103 research outputs found
Explicit Green's Function of a Boundary Value Problem for a Sphere and Trapped Flux Analysis in Gravity Probe B Experiment
Magnetic flux trapped on the surface of superconducting rotors of the Gravity
Probe B (GP-B) experiment produces some signal in the SQUID readout. For the
needs of GP-B error analysis and simulation of data reduction, this signal is
calculated and analyzed in the paper. We first solve a magnetostatic problem
for a point source (fluxon) on the surface of a sphere, finding the closed form
elementary expression for the corresponding Green's function. Second, we
calculate the flux through the pick-up loop as a function of the fluxon
position. Next, the time dependence of a fluxon position, caused by rotor
motion according to a symmetric top model, and thus the time signature of the
flux are determined, and the spectrum of the trapped flux signal is analyzed.
Finally, a multi-purpose program of trapped flux signal generation based on the
above results is described, various examples of the signal obtained by means of
this program are given, and their features are discussed.Comment: 14 pages, including 7 figures. Submitted to: "Journal of Applied
Physics
Gain control in molecular information processing: Lessons from neuroscience
Statistical properties of environments experienced by biological signaling
systems in the real world change, which necessitate adaptive responses to
achieve high fidelity information transmission. One form of such adaptive
response is gain control. Here we argue that a certain simple mechanism of gain
control, understood well in the context of systems neuroscience, also works for
molecular signaling. The mechanism allows to transmit more than one bit (on or
off) of information about the signal independently of the signal variance. It
does not require additional molecular circuitry beyond that already present in
many molecular systems, and, in particular, it does not depend on existence of
feedback loops. The mechanism provides a potential explanation for abundance of
ultrasensitive response curves in biological regulatory networks.Comment: 10 pages, 5 figure
Minimal subtraction and the Callan-Symanzik equation
The usual proof of renormalizability using the Callan-Symanzik equation makes
explicit use of normalization conditions. It is shown that demanding that the
renormalization group functions take the form required for minimal subtraction
allows one to prove renormalizability using the Callan-Symanzik equation,
without imposing normalization conditions. Scalar field theory and quantum
electrodynamics are treated.Comment: 6 pages, plain Te
Modular Acquisition and Stimulation System for Timestamp-Driven Neuroscience Experiments
Dedicated systems are fundamental for neuroscience experimental protocols
that require timing determinism and synchronous stimuli generation. We
developed a data acquisition and stimuli generator system for neuroscience
research, optimized for recording timestamps from up to 6 spiking neurons and
entirely specified in a high-level Hardware Description Language (HDL). Despite
the logic complexity penalty of synthesizing from such a language, it was
possible to implement our design in a low-cost small reconfigurable device.
Under a modular framework, we explored two different memory arbitration schemes
for our system, evaluating both their logic element usage and resilience to
input activity bursts. One of them was designed with a decoupled and latency
insensitive approach, allowing for easier code reuse, while the other adopted a
centralized scheme, constructed specifically for our application. The usage of
a high-level HDL allowed straightforward and stepwise code modifications to
transform one architecture into the other. The achieved modularity is very
useful for rapidly prototyping novel electronic instrumentation systems
tailored to scientific research.Comment: Preprint submitted to ARC 2015. Extended: 16 pages, 10 figures. The
final publication is available at link.springer.co
On the detectability of quantum spacetime foam with gravitational-wave interferometers
We discuss a recent provocative suggestion by Amelino-Camelia and others that
classical spacetime may break down into ``quantum foam'' on distance scales
many orders of magnitude larger than the Planck length, leading to effects
which could be detected using large gravitational wave interferometers. This
suggestion is based on a quantum uncertainty limit obtained by Wigner using a
quantum clock in a gedanken timing experiment. Wigner's limit, however, is
based on two unrealistic and unneccessary assumptions: that the clock is free
to move, and that it does not interact with the environment. Removing either of
these assumptions makes the uncertainty limit invalid, and removes the basis
for Amelino-Camelia's suggestion.Comment: Submitted to Phys. Lett.
Serially-regulated biological networks fully realize a constrained set of functions
We show that biological networks with serial regulation (each node regulated
by at most one other node) are constrained to {\it direct functionality}, in
which the sign of the effect of an environmental input on a target species
depends only on the direct path from the input to the target, even when there
is a feedback loop allowing for multiple interaction pathways. Using a
stochastic model for a set of small transcriptional regulatory networks that
have been studied experimentally, we further find that all networks can achieve
all functions permitted by this constraint under reasonable settings of
biochemical parameters. This underscores the functional versatility of the
networks.Comment: 9 pages, 3 figure
Statistical properties of multistep enzyme-mediated reactions
Enzyme-mediated reactions may proceed through multiple intermediate
conformational states before creating a final product molecule, and one often
wishes to identify such intermediate structures from observations of the
product creation. In this paper, we address this problem by solving the
chemical master equations for various enzymatic reactions. We devise a
perturbation theory analogous to that used in quantum mechanics that allows us
to determine the first () and the second (variance) cumulants of the
distribution of created product molecules as a function of the substrate
concentration and the kinetic rates of the intermediate processes. The mean
product flux V=d/dt (or "dose-response" curve) and the Fano factor
F=variance/ are both realistically measurable quantities, and while the mean
flux can often appear the same for different reaction types, the Fano factor
can be quite different. This suggests both qualitative and quantitative ways to
discriminate between different reaction schemes, and we explore this
possibility in the context of four sample multistep enzymatic reactions. We
argue that measuring both the mean flux and the Fano factor can not only
discriminate between reaction types, but can also provide some detailed
information about the internal, unobserved kinetic rates, and this can be done
without measuring single-molecule transition events.Comment: 8 pages, 3 figure
- …