2,380 research outputs found
Evaluation of a computer-generated perspective tunnel display for flight path following
The display was evaluated by monitoring pilot performance in a fixed base simulator with the vehicle dynamics of a CH-47 tandem rotor helicopter. Superposition of the predicted future vehicle position on the tunnel image was also investigated to determine whether, and to what extent, it contributes to better system performance (the best predicted future vehicle position was sought). Three types of simulator experiments were conducted: following a desired trajectory in the presence of disturbances; entering the trajectory from a random position, outside the trajectory; detecting and correcting failures in automatic flight. The tunnel display with superimposed predictor/director symbols was shown to be a very successful combination, which outperformed the other two displays in all three experiments. A prediction time of 4 to 7 sec. was found to optimize trajectory tracking for the given vehicle dynamics and flight condition. Pilot acceptance of the tunnel plus predictor/director display was found to be favorable and the time the pilot needed for familiarization with the display was found to be relatively short
Edges and Diffractive Effects in Casimir Energies
The prototypical Casimir effect arises when a scalar field is confined
between parallel Dirichlet boundaries. We study corrections to this when the
boundaries themselves have apertures and edges. We consider several geometries:
a single plate with a slit in it, perpendicular plates separated by a gap, and
two parallel plates, one of which has a long slit of large width, related to
the case of one plate being semi-infinite. We develop a general formalism for
studying such problems, based on the wavefunctional for the field in the gap
between the plates. This formalism leads to a lower dimensional theory defined
on the open regions of the plates or boundaries. The Casimir energy is then
given in terms of the determinant of the nonlocal differential operator which
defines the lower dimensional theory. We develop perturbative methods for
computing these determinants. Our results are in good agreement with known
results based on Monte Carlo simulations. The method is well suited to
isolating the diffractive contributions to the Casimir energy.Comment: 32 pages, LaTeX, 9 figures. v2: additional discussion of
renormalization procedure, version to appear in PRD. v3: corrected a sign
error in (70
Linux kernel compaction through cold code swapping
There is a growing trend to use general-purpose operating systems like Linux in embedded systems. Previous research focused on using compaction and specialization techniques to adapt a general-purpose OS to the memory-constrained environment, presented by most, embedded systems. However, there is still room for improvement: it has been shown that even after application of the aforementioned techniques more than 50% of the kernel code remains unexecuted under normal system operation. We introduce a new technique that reduces the Linux kernel code memory footprint, through on-demand code loading of infrequently executed code, for systems that support virtual memory. In this paper, we describe our general approach, and we study code placement algorithms to minimize the performance impact of the code loading. A code, size reduction of 68% is achieved, with a 2.2% execution speedup of the system-mode execution time, for a case study based on the MediaBench II benchmark suite
Spatially asymptotic S-matrix from general boundary formulation
We construct a new type of S-matrix in quantum field theory using the general
boundary formulation. In contrast to the usual S-matrix the space of free
asymptotic states is located at spatial rather than at temporal infinity.
Hence, the new S-matrix applies to situations where interactions may remain
important at all times, but become negligible with distance. We show that the
new S-matrix is equivalent to the usual one in situations where both apply.
This equivalence is mediated by an isomorphism between the respective
asymptotic state spaces that we construct. We introduce coherent states that
allow us to obtain explicit expressions for the new S-matrix. In our formalism
crossing symmetry becomes a manifest rather than a derived feature of the
S-matrix.Comment: 27 pages, LaTeX + revtex4; v2: various corrections, references
update
Partition Function for (2+1)-Dimensional Einstein Gravity
Taking (2+1)-dimensional pure Einstein gravity for arbitrary genus as a
model, we investigate the relation between the partition function formally
defined on the entire phase space and the one written in terms of the reduced
phase space. In particular the case of is analyzed in detail.
By a suitable gauge-fixing, the partition function basically reduces to
the partition function defined for the reduced system, whose dynamical
variables are . [The 's are the Teichm\"uller
parameters, and the 's are their conjugate momenta.]
As for the case of , we find out that is also related with another
reduced form, whose dynamical variables are and .
[Here is a conjugate momentum to 2-volume .] A nontrivial factor
appears in the measure in terms of this type of reduced form. The factor turns
out to be a Faddeev-Popov determinant coming from the time-reparameterization
invariance inherent in this type of formulation. Thus the relation between two
reduced forms becomes transparent even in the context of quantum theory.
Furthermore for , a factor coming from the zero-modes of a differential
operator can appear in the path-integral measure in the reduced
representation of . It depends on the path-integral domain for the shift
vector in : If it is defined to include , the nontrivial factor
does not appear. On the other hand, if the integral domain is defined to
exclude , the factor appears in the measure. This factor can depend
on the dynamical variables, typically as a function of , and can influence
the semiclassical dynamics of the (2+1)-dimensional spacetime.
These results shall be significant from the viewpoint of quantum gravity.Comment: 21 pages. To appear in Physical Review D. The discussion on the
path-integral domain for the shift vector has been adde
Mineral sinks within ripening grape berries (Vitis vinifera L.)
Trends in the accumulation of mineral elements into the grape berry components give information about vascular flow into the berry. Shiraz berries were dissected into receptacle, skin, pulp, brush and seeds and the accumulation of 10 mineral elements into these components was followed through development. The elements were separated into two categories according to their accumulation pattern into the berry. The first group of elements continued to accumulate throughout berry growth and ripening, and was comprised of phloem-mobile potassium, phosphorus, sulphur, magnesium, boron, iron and copper. The second group of elements accumulated mostly prior to veraison, and included the xylem-mobile minerals calcium, manganese and zinc. These results indicate that the xylem contribution to berry growth diminished after veraison. Berry fresh weight, dry weight, as well as berry sugar content, were all highly correlated with berry potassium content. While the pulp and skin were the strongest sinks for potassium and boron, seeds were the strongest sinks for calcium, phosphorus, sulphur, manganese and zinc. With the exception of calcium and manganese, seeds ceased to accumulate most elements during late ripening. The berry receptacle and brush did not accumulate any of the elements to levels above those of the other berry components at any stage of development. Therefore, they did not act as sinks for xylem- or phloem-mobile elements as vascular flow to the pulp and skin slowed.
Valence band offset of the ZnO/AlN heterojunction determined by X-ray photoemission spectroscopy
The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values
The use of exp(iS[x]) in the sum over histories
The use of as the generic form for a sum over histories in
configuration space is discussed critically and placed in its proper context.
The standard derivation of the sum over paths by discretizing the paths is
reviewed, and it is shown that the form is justified only
for Schrodinger-type systems which are at most second order in the momenta.
Extending this derivation to the relativistic free particle, the causal Green's
function is expressed as a sum over timelike paths, and the Feynman Green's
function is expressed both as a sum over paths which only go one way in time
and as a sum over paths which move forward and backward in time. The weighting
of the paths is shown not to be in any of these cases. The role
of the inner product and the operator ordering of the wave equation in defining
the sum over histories is discussed.Comment: 22 pages, Latex, Imperial-TP-92-93-4
Deformation Quantization of Geometric Quantum Mechanics
Second quantization of a classical nonrelativistic one-particle system as a
deformation quantization of the Schrodinger spinless field is considered. Under
the assumption that the phase space of the Schrodinger field is ,
both, the Weyl-Wigner-Moyal and Berezin deformation quantizations are discussed
and compared. Then the geometric quantum mechanics is also quantized using the
Berezin method under the assumption that the phase space is
endowed with the Fubini-Study Kahlerian metric. Finally, the Wigner function
for an arbitrary particle state and its evolution equation are obtained. As is
shown this new "second quantization" leads to essentially different results
than the former one. For instance, each state is an eigenstate of the total
number particle operator and the corresponding eigenvalue is always .Comment: 27+1 pages, harvmac file, no figure
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