11,754 research outputs found
Methods Matter: Beating the Backward Clock
In “Beat the (Backward) Clock,” we argued that John Williams and Neil Sinhababu’s Backward Clock Case fails to be a counterexample to Robert Nozick’s or Fred Dretske’s Theories of Knowledge. Williams’ reply to our paper, “There’s Nothing to Beat a Backward Clock: A Rejoinder to Adams, Barker and Clarke,” is a further attempt to defend their counterexample against a range of objections. In this paper, we argue that, despite the number and length of footnotes, Williams is still wrong
Beat the (Backward) Clock
In a recent very interesting and important challenge to tracking theories of knowledge, Williams & Sinhababu claim to have devised a counter-example to tracking theories of knowledge of a sort that escapes the defense of those theories by Adams & Clarke. In this paper we will explain why this is not true. Tracking theories are not undermined by the example of the backward clock, as interesting as the case is
Simulating Cortical Feedback Modulation as Changes in Excitation and Inhibition in a Cortical Circuit Model.
Cortical feedback pathways are hypothesized to distribute context-dependent signals during flexible behavior. Recent experimental work has attempted to understand the mechanisms by which cortical feedback inputs modulate their target regions. Within the mouse whisker sensorimotor system, cortical feedback stimulation modulates spontaneous activity and sensory responsiveness, leading to enhanced sensory representations. However, the cellular mechanisms underlying these effects are currently unknown. In this study we use a simplified neural circuit model, which includes two recurrent excitatory populations and global inhibition, to simulate cortical modulation. First, we demonstrate how changes in the strengths of excitation and inhibition alter the input-output processing responses of our model. Second, we compare these responses with experimental findings from cortical feedback stimulation. Our analyses predict that enhanced inhibition underlies the changes in spontaneous and sensory evoked activity observed experimentally. More generally, these analyses provide a framework for relating cellular and synaptic properties to emergent circuit function and dynamic modulation
Morphology of Hydrodynamic Winds: A Study of Planetary Winds in Stellar Environments
Bathed in intense ionizing radiation, close-in gaseous planets undergo
hydrodynamic atmospheric escape, which ejects the upper extent of their
atmospheres into the interplanetary medium. Ultraviolet detections of escaping
gas around transiting planets corroborate such a framework. Exposed to the
stellar environment, the outflow is shaped by its interaction with the stellar
wind and by the planet's orbit. We model these effects using Athena to perform
3-D radiative-hydrodynamic simulations of tidally-locked hydrogen atmospheres
receiving large amounts of ionizing extreme-ultraviolet flux in various stellar
environments for the low-magnetic-field case. Through a step-by-step
exploration of orbital and stellar wind effects on the planetary outflow, we
find three structurally distinct stellar wind regimes: weak, intermediate, and
strong. We perform synthetic Lyman- observations and find unique
observational signatures for each regime. A weak stellar
windwhich cannot confine the planetary outflow, leading to a
torus of material around the starhas a pre-transit, red-shifted
dayside arm and a slightly redward-skewed spectrum during transit. The
intermediate regime truncates the dayside outflow at large distances from the
planet and causes periodic disruptions of the outflow, producing observational
signatures that mimic a double transit. The first of these dips is blue-shifted
and precedes the optical transit. Finally, strong stellar winds completely
confine the outflow into a cometary tail and accelerate the outflow outwards,
producing large blue-shifted signals post-transit. Across all three regimes,
large signals occur far outside of transit, offering motivation to continue
ultraviolet observations outside of direct transit.Comment: 33 pages, 21 figures (7 of which have embedded movies viewable with
Adobe Acrobat Pro), Submitted to Ap
Narrow-line Laser Cooling by Adiabatic Transfer
We propose and demonstrate a novel laser cooling mechanism applicable to
particles with narrow-linewidth optical transitions. By sweeping the frequency
of counter-propagating laser beams in a sawtooth manner, we cause adiabatic
transfer back and forth between the ground state and a long-lived optically
excited state. The time-ordering of these adiabatic transfers is determined by
Doppler shifts, which ensures that the associated photon recoils are in the
opposite direction to the particle's motion. This ultimately leads to a robust
cooling mechanism capable of exerting large forces via a weak transition and
with reduced reliance on spontaneous emission. We present a simple intuitive
model for the resulting frictional force, and directly demonstrate its efficacy
for increasing the total phase-space density of an atomic ensemble. We rely on
both simulation and experimental studies using the 7.5~kHz linewidth S
to P transition in Sr. The reduced reliance on spontaneous
emission may allow this adiabatic sweep method to be a useful tool for cooling
particles that lack closed cycling transitions, such as molecules.Comment: 5 pages, 4 figure
I\u27ll Be Your Valentine
https://digitalcommons.library.umaine.edu/mmb-vp/4135/thumbnail.jp
You Are My Rain-Beau
https://digitalcommons.library.umaine.edu/mmb-vp/2784/thumbnail.jp
Come To Bohemia
https://digitalcommons.library.umaine.edu/mmb-vp/3861/thumbnail.jp
Sweetheart Lane
https://digitalcommons.library.umaine.edu/mmb-vp/2562/thumbnail.jp
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