7,584 research outputs found
More on the covariant retarded Green's function for the electromagnetic field in de Sitter spacetime
In a recent paper (Phys. Rev. D78, 084031 (2008), arXiv:0808.0642, Ref. [1])
it was shown in examples that the covariant retarded Green's functions in
particular gauges for electromagnetism and linearized gravity can be used to
reproduce field configurations correctly in spite of the spacelike nature of
past infinity in de Sitter spacetime. In this paper we extend the work of Ref.
[1] concerning the electromagnetic field and show that the covariant retarded
Green's function with an arbitrary value of the gauge parameter reproduces the
electromagnetic field from two opposite charges at antipodal points of de
Sitter spacetime.Comment: 4 pages, addendum to PRD 78, 084031 (2008) [arXiv:0808.0642
Meteoritic material on the moon
Three types of meteoritic material are found on the moon: micrometeorites, ancient planetesimal debris from the "early intense bombardment," and debris of recent, craterforming projectiles. Their amounts and compositions have been determined from trace element studies. The micrometeorite component is uniformly distributed over the entire lunar surface, but is seen most clearly in mare soils. It has a primitive, C1-chondrite-like composition, and comprises 1 to 1.5 percent of mature soils. Apparently it represents cometary debris. The ancient component is seen in highland breccias and soils. Six varieties have been recognized, differing in their proportions of refractories (Ir, Re), volatiles (Ge, Sb), and Au. All have a fractionated composition, with volatiles depleted relative to siderophiles. The abundance patterns do not match those of the known meteorite classes. These ancient meteoritic components seem to represent the debris of an extinct population of bodies (planetisimals, moonlets) that produced the mare basins during the first 700 Myr of the moon's history. On the basis of their stratigraphy and geographic distribution, five of the six groups are tentatively assigned to specific mare basins: Imbrium, Serenitatis, Crisium, Nectaris, and Humorum or Nubium
Meteoritic material on the moon
Micrometeorites, ancient planetesimal debris from the early intense bombardment, and debris of recent, crater-forming projectiles are discussed and their amounts and compositions have been determined from trace element studies. The micrometeorite component is uniformly distrubuted over the entire lunar surface, but is seen most clearly in mare soils whereas, the ancient component is seen in highland breccias and soils. A few properties of the basin-forming objects are inferred from the trace element data. An attempt is made to reconstruct the bombardment history of the moon from the observation that only basin-forming objects fell on the moon after crustal differentiation. The apparent half-life of basin-forming bodies is close to the calculated value for earth-crossing planetesimals. It is shown that a gap in radiometric ages is expected between the Imbrium and Nectaris impacts, because all 7 basins formed in this interval lie on the farside or east limb
Almost Ideal Clocks in Quantum Cosmology: A Brief Derivation of Time
A formalism for quantizing time reparametrization invariant dynamics is
considered and applied to systems which contain an `almost ideal clock.'
Previously, this formalism was successfully applied to the Bianchi models and,
while it contains no fundamental notion of `time' or `evolution,' the approach
does contain a notion of correlations. Using correlations with the almost ideal
clock to introduce a notion of time, the work below derives the complete
formalism of external time quantum mechanics. The limit of an ideal clock is
found to be closely associated with the Klein-Gordon inner product and the
Newton-Wigner formalism and, in addition, this limit is shown to fail for a
clock that measures metric-defined proper time near a singularity in Bianchi
models.Comment: 16 pages ReVTeX (35 preprint pages
Optical control of magnetization of micron-size domains in antiferromagnetic NiO single crystals
We propose Raman-induced collinear difference-frequency generation (DFG) as a
method to manipulate dynamical magnetization. When a fundamental beam
propagates along a threefold rotational axis, this coherent second-order
optical process is permitted by angular momentum conservation through the
rotational analogue of the Umklapp process. As a demonstration, we
experimentally obtained polarization properties of collinear magnetic DFG along
a [111] axis of a single crystal of antiferromagnetic NiO with micro
multidomain structure, which excellently agreed with the theoretical
prediction.Comment: 11 pages, 3 figures, submitted to Physical Review Letter
Low-energy sector quantization of a massless scalar field outside a Reissner-Nordstrom black hole and static sources
We quantize the low-energy sector of a massless scalar field in the
Reissner-Nordstrom spacetime. This allows the analysis of processes involving
soft scalar particles occurring outside charged black holes. In particular, we
compute the response of a static scalar source interacting with Hawking
radiation using the Unruh (and the Hartle-Hawking) vacuum. This response is
compared with the one obtained when the source is uniformly accelerated in the
usual vacuum of the Minkowski spacetime with the same proper acceleration. We
show that both responses are in general different in opposition to the result
obtained when the Reissner-Nordstrom black hole is replaced by a Schwarzschild
one. The conceptual relevance of this result is commented.Comment: 12 pages (REVTEX), no figure
SO(4) Invariant States in Quantum Cosmology
The phenomenon of linearisation instability is identified in models of
quantum cosmology that are perturbations of mini-superspace models. In
particular, constraints that are second order in the perturbations must be
imposed on wave functions calculated in such models. It is shown explicitly
that in the case of a model which is a perturbation of the mini-superspace
which has spatial sections these constraints imply that any wave
functions calculated in this model must be SO(4) invariant. (This replaces the
previous corrupted version.)Comment: 15 page
On the response of detectors in classical electromagnetic backgrounds
I study the response of a detector that is coupled non-linearly to a
quantized complex scalar field in different types of classical electromagnetic
backgrounds. Assuming that the quantum field is in the vacuum state, I show
that, when in {\it inertial} motion, the detector responds {\it only} when the
electromagnetic background produces particles. However, I find that the
response of the detector is {\it not} proportional to the number of particles
produced by the background.Comment: 10 pages, LaTeX, Final versio
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