941 research outputs found
First electrostatic probe results from Explorer 17
Electrostatic probe results from ionospheric sounding by Explorer XVII satellit
Nonabelian Gauge Theories on Noncommutative Spaces
In this paper, we describe a method for obtaining the nonabelian
Seiberg-Witten map for any gauge group and to any order in theta. The equations
defining the Seiberg-Witten map are expressed using a coboundary operator, so
that they can be solved by constructing a corresponding homotopy operator. The
ambiguities, of both the gauge and covariant type, which arise in this map are
manifest in our formalism.Comment: 14 pages, latex, Talk presented at 2001: A Spacetime Odyssey -
Michigan Center for Theoretical Physics, some typos correcte
Some winter characteristics of the northern high latitude ionosphere
Langmuir probe measurements of ionospheric electron density and temperature during winte
Recommended from our members
Assessing time knowledge in children aged 10 to 11 years
The acquisition of time knowledge involves learning how to read clocks, estimate time, read dates and learn about temporal sequences. Evidence suggests that many of these competencies are acquired by 10 years of age although not all children may follow this developmental path. The main purpose of this study was to collect normative data for a screening tool that assesses time knowledge. These data identify the prevalence and pattern of difficulties with time knowledge among a UK sample of Year 6 pupils (aged 10 to 11 years). The Time Screening Assessment tool (Doran, Dutt & Pembery, 2015), designed to assess time knowledge, was administered individually to a sample of 79 children. Findings revealed a median overall score of 32 out of a maximum score of 36. 25% of children performed at or close to ceiling, however seven children scored more than -1.5 standard deviations below the mean. The value of these findings to practitioners working with children in schools is discussed
Seiberg-Witten maps and noncommutative Yang-Mills theories for arbitrary gauge groups
Seiberg-Witten maps and a recently proposed construction of noncommutative
Yang-Mills theories (with matter fields) for arbitrary gauge groups are
reformulated so that their existence to all orders is manifest. The ambiguities
of the construction which originate from the freedom in the Seiberg-Witten map
are discussed with regard to the question whether they can lead to inequivalent
models, i.e., models not related by field redefinitions.Comment: 12 pages; references added, minor misprints correcte
Ionosphere Electron Temperature Measurements and Their Implications
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147864/1/jgrtb00020.pd
Boundary States for Supertubes in Flat Spacetime and Godel Universe
We construct boundary states for supertubes in the flat spacetime. The T-dual
objects of supertubes are moving spiral D1-branes (D-helices). Since we can
obtain these D-helices from the usual D1-branes via null deformation, we can
construct the boundary states for these moving D-helices in the covariant
formalism. Using these boundary states, we calculate the vacuum amplitude
between two supertubes in the closed string channel and read the open string
spectrum via the open closed duality. We find there are critical values of the
energy for on-shell open strings on the supertubes due to the non-trivial
stringy correction. We also consider supertubes in the type IIA Godel universe
in order to use them as probes of closed timelike curves. This universe is the
T-dual of the maximally supersymmetric type IIB PP-wave background. Since the
null deformations of D-branes are also allowed in this PP-wave, we can
construct the boundary states for supertubes in the type IIA Godel universe in
the same way. We obtain the open string spectrum on the supertube from the
vacuum amplitude between supertubes. As a consequence, we find that the
tachyonic instability of open strings on the supertube, which is the signal of
closed time like curves, disappears due to the stringy correction.Comment: 26 pages, 3 figures, v2: explanations added, references added, v3:
explanations adde
The Seiberg-Witten Map for a Time-dependent Background
In this paper the Seiberg-Witten map for a time-dependent background related
to a null-brane orbifold is studied. The commutation relations of the
coordinates are linear, i.e. it is an example of the Lie algebra type. The
equivalence map between the Kontsevich star product for this background and the
Weyl-Moyal star product for a background with constant noncommutativity
parameter is also studied.Comment: latex, 13 pages, references added and some misprints correcte
Towards an explicit expression of the Seiberg-Witten map at all orders
The Seiberg-Witten map links noncommutative gauge theories to ordinary gauge
theories, and allows to express the noncommutative variables in terms of the
commutative ones. Its explicit form can be found order by order in the
noncommutative parameter theta and the gauge potential A by the requirement
that gauge orbits are mapped on gauge orbits. This of course leaves
ambiguities, corresponding to gauge transformations, and there is an infinity
of solutions. Is there one better, clearer than the others ? In the abelian
case, we were able to find a solution, linked by a gauge transformation to
already known formulas, which has the property of admitting a recursive
formulation, uncovering some pattern in the map. In the special case of a pure
gauge, both abelian and non-abelian, these expressions can be summed up, and
the transformation is expressed using the parametrisation in terms of the gauge
group.Comment: 17 pages. Latex, 1 figure. v2: minor changes, published versio
Hagedorn transition and chronology protection in string theory
We conjecture chronology is protected in string theory due to the
condensation of light winding strings near closed null curves. This
condensation triggers a Hagedorn phase transition, whose end-point target space
geometry should be chronological. Contrary to conventional arguments,
chronology is protected by an infrared effect. We support this conjecture by
studying strings in the O-plane orbifold, where we show that some winding
string states are unstable and condense in the non-causal region of spacetime.
The one-loop string partition function has infrared divergences associated to
the condensation of these states.Comment: 40 pages, 11 figures. Expanded discussion on evolution of on-shell
modes and added appendi
- …