical systems,
or involve a breakdown of the test-particle approximation leading to the
Papapetrou equations (or both). As a result, the gravitational radiation from
bodies spiraling into much more massive black holes (as detectable, for
example, by LISA, the Laser Interferometer Space Antenna) should not exhibit
any signs of chaos.Comment: Submitted to Phys. Rev. D. Follow-up to gr-qc/0210042. Figures are
low-resolution in order to satisfy archive size constraints; a
high-resolution version is available at http://www.michaelhartl.com/papers
String-net condensation: A physical mechanism for topological phases
We show that quantum systems of extended objects naturally give rise to a
large class of exotic phases - namely topological phases. These phases occur
when the extended objects, called ``string-nets'', become highly fluctuating
and condense. We derive exactly soluble Hamiltonians for 2D local bosonic
models whose ground states are string-net condensed states. Those ground states
correspond to 2D parity invariant topological phases. These models reveal the
mathematical framework underlying topological phases: tensor category theory.
One of the Hamiltonians - a spin-1/2 system on the honeycomb lattice - is a
simple theoretical realization of a fault tolerant quantum computer. The higher
dimensional case also yields an interesting result: we find that 3D string-net
condensation naturally gives rise to both emergent gauge bosons and emergent
fermions. Thus, string-net condensation provides a mechanism for unifying gauge
bosons and fermions in 3 and higher dimensions.Comment: 21 pages, RevTeX4, 19 figures. Homepage http://dao.mit.edu/~we
Collective states of non-abelian quasiparticles in a magnetic field
Motivated by the physics of the Moore-Read \nu = 1/2 state away from
half-filling, we investigate collective states of non-abelian e/4
quasiparticles in a magnetic field. We consider two types of collective states:
incompressible liquids and Wigner crystals. In the incompressible liquid case,
we construct a natural series of states which can be thought of as a
non-abelian generalization of the Laughlin states. These states are associated
with a series of hierarchical states derived from the Moore-Read state - the
simplest of which occur at filling fraction 8/17 and 7/13. Interestingly, we
find that the hierarchical states are abelian even though their parent state is
non-abelian. In the Wigner crystal case, we construct two candidate states. We
find that they, too, are abelian - in agreement with previous analysis.Comment: 11 page
The Phenology of Ticks and the Effects of Long-Term Prescribed Burning on Tick Population Dynamics in Southwestern Georgia and Northwestern Florida
Some tick populations have increased dramatically in the past several decades leading to an increase in the incidence and emergence of tick-borne diseases. Management strategies that can effectively reduce tick populations while better understanding regional tick phenology is needed. One promising management strategy is prescribed burning. However, the efficacy of prescribed burning as a mechanism for tick control is unclear because past studies have provided conflicting data, likely due to a failure of some studies to simulate operational management scenarios and/or account for other predictors of tick abundance. Therefore, our study was conducted to increase knowledge of tick population dynamics relative to long-term prescribed fire management. Furthermore, we targeted a region, southwestern Georgia and northwestern Florida (USA), in which little is known regarding tick dynamics so that basic phenology could be determined. Twenty-one plots with varying burn regimes (burned surrounded by burned [BB], burned surrounded by unburned [BUB], unburned surrounded by burned [UBB], and unburned surrounded by unburned [UBUB]) were sampled monthly for two years while simultaneously collecting data on variables that can affect tick abundance (e.g., host abundance, vegetation structure, and micro- and macro-climatic conditions). In total, 47,185 ticks were collected, of which, 99% were Amblyomma americanum, 0.7% were Ixodes scapularis, and fewer numbers of Amblyomma maculatum, Ixodes brunneus, and Dermacentor variabilis. Monthly seasonality trends were similar between 2010 and 2011. Long-term prescribed burning consistently and significantly reduced tick counts (overall and specifically for A. americanum and I. scapularis) regardless of the burn regimes and variables evaluated. Tick species composition varied according to burn regime with A. americanum dominating at UBUB, A. maculatum at BB, I. scapularis at UBB, and a more even composition at BUB. These data indicate that regular prescribed burning is an effective tool for reducing tick populations and ultimately may reduce risk of tick-borne disease
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