4,714 research outputs found
Improved molecular sorbent trap for high-vacuum systems
Closed cycle refrigeration loop in which trays holding molecular sorbent are made to serve as cooling baffles improves the performance of high vacuum systems. High performance is obtained with almost no decrease in pumping speed
Null Cones in Lorentz-Covariant General Relativity
The oft-neglected issue of the causal structure in the flat spacetime
approach to Einstein's theory of gravity is considered. Consistency requires
that the flat metric's null cone be respected, but this does not happen
automatically. After reviewing the history of this problem, we introduce a
generalized eigenvector formalism to give a kinematic description of the
relation between the two null cones, based on the Segre' classification of
symmetric rank 2 tensors with respect to a Lorentzian metric. Then we propose a
method to enforce special relativistic causality by using the naive gauge
freedom to restrict the configuration space suitably. A set of new variables
just covers this smaller configuration space and respects the flat metric's
null cone automatically. In this smaller space, gauge transformations do not
form a group, but only a groupoid. Respecting the flat metric's null cone
ensures that the spacetime is globally hyperbolic, indicating that the Hawking
black hole information loss paradox does not arise.Comment: groupoid nature of gauge transformations explained; shortened, new
references, 102 page
Null Cones and Einstein's Equations in Minkowski Spacetime
If Einstein's equations are to describe a field theory of gravity in
Minkowski spacetime, then causality requires that the effective curved metric
must respect the flat background metric's null cone. The kinematical problem is
solved using a generalized eigenvector formalism based on the Segr\'{e}
classification of symmetric rank 2 tensors with respect to a Lorentzian metric.
Securing the correct relationship between the two null cones dynamically
plausibly is achieved using the naive gauge freedom. New variables tied to the
generalized eigenvector formalism reduce the configuration space to the
causality-respecting part. In this smaller space, gauge transformations do not
form a group, but only a groupoid. The flat metric removes the difficulty of
defining equal-time commutation relations in quantum gravity and guarantees
global hyperbolicity
Two force component measuring device Patent
Development of two force component measuring devic
Light Cone Consistency in Bimetric General Relativity
General relativity can be formally derived as a flat spacetime theory, but
the consistency of the resulting curved metric's light cone with the flat
metric's null cone has not been adequately considered. If the two are
inconsistent, then gravity is not just another field in flat spacetime after
all.
Here we discuss recent progress in describing the conditions for consistency
and prospects for satisfying those conditions.Comment: contribution to the Proceedings of the 20th Texas Symposium on
Relativistic Astrophysics; 3 pages, 1 figur
Composite multilayer insulations for thermal protection of aerospace vehicles
Composite flexible multilayer insulation systems (MLI), consisting of alternating layers of metal foil and scrim cloth or insulation quilted together using ceramic thread, were evaluated for thermal performance and compared with a silica fibrous (baseline) insulation system. The systems studied included: (1) alternating layers of aluminoborosilicate (ABS) scrim cloth and stainless steel foil, with silica, ABS, or alumina insulation; (2) alternating layers of scrim cloth and aluminum foil, with silica or ABS insulation; (3) alternating layers of alumininum foil and silica or ABS insulation; and (4) alternating layers of aluminum-coated polyimide placed on the bottom of the silica insulation. The MLIs containing aluminum were the most efficient, measuring as little as half the backface temperature increase of the baseline system
On thermal stress failure of the SNAP-19A RTG heat shield
Results of a study on thermal stress problems in an amorphous graphite heat shield that is part of the launch-abort protect system for the SNAP-19A radio-isotope thermoelectric generators (RTG) that will be used on the Viking Mars Lander are presended. The first result is from a thermal stress analysis of a full-scale RTG heat source that failed to survive a suborbital entry flight test, possibly due to thermal stress failure. It was calculated that the maximum stress in the heat shield was only 50 percent of the ultimate strength of the material. To provide information on the stress failure criterion used for this calculation, some heat shield specimens were fractured under abort entry conditions in a plasma arc facility. It was found that in regions free of stress concentrations the POCO graphite heat shield material did fracture when the local stress reached the ultimate uniaxial stress of the material
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