2,819 research outputs found
Comment on "Relativistic Effects of Light in Moving Media with Extremely Low Group Velocity"
In [cond-mat/9906332; Phys. Rev. Lett. 84, 822 (2000)] and [physics/9906038;
Phys. Rev. A 60, 4301 (1999)] Leonhardt and Piwnicki have presented an
interesting analysis of how to use a flowing dielectric fluid to generate a
so-called "optical black hole". Qualitatively similar phenomena using
acoustical processes have also been much investigated. Unfortunately there is a
subtle misinterpretation in the Leonhardt-Piwnicki analysis regarding these
"optical black holes": While it is clear that "optical black holes" can
certainly exist as theoretical constructs, and while the experimental prospects
for actually building them in the laboratory are excellent, the particular
model geometries that Leonhardt and Piwnicki write down as alleged examples of
"optical black holes" are in fact not black holes at all.Comment: one page comment, uses ReV_TeX 3; discussion clarified; basic
physical results unaltere
The Reality and Measurement of the Wavefunction
Using a simple version of the model for the quantum measurement of a two
level system, the contention of Aharonov, Anandan, and Vaidman that one must in
certain circumstances give the wavefunction an ontological as well as an
epistemological significance is examined. I decide that their argument that the
wave function of a system can be measured on a single system fails to establish
the key point and that what they demonstrate is the ontological significance of
certain operators in the theory, with the wave function playing its usual
epistemological role.Comment: 10p
Early history of the moon: Implications of U-Th-Pb and Rb-Sr systematics
Anorthosite 60015 contains the lowest initial Sr-87/Sr-86 ratio yet reported for a lunar sample. The initial ratio is equal to that of the achondrite Angra dos Reis and slightly higher than the lowest measured Sr-87/Sr-86 ratio for an inclusion in the C3 carbonaceous chondrite Allende. The Pb-Pb ages of both Angra does Reis and Allende are 4.62 X 10 to the ninth power yr. Thus, the initial Sr/87/Sr-86 ratio found in lunar anorthosite 60015 strongly supports the hypothesis that the age of the Moon is about 4.65 b.y. The U-238/Pb-204 value estimated for the source of the excess lead in "orange soil" 74220 is approximately 35 and lower than the values estimated for the sources of KREEP (600-1000), high-K (300-600), and low-K (100-300) basalts. From these and other physical, chemical and petrographic results it was hypothesized that (1) the moon formed approximately 4.65 b.y. ago; (2) a global-scale gravitational differentiation occurred at the beginning of lunar history; and (3) the differentiation resulted in a radical chemical and mineralogical zoning in which the U-238/Pb-204 ratios increased toward the surface, with the exception of the low U-238/Pb-204 surficial anorthositic layer which "floated" at the beginning of the differentiation relative to the denser pyroxene-rich material
Early history of the moon: Implications of U-Th-Pb and Rb-Sr systematics
Anorthosite 60015 contains the lowest initial Sr-87/Sr-86 ratio (0.69884 + or - 0.00004) yet reported for a lunar sample. The initial ratio is equal to that of the achondrite Angra dos Reis and slightly higher than the lowest measured Sr-87/Sr-86 ratio for an inclusion in the C3 carbonaceous chondrite Allende. The Pb-Pb ages of both Angra dos Reis and Allende are 4.62 x 10 to the 9th power years (4.62 billion years). Thus, the initial Sr-87/Sr-86 ratio found in lunar anorthosite 60015 strongly supports the hypothesis that the age of the moon is about 4.65 b.y. The U-238/Pb-204 value estimated for the source of the excess lead in orange soil 74220 is lower than the values estimated for the sources of KREEP (600-1000), high K (300-600) and low K (100-300) basalts
Radiation Reaction fields for an accelerated dipole for scalar and electromagnetic radiation
The radiation reaction fields are calculated for an accelerated changing
dipole in scalar and electromagnetic radiation fields. The acceleration
reaction is shown to alter the damping of a time varying dipole in the EM case,
but not the scalar case. In the EM case, the dipole radiation reaction field
can exert a force on an accelerated monopole charge associated with the
accelerated dipole. The radiation reaction of an accelerated charge does not
exert a torque on an accelerated magnetic dipole, but an accelerated dipole
does exert a force on the charge. The technique used is that originally
developed by Penrose for non-singular fields and extended by the author for an
accelerated monopole charge.Comment: 11 page
A comparison of measured and simulated solar network contrast
Long-term trends in the solar spectral irradiance are important to determine
the impact on Earth's climate. These long-term changes are thought to be caused
mainly by changes in the surface area covered by small-scale magnetic elements.
The direct measurement of the contrast to determine the impact of these
small-scale magnetic elements is, however, limited to a few wavelengths, and
is, even for space instruments, affected by scattered light and instrument
defocus. In this work we calculate emergent intensities from 3-D simulations of
solar magneto-convection and validate the outcome by comparing with
observations from Hinode/SOT. In this manner we aim to construct the contrast
at wavelengths ranging from the NUV to the FIR.Comment: Proceedings paper, IAU XXVII, Symposium 264, 3 page
Difficulties with Recollapsing models in Closed Isotropic Loop Quantum Cosmology
The use of techniques from loop quantum gravity for cosmological models may
solve some difficult problems in quantum cosmology. The solutions under a
number of circumstances have been well studied. We will analyse the behaviour
of solutions in the closed model, focusing on the behaviour of a universe
containing a massless scalar field. The asymptotic behaviour of the solutions
is examined, and is used to determine requirements of the initial conditions.Comment: 10 pages, accepted to Phys. Rev.
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