23,304 research outputs found
The production of surface-active material by marine phytoplankton cultures
The production of surface-active materials (surfactants) by axenic cultures of five marine diatoms, a dinoflagellate, and a coccolithophorid (all grown in uniform culture conditions) was determined by measuring the volume and stability of the foam produced by shaking...
Gravitational wave detectors based on matter wave interferometers (MIGO) are no better than laser interferometers (LIGO)
We show that a recent claim that matter wave interferometers have a much
higher sensitivity than laser interferometers for a comparable physical setup
is unfounded. We point out where the mistake in the earlier analysis is made.
We also disprove the claim that only a description based on the geodesic
deviation equation can produce the correct physical result. The equations for
the quantum dynamics of non-relativistic massive particles in a linearly
perturbed spacetime derived here are useful for treating a wider class of
related physical problems. A general discussion on the use of atom
interferometers for the detection of gravitational waves is also provided.Comment: 16 pages, REVTeX4; minor changes, one figure and a few references
were added, an additional appendix was included where we explain why,
contrary to the claims in gr-qc/0409099, the effects due to the reflection
off the mirrors cancel out in the final result for the phase shif
Resilience of the Internet to random breakdowns
A common property of many large networks, including the Internet, is that the
connectivity of the various nodes follows a scale-free power-law distribution,
P(k)=ck^-a. We study the stability of such networks with respect to crashes,
such as random removal of sites. Our approach, based on percolation theory,
leads to a general condition for the critical fraction of nodes, p_c, that need
to be removed before the network disintegrates. We show that for a<=3 the
transition never takes place, unless the network is finite. In the special case
of the Internet (a=2.5), we find that it is impressively robust, where p_c is
approximately 0.99.Comment: latex, 3 pages, 1 figure (eps), explanations added, Phys. Rev. Lett.,
in pres
Batalin-Vilkovisky Integrals in Finite Dimensions
The Batalin-Vilkovisky method (BV) is the most powerful method to analyze
functional integrals with (infinite-dimensional) gauge symmetries presently
known. It has been invented to fix gauges associated with symmetries that do
not close off-shell. Homological Perturbation Theory is introduced and used to
develop the integration theory behind BV and to describe the BV quantization of
a Lagrangian system with symmetries. Localization (illustrated in terms of
Duistermaat-Heckman localization) as well as anomalous symmetries are discussed
in the framework of BV.Comment: 35 page
This elusive objective existence
Zurek's existential interpretation of quantum mechanics suffers from three
classical prejudices, including the belief that space and time are
intrinsically and infinitely differentiated. They compel him to relativize the
concept of objective existence in two ways. The elimination of these prejudices
makes it possible to recognize the quantum formalism's ontological implications
- the relative and contingent reality of spatiotemporal distinctions and the
extrinsic and finite spatiotemporal differentiation of the physical world -
which in turn makes it possible to arrive at an unqualified objective
existence. Contrary to a widespread misconception, viewing the quantum
formalism as being fundamentally a probability algorithm does not imply that
quantum mechanics is concerned with states of knowledge rather than states of
Nature. On the contrary, it makes possible a complete and strongly objective
description of the physical world that requires no reference to observers. What
objectively exists, in a sense that requires no qualification, is the
trajectories of macroscopic objects, whose fuzziness is empirically irrelevant,
the properties and values of whose possession these trajectories provide
indelible records, and the fuzzy and temporally undifferentiated states of
affairs that obtain between measurements and are described by counterfactual
probability assignments.Comment: To appear in IJQI; 21 pages, LaTe
Ion heating in the presheath
A one-dimensional model of a small plasma ion source (10cm long) is studied. A hybrid simulation where ions are treated as particles and electrons as a fluid obeying the Boltzmann relation is used to investigate ion heating in the plasma presheath. At low pressure (below a few mTorr), the ion velocity distribution is Maxwellian in the bulk and becomes a drifting Maxwellian distribution while transiting the presheath. The distribution remains essentially isotropic as the ions are accelerated through the presheath to satisfy the Bohm criterion. At intermediate pressures (around 10mTorr), ion-neutral collisions scatter a significant part of the ion kinetic energy from the parallel direction to the perpendicular direction, leading to a net heating of the ions. In addition, the ion velocity distribution becomes distinctly anisotropic. At higher pressure (above a few tens of mTorr), ion heating is still observed, but yields isotropic ion velocity distributions
Stepping Responses of Young and Old Adults to Postural Disturbances: Kinematics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111171/1/j.1532-5415.1994.tb04972.x.pd
Optimization of robustness of scale-free network to random and targeted attacks
The scale-fee networks, having connectivity distribution (where is the site connectivity), is very resilient to random
failures but fragile to intentional attack. The purpose of this paper is to
find the network design guideline which can make the robustness of the network
to both random failures and intentional attack maximum while keeping the
average connectivity per node constant. We find that when $=3$ the
robustness of the scale-free networks reach its maximum value if the minimal
connectivity $m=1$, but when is larger than four, the networks will
become more robust to random failures and targeted attacks as the minimal
connectivity gets larger
Tests of Two-Body Dirac Equation Wave Functions in the Decays of Quarkonium and Positronium into Two Photons
Two-Body Dirac equations of constraint dynamics provide a covariant framework
to investigate the problem of highly relativistic quarks in meson bound states.
This formalism eliminates automatically the problems of relative time and
energy, leading to a covariant three dimensional formalism with the same number
of degrees of freedom as appears in the corresponding nonrelativistic problem.
It provides bound state wave equations with the simplicity of the
nonrelativistic Schroedinger equation. Here we begin important tests of the
relativistic sixteen component wave function solutions obtained in a recent
work on meson spectroscopy, extending a method developed previously for
positronium decay into two photons. Preliminary to this we examine the
positronium decay in the 3P_{0,2} states as well as the 1S_0. The two-gamma
quarkonium decays that we investigate are for the \eta_{c}, \eta_{c}^{\prime},
\chi_{c0}, \chi_{c2}, \pi^{0}, \pi_{2}, a_{2}, and f_{2}^{\prime} mesons. Our
results for the four charmonium states compare well with those from other quark
models and show the particular importance of including all components of the
wave function as well as strong and CM energy dependent potential effects on
the norm and amplitude. The results for the \pi^{0}, although off the
experimental rate by 15%, is much closer than the usual expectations from a
potential model. We conclude that the Two-Body Dirac equations lead to wave
functions which provide good descriptions of the two-gamma decay amplitude and
can be used with some confidence for other purposes.Comment: 79 pages, included new sections on covariant scalar product and added
pages on positronium decay for 3P0 and 3P_2 state
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