8,527 research outputs found
Feedback control of unsupported standing
This paper presents the results of continuing work on feedback control of unsupported standing in paraplegia. Our experimental setup considers a situation in which all joints above the ankle are braced, and stabilising torque at the ankle is generated by stimulation of the plantarflexors. A previous study showed that short periods of unsupported standing with paraplegic subjects could be achieved. In order to improve consistency and reliability of unsupported standing we are currently investigating several modifications to the control strategy. The paper reports progress towards this goal
Exotic Smoothness and Physics
The essential role played by differentiable structures in physics is reviewed
in light of recent mathematical discoveries that topologically trivial
space-time models, especially the simplest one, , possess a rich
multiplicity of such structures, no two of which are diffeomorphic to each
other and thus to the standard one. This means that physics has available to it
a new panoply of structures available for space-time models. These can be
thought of as source of new global, but not properly topological, features.
This paper reviews some background differential topology together with a
discussion of the role which a differentiable structure necessarily plays in
the statement of any physical theory, recalling that diffeomorphisms are at the
heart of the principle of general relativity. Some of the history of the
discovery of exotic, i.e., non-standard, differentiable structures is reviewed.
Some new results suggesting the spatial localization of such exotic structures
are described and speculations are made on the possible opportunities that such
structures present for the further development of physical theories.Comment: 13 pages, LaTe
A statistical investigation of the effects contributed by tape recorders and by wow and flutter of magnetic tape on the accuracy of a telemetry system Technical report no. 15
Effects contributed by tape recorders and by wow and flutter of magnetic tape on accuracy of telemetry syste
N=2 Topological Yang-Mills Theory on Compact K\"{a}hler Surfaces
We study a topological Yang-Mills theory with fermionic symmetry. Our
formalism is a field theoretical interpretation of the Donaldson polynomial
invariants on compact K\"{a}hler surfaces. We also study an analogous theory on
compact oriented Riemann surfaces and briefly discuss a possible application of
the Witten's non-Abelian localization formula to the problems in the case of
compact K\"{a}hler surfaces.Comment: ESENAT-93-01 & YUMS-93-10, 34pages: [Final Version] to appear in
Comm. Math. Phy
The effect of prolonged simulated non- gravitational environment on mineral balance in the adult male, volume 1 Final report
Effect of prolonged bed rest with simulated weightlessness on mineral balance in male adult - Vol.
Optical Coronagraphic Spectroscopy of AU Mic: Evidence of Time Variable Colors?
We present coronagraphic long slit spectra of AU Mic's debris disk taken with
the STIS instrument aboard the Hubble Space Telescope (HST). Our spectra are
the first spatially resolved, scattered light spectra of the system's disk,
which we detect at projected distances between approximately 10 and 45 AU. Our
spectra cover a wavelength range between 5200 and 10200 angstroms. We find that
the color of AU Mic's debris disk is bluest at small (12-35 AU) projected
separations. These results both confirm and quantify the findings qualitatively
noted by Krist et al. (2005), and are different than IR observations that
suggested a uniform blue or gray color as a function of projected separation in
this region of the disk. Unlike previous literature that reported the color of
AU Mic's disk became increasingly more blue as a function of projected
separation beyond approximately 30 AU, we find the disk's optical color between
35-45 AU to be uniformly blue on the southeast side of the disk and
decreasingly blue on the northwest side. We note that this apparent change in
disk color at larger projected separations coincides with several fast, outward
moving "features" that are passing through this region of the southeast side of
the disk. We speculate that these phenomenon might be related, and that the
fast moving features could be changing the localized distribution of sub-micron
sized grains as they pass by, thereby reducing the blue color of the disk in
the process. We encourage follow-up optical spectroscopic observations of the
AU Mic to both confirm this result, and search for further modifications of the
disk color caused by additional fast moving features propagating through the
disk.Comment: Accepted by AJ, 13 pages, 8 figures, 1 tabl
S-duality and Topological Strings
In this paper we show how S-duality of type IIB superstrings leads to an
S-duality relating A and B model topological strings on the same Calabi-Yau as
had been conjectured recently: D-instantons of the B-model correspond to
A-model perturbative amplitudes and D-instantons of the A-model capture
perturbative B-model amplitudes.
Moreover this confirms the existence of new branes in the two models.
As an application we explain the recent results concerning A-model
topological strings on Calabi-Yau and its equivalence to the statistical
mechanical model of melting crystal.Comment: 13 page
Development of a new laser Doppler velocimeter for the Ames High Reynolds Channel No. 2
A new two-channel laser Doppler velocimeter developed for the Ames High Reynolds Channel No. 2 is described. Design features required for the satisfactory operation of the optical system in the channel environment are discussed. Fiber optics are used to transmit the megahertz Doppler signal to the photodetectors located outside the channel pressure vessel, and provision is made to isolate the optical system from pressure and thermal strain effects. Computer-controlled scanning mirrors are used to position the laser beams in the channel flow. Techniques used to seed the flow with 0.5-micron-diam polystyrene spheres avoiding deposition on the test-section windows and porous boundary-layer removal panels are described. Preliminary results are presented with a discussion of several of the factors affecting accuracy
Localized Exotic Smoothness
Gompf's end-sum techniques are used to establish the existence of an infinity
of non-diffeomorphic manifolds, all having the same trivial
topology, but for which the exotic differentiable structure is confined to a
region which is spatially limited. Thus, the smoothness is standard outside of
a region which is topologically (but not smoothly) ,
where is the compact three ball. The exterior of this region is
diffeomorphic to standard . In a
space-time diagram, the confined exoticness sweeps out a world tube which, it
is conjectured, might act as a source for certain non-standard solutions to the
Einstein equations. It is shown that smooth Lorentz signature metrics can be
globally continued from ones given on appropriately defined regions, including
the exterior (standard) region. Similar constructs are provided for the
topology, of the Kruskal form of the Schwarzschild
solution. This leads to conjectures on the existence of Einstein metrics which
are externally identical to standard black hole ones, but none of which can be
globally diffeomorphic to such standard objects. Certain aspects of the Cauchy
problem are also discussed in terms of \models which are
``half-standard'', say for all but for which cannot be globally
smooth.Comment: 8 pages plus 6 figures, available on request, IASSNS-HEP-94/2
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