9,894 research outputs found
An investigation of S-band signal transmission through an Apollo earth-entry plasma
Investigation of S band signal transmission through Apollo earth entry plasm
Self-organization, scaling and collapse in a coupled automaton model of foragers and vegetation resources with seed dispersal
We introduce a model of traveling agents ({\it e.g.} frugivorous animals) who
feed on randomly located vegetation patches and disperse their seeds, thus
modifying the spatial distribution of resources in the long term. It is assumed
that the survival probability of a seed increases with the distance to the
parent patch and decreases with the size of the colonized patch. In turn, the
foraging agents use a deterministic strategy with memory, that makes them visit
the largest possible patches accessible within minimal travelling distances.
The combination of these interactions produce complex spatio-temporal patterns.
If the patches have a small initial size, the vegetation total mass (biomass)
increases with time and reaches a maximum corresponding to a self-organized
critical state with power-law distributed patch sizes and L\'evy-like movement
patterns for the foragers. However, this state collapses as the biomass sharply
decreases to reach a noisy stationary regime characterized by corrections to
scaling. In systems with low plant competition, the efficiency of the foraging
rules leads to the formation of heterogeneous vegetation patterns with
frequency spectra, and contributes, rather counter-intuitively,
to lower the biomass levels.Comment: 11 pages, 5 figure
The Paradoxical Forces for the Classical Electromagnetic Lag Associated with the Aharonov-Bohm Phase Shift
The classical electromagnetic lag assocated with the Aharonov-Bohm phase
shift is obtained by using a Darwin-Lagrangian analysis similar to that given
by Coleman and Van Vleck to identify the puzzling forces of the Shockley-James
paradox. The classical forces cause changes in particle velocities and so
produce a relative lag leading to the same phase shift as predicted by Aharonov
and Bohm and observed in experiments. An experiment is proposed to test for
this lag aspect implied by the classical analysis but not present in the
currently-accepted quantum topological description of the phase shift.Comment: 8 pages, 3 figure
Some Heuristic Semiclassical Derivations of the Planck Length, the Hawking Effect and the Unruh Effect
The formulae for Planck length, Hawking temperature and Unruh-Davies
temperature are derived by using only laws of classical physics together with
the Heisenberg principle. Besides, it is shown how the Hawking relation can be
deduced from the Unruh relation by means of the principle of equivalence; the
deep link between Hawking effect and Unruh effect is in this way clarified.Comment: LaTex file, 6 pages, no figure
Scaling Symmetries of Scatterers of Classical Zero-Point Radiation
Classical radiation equilibrium (the blackbody problem) is investigated by
the use of an analogy. Scaling symmetries are noted for systems of classical
charged particles moving in circular orbits in central potentials V(r)=-k/r^n
when the particles are held in uniform circular motion against radiative
collapse by a circularly polarized incident plane wave. Only in the case of a
Coulomb potential n=1 with fixed charge e is there a unique scale-invariant
spectrum of radiation versus frequency (analogous to zero-point radiation)
obtained from the stable scattering arrangement. These results suggest that
non-electromagnetic potentials are not appropriate for discussions of classical
radiation equilibrium.Comment: 13 page
Using the Tip of the Red Giant Branch as a Distance Indicator in the Near Infrared
The tip of the red giant branch (TRGB) is a well-established standard candle
used to measure distances to nearby galaxies. The TRGB luminosity is typically
measured in the I-band, where the luminosity has little dependency on stellar
age or stellar metallicity. As the TRGB is brighter at wavelengths redder than
the I-band, observational gains can be made if the TRGB luminosity can be
robustly calibrated at longer wavelengths. This is of particular interest given
the infrared capabilities that will be available with the James Webb Space
Telescope and an important calibration consideration for using TRGB distances
as part of an independent measurement of the Hubble constant. Here, we use
simulated photometry to investigate the dependency of the TRGB luminosity on
stellar age and metallicity as a function of wavelength (475 nm - 4.5 micron).
We find intrinsic variations in the TRGB magnitude to increase from a few
hundredths of a magnitude at 800-900 nm to ~0.6 mag by 1.5 micron. We show that
variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag
(1-2% accuracy in distance) with careful calibrations that account for changes
in age and metal content. These represent the minimum uncertainties;
observational uncertainties will be higher. Such calibration efforts may also
provide independent constraints of the age and metallicity of stellar halos
where TRGB distances are best measured. At 3.6 and 4.5 micron, the TRGB
magnitude is predicted to vary up to ~0.15 mag even after corrections for
stellar age and metallicity, making these wavelengths less suitable for
precision distances.Comment: 11 pages, 7 figures, 1 table, Accepted to the Astrophysical Journa
Deeply subrecoil two-dimensional Raman cooling
We report the implementation of a two-dimensional Raman cooling scheme using
sequential excitations along the orthogonal axes. Using square pulses, we have
cooled a cloud of ultracold Cesium atoms down to an RMS velocity spread of
0.39(5) recoil velocity, corresponding to an effective temperature of 30 nK
(0.15 T_rec). This technique can be useful to improve cold atom atomic clocks,
and is particularly relevant for clocks in microgravity.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
Randomizing world trade. II. A weighted network analysis
Based on the misleading expectation that weighted network properties always
offer a more complete description than purely topological ones, current
economic models of the International Trade Network (ITN) generally aim at
explaining local weighted properties, not local binary ones. Here we complement
our analysis of the binary projections of the ITN by considering its weighted
representations. We show that, unlike the binary case, all possible weighted
representations of the ITN (directed/undirected, aggregated/disaggregated)
cannot be traced back to local country-specific properties, which are therefore
of limited informativeness. Our two papers show that traditional macroeconomic
approaches systematically fail to capture the key properties of the ITN. In the
binary case, they do not focus on the degree sequence and hence cannot
characterize or replicate higher-order properties. In the weighted case, they
generally focus on the strength sequence, but the knowledge of the latter is
not enough in order to understand or reproduce indirect effects.Comment: See also the companion paper (Part I): arXiv:1103.1243
[physics.soc-ph], published as Phys. Rev. E 84, 046117 (2011
The effects of bioturbation on the initiation of motion of intertidal sands
The results of laboratory flume experiments on initiation of sediment motion in natural marine sediments (fine sand) are reported. Sediment cores were taken from an intertidal region during all seasons of the year over a four-year period…
An auxiliary capacitor based ultra-fast drive circuit for shear piezoelectric motors
Shear piezoelectric motors frequently require large voltage changes on very short time scales. Since piezos behave electrically as capacitors, this requires a drive circuit capable of quickly sourcing or sinking a large amount of current at high voltages. Here we describe a novel circuit design using a high voltage amplifier, transistor switching stage, and auxiliary capacitor. This circuit can drive piezoelectric motors at higher speeds and lower costs than conventional methods and with greater flexibility for computer automation. We illustrate its application in a controller for a scanning tunneling microscope coarse approach mechanism and discuss other possible applications and modifications of this circuit.National Science Foundation CAREER programNational Science Foundation MRSEC ProgramResearch Corporation Cottrell Scholarshi
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