36,175 research outputs found
Stress-energy Tensor Correlators in N-dim Hot Flat Spaces via the Generalized Zeta-Function Method
We calculate the expectation values of the stress-energy bitensor defined at
two different spacetime points of a massless, minimally coupled scalar
field with respect to a quantum state at finite temperature in a flat
-dimensional spacetime by means of the generalized zeta-function method.
These correlators, also known as the noise kernels, give the fluctuations of
energy and momentum density of a quantum field which are essential for the
investigation of the physical effects of negative energy density in certain
spacetimes or quantum states. They also act as the sources of the
Einstein-Langevin equations in stochastic gravity which one can solve for the
dynamics of metric fluctuations as in spacetime foams. In terms of
constitutions these correlators are one rung above (in the sense of the
correlation -- BBGKY or Schwinger-Dyson -- hierarchies) the mean (vacuum and
thermal expectation) values of the stress-energy tensor which drive the
semiclassical Einstein equation in semiclassical gravity. The low and the high
temperature expansions of these correlators are also given here: At low
temperatures, the leading order temperature dependence goes like while
at high temperatures they have a dependence with the subleading terms
exponentially suppressed by . We also discuss the singular behaviors of
the correlators in the coincident limit as was done before
for massless conformal quantum fields.Comment: 23 pages, no figures. Invited contribution to a Special Issue of
Journal of Physics A in honor of Prof. J. S. Dowke
RTT relations, a modified braid equation and noncommutative planes
With the known group relations for the elements of a quantum
matrix as input a general solution of the relations is sought without
imposing the Yang - Baxter constraint for or the braid equation for
. For three biparametric deformatios, and , the standard,the nonstandard and the
hybrid one respectively, or is found to depend, apart from the
two parameters defining the deformation in question, on an extra free parameter
,such that only for two values of , given explicitly for each case, one
has the braid equation. Arbitray corresponds to a class (conserving the
group relations independent of ) of the MQYBE or modified quantum YB
equations studied by Gerstenhaber, Giaquinto and Schak. Various properties of
the triparametric , and are
studied. In the larger space of the modified braid equation (MBE) even
can satisfy outside braid equation (BE)
subspace. A generalized, - dependent, Hecke condition is satisfied by each
3-parameter . The role of in noncommutative geometries of the
, and deformed planes is studied. K is found to
introduce a "soft symmetry breaking", preserving most interesting properties
and leading to new interesting ones. Further aspects to be explored are
indicated.Comment: Latex, 17 pages, minor change
Time-division SQUID multiplexers with reduced sensitivity to external magnetic fields
Time-division SQUID multiplexers are used in many applications that require
exquisite control of systematic error. One potential source of systematic error
is the pickup of external magnetic fields in the multiplexer. We present
measurements of the field sensitivity figure of merit, effective area, for both
the first stage and second stage SQUID amplifiers in three NIST SQUID
multiplexer designs. These designs include a new variety with improved
gradiometry that significantly reduces the effective area of both the first and
second stage SQUID amplifiers.Comment: 4 pages, 7 figures. Submitted for publication in the IEEE
Transactions on Applied Superconductivity, August 201
Purification through Zeno-like Measurements
A series of frequent measurements on a quantum system (Zeno-like
measurements) is shown to result in the ``purification'' of another quantum
system in interaction with the former. Even though the measurements are
performed on the former system, their effect drives the latter into a pure
state, irrespectively of its initial (mixed) state, provided certain conditions
are satisfied.Comment: REVTeX4, 4 pages, 1 figure; to be published in Phys. Rev. Lett.
(2003
Neuromuscular control of wingbeat kinematics in Anna's hummingbirds (Calypte anna)
Hummingbirds can maintain the highest wingbeat frequencies of any flying vertebrate – a feat accomplished by the large pectoral muscles that power the wing strokes. An unusual feature of these muscles is that they are activated by one or a few spikes per cycle as revealed by electromyogram recordings (EMGs). The relatively simple nature of this activation pattern provides an opportunity to understand how motor units are recruited to modulate limb kinematics. Hummingbirds made to fly in low-density air responded by moderately increasing wingbeat frequency and substantially increasing the wing stroke amplitude as compared with flight in normal air. There was little change in the number of spikes per EMG burst in the pectoralis major muscle between flight in normal and low-density heliox (mean=1.4 spikes cycle^(–1)). However the spike amplitude, which we take to be an indication of the number of active motor units, increased in concert with the wing stroke amplitude, 1.7 times the value in air. We also challenged the hummingbirds using transient load lifting to elicit maximum burst performance. During maximum load lifting, both wing stroke amplitude and wingbeat frequency increased substantially above those values during hovering flight. The number of spikes per EMG burst increased to a mean of 3.3 per cycle, and the maximum spike amplitude increased to approximately 1.6 times those values during flight in heliox. These results suggest that hummingbirds recruit additional motor units (spatial recruitment) to regulate wing stroke amplitude but that temporal recruitment is also required to maintain maximum stroke amplitude at the highest wingbeat frequencies
The phase-dependent Infrared brightness of the extrasolar planet upsilon Andromedae b
The star upsilon Andromeda is orbited by three known planets, the innermost
of which has an orbital period of 4.617 days and a mass at least 0.69 that of
Jupiter. This planet is close enough to its host star that the radiation it
absorbs overwhelms its internal heat losses. Here we present the 24 micron
light curve of this system, obtained with the Spitzer Space Telescope. It shows
a clear variation in phase with the orbital motion of the innermost planet.
This is the first demonstration that such planets possess distinct hot
substellar (day) and cold antistellar (night) faces.Comment: "Director's cut" of paper to appear in Science, 27 October, 200
Color Reflection Invariance and Monopole Condensation in QCD
We review the quantum instability of the Savvidy-Nielsen-Olesen (SNO) vacuum
of the one-loop effective action of SU(2) QCD, and point out a critical defect
in the calculation of the functional determinant of the gluon loop in the SNO
effective action. We prove that the gauge invariance, in particular the color
reflection invariance, exclude the unstable tachyonic modes from the gluon loop
integral. This guarantees the stability of the magnetic condensation in QCD.Comment: 28 pages, 3 figures, JHEP styl
Quantum Mechanics on the h-deformed Quantum Plane
We find the covariant deformed Heisenberg algebra and the Laplace-Beltrami
operator on the extended -deformed quantum plane and solve the Schr\"odinger
equations explicitly for some physical systems on the quantum plane. In the
commutative limit the behaviour of a quantum particle on the quantum plane
becomes that of the quantum particle on the Poincar\'e half-plane, a surface of
constant negative Gaussian curvature. We show the bound state energy spectra
for particles under specific potentials depend explicitly on the deformation
parameter . Moreover, it is shown that bound states can survive on the
quantum plane in a limiting case where bound states on the Poincar\'e
half-plane disappear.Comment: 16pages, Latex2e, Abstract and section 4 have been revise
Gauge Independent Trace Anomaly for Gravitons
We show that the trace anomaly for gravitons calculated using the usual
effective action formalism depends on the choice of gauge when the background
spacetime is not a solution of the classical equation of motion, that is, when
off-shell. We then use the gauge independent Vilkovisky-DeWitt effective action
to restore gauge independence to the off-shell case. Additionally we explicitly
evaluate trace anomalies for some N-sphere background spacetimes.Comment: 19 pages, additional references and title chang
Magnetic Moments of Heavy Baryons
First non-trivial chiral corrections to the magnetic moments of triplet (T)
and sextet (S^(*)) heavy baryons are calculated using Heavy Hadron Chiral
Perturbation Theory. Since magnetic moments of the T-hadrons vanish in the
limit of infinite heavy quark mass (m_Q->infinity), these corrections occur at
order O(1/(m_Q \Lambda_\chi^2)) for T-baryons while for S^(*)-baryons they are
of order O(1/\Lambda_\chi^2). The renormalization of the chiral loops is
discussed and relations among the magnetic moments of different hadrons are
provided. Previous results for T-baryons are revised.Comment: 11 Latex pages, 2 figures, to be published in Phys.Rev.
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