69,886 research outputs found
Induced scattering of short radio pulses
Effect of the induced Compton and Raman scattering on short, bright radio
pulses is investigated. It is shown that when a single pulse propagates through
the scattering medium, the effective optical depth is determined by the
duration of the pulse but not by the scale of the medium. The induced
scattering could hinder propagation of the radio pulse only if close enough to
the source a dense enough plasma is presented. The induced scattering within
the relativistically moving source places lower limits on the Lorentz factor of
the source. The results are applied to the recently discovered short
extragalactic radio pulse.Comment: submitted to Ap
On the Use of Group Theoretical and Graphical Techniques toward the Solution of the General N-body Problem
Group theoretic and graphical techniques are used to derive the N-body wave
function for a system of identical bosons with general interactions through
first-order in a perturbation approach. This method is based on the maximal
symmetry present at lowest order in a perturbation series in inverse spatial
dimensions. The symmetric structure at lowest order has a point group
isomorphic with the S_N group, the symmetric group of N particles, and the
resulting perturbation expansion of the Hamiltonian is order-by-order invariant
under the permutations of the S_N group. This invariance under S_N imposes
severe symmetry requirements on the tensor blocks needed at each order in the
perturbation series. We show here that these blocks can be decomposed into a
basis of binary tensors invariant under S_N. This basis is small (25 terms at
first order in the wave function), independent of N, and is derived using
graphical techniques. This checks the N^6 scaling of these terms at first order
by effectively separating the N scaling problem away from the rest of the
physics. The transformation of each binary tensor to the final normal
coordinate basis requires the derivation of Clebsch-Gordon coefficients of S_N
for arbitrary N. This has been accomplished using the group theory of the
symmetric group. This achievement results in an analytic solution for the wave
function, exact through first order, that scales as N^0, effectively
circumventing intensive numerical work. This solution can be systematically
improved with further analytic work by going to yet higher orders in the
perturbation series.Comment: This paper was submitted to the Journal of Mathematical physics, and
is under revie
Correspondence between sound propagation in discrete and continuous random media with application to forest acoustics
Although sound propagation in a forest is important in several applications, there are currently no rigorous yet computationally tractable prediction methods. Due to the complexity of sound scattering in a forest, it is natural to formulate the problem stochastically. In this paper, it is demonstrated that the equations for the statistical moments of the sound field propagating in a forest have the same form as those for sound propagation in a turbulent atmosphere if the scattering properties of the two media are expressed in terms of the differential scattering and total cross sections. Using the existing theories for sound propagation in a turbulent atmosphere, this analogy enables the derivation of several results for predicting forest acoustics. In particular, the second-moment parabolic equation is formulated for the spatial correlation function of the sound field propagating above an impedance ground in a forest with micrometeorology. Effective numerical techniques for solving this equation have been developed in atmospheric acoustics. In another example, formulas are obtained that describe the effect of a forest on the interference between the direct and ground-reflected waves. The formulated correspondence between wave propagation in discrete and continuous random media can also be used in other fields of physics
Measurement of non-Gaussian shot noise: influence of the environment
We present the first measurements of the third moment of the voltage
fluctuations in a conductor. This technique can provide new and complementary
information on the electronic transport in conducting systems. The measurement
was performed on non-superconducting tunnel junctions as a function of voltage
bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate
the significant effect of the electromagnetic environment of the sample.Comment: 13 pages, for the SPIE International Symposium on Fluctuations and
Noise, Maspalomas, Gran Canaria, Spain (May 2004
Quantum state preparation and control of single molecular ions
Preparing molecules at rest and in a highly pure quantum state is a long
standing dream in chemistry and physics, so far achieved only for a select set
of molecules in dedicated experimental setups. Here, a quantum-limited
combination of mass spectrometry and Raman spectroscopy is proposed that should
be applicable to a wide range of molecular ions. Excitation of electrons in the
molecule followed by uncontrolled decay and branching into several lower energy
states is avoided. Instead, the molecule is always connected to rotational
states within the electronic and vibrational ground-state manifold, while a
co-trapped atomic ion provides efficient entropy removal and allows for
extraction of information on the molecule. The outlined techniques might enable
preparation, manipulation and measurement of a large multitude of molecular ion
species with the same instrument, with applications including, but not limited
to, precise determination of molecular properties and fundamental tests of
physics.Comment: 12 pages, 2 figures, reformatted for resubmissio
RV "John Murray" Cruise 6, 21 April - 30 May 1972: Tidal recording north and east of Shetland, and geological sampling along the North British Shelf
Science is perception: what can our sense of smell tell us about ourselves and the world around us?
Human sensory processes are well understood: hearing, seeing, perhaps even tasting and touch—but we do not understand smell—the elusive sense. That is, for the others we know what stimuli causes what response, and why and how. These fundamental questions are not answered within the sphere of smell science; we do not know what it is about a molecule that … smells. I report, here, the status quo theories for olfaction, highlighting what we do not know, and explaining why dismissing the perception of the input as ‘too subjective’ acts as a roadblock not conducive to scientific inquiry. I outline the current and new theory that conjectures a mechanism for signal transduction based on quantum mechanical phenomena, dubbed the ‘swipe card’, which is perhaps controversial but feasible. I show that such lines of thinking may answer some questions, or at least pose the right questions. Most importantly, I draw links and comparisons as to how better understanding of how small (10’s of atoms) molecules can interact so specially with large (10 000’s of atoms) proteins in a way that is so integral to healthy living. Repercussions of this work are not just important in understanding a basic scientific tool used by us all, but often taken for granted, it is also a step closer to understanding generic mechanisms between drug and receptor, for example
Remote measurement utilizing NASA's scanning laser Doppler systems. Volume 1. Laser Doppler wake vortex tracking at Kennedy Airport
Test operations of the Scanning Laser Doppler System (SLDS) at Kennedy International Airport (KIA) during August 1974 through June 1975 are reported. A total of 1,619 data runs was recorded with a totally operational system during normal landing operations at KIA. In addition, 53 data runs were made during cooperative flybys with the C880 for a grand total of 1672 recorded vortex tracks. Test crews were in attendance at KIA for 31 weeks, of which 25 weeks were considered operational and the other six were packing, unpacking, setup and check out. Although average activity equates to 67 recorded landing operations per week, two periods of complete runway inactivity spanned 20 days and 13 days, respectively. The operation frequency therefore averaged about 88 operations per week
Assessing habitat stability for disturbance-prone species by evaluating landscape dynamics along the Virginia barrier islands
Tunable Holstein model with cold polar molecules
We show that an ensemble of polar molecules trapped in an optical lattice can
be considered as a controllable open quantum system. The coupling between
collective rotational excitations and the motion of the molecules in the
lattice potential can be controlled by varying the strength and orientation of
an external DC electric field as well as the intensity of the trapping laser.
The system can be described by a generalized Holstein Hamiltonian with tunable
parameters and can be used as a quantum simulator of excitation energy transfer
and polaron phenomena. We show that the character of excitation energy transfer
can be modified by tuning experimental parameters.Comment: 5 pages, 3 figures (accepted in as a Rapid Communication in
Phys.Rev.A
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