11,674 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
Thin-shell wormholes: Linearization stability
The class of spherically-symmetric thin-shell wormholes provides a
particularly elegant collection of exemplars for the study of traversable
Lorentzian wormholes. In the present paper we consider linearized (spherically
symmetric) perturbations around some assumed static solution of the Einstein
field equations. This permits us to relate stability issues to the (linearized)
equation of state of the exotic matter which is located at the wormhole throat.Comment: 4 pages; ReV_TeX 3.0; one postscript figur
Chemistry of pyrrolizines; reactions with cyanogen bromide and trifluoroacetic anhydride
Interaction of the pyrrolizine 3 with cyanogen bromide in a tetrahydrofuran/water mixture affords addition to the enamine double bond with formation of 5 which can be aromatized to 6 by silica gel. Reaction of 6 with cyanogen bromide in the same solvent mixture yields the indoline 8a which structure is proved in a chemical way by conversion of the product into the aldehyde 8d. The different reaction pathway is discussed in terms of steric hindrance by the ester groups. Treatment of 6 with trifluoroacetic anhydride gives the trifluoroacetylated compound 11. Removal of the sterically hindered ester groups in 6, with acetic acid in quinoline at 200°, is accompanied by the simultaneous decarboxylation to yield the pyrrolo[1,2-a]indole 1
Traversable wormholes: minimum violation of null energy condition revisited
It was argued in literature that traversable wormholes can exist with
arbitrarily small violation of null energy conditions. We show that if the
amount of exotic material near the wormhole throat tends to zero, either this
leads to a horn instead of a wormhole or the throat approaches the horizon in
such a way that infnitely large stresses develop on the throat.Comment: 12 pages. To appear in PR
Tolman wormholes violate the strong energy condition
For an arbitrary Tolman wormhole, unconstrained by symmetry, we shall define
the bounce in terms of a three-dimensional edgeless achronal spacelike
hypersurface of minimal volume. (Zero trace for the extrinsic curvature plus a
"flare-out" condition.) This enables us to severely constrain the geometry of
spacetime at and near the bounce and to derive general theorems regarding
violations of the energy conditions--theorems that do not involve geodesic
averaging but nevertheless apply to situations much more general than the
highly symmetric FRW-based subclass of Tolman wormholes. [For example: even
under the mildest of hypotheses, the strong energy condition (SEC) must be
violated.] Alternatively, one can dispense with the minimal volume condition
and define a generic bounce entirely in terms of the motion of test particles
(future-pointing timelike geodesics), by looking at the expansion of their
timelike geodesic congruences. One re-confirms that the SEC must be violated at
or near the bounce. In contrast, it is easy to arrange for all the other
standard energy conditions to be satisfied.Comment: 8 pages, ReV-TeX 3.
Novel applications of the "t-amino effect" in heterocyclic chemistry. Synthesis of a pyrrolo[1,2-a]quinazoline and 5H-pyrrolo[1,2-a][3,1]benzothiazines
1-(1-Pyrrolidinyl)benzenes substituted with an imino- or an in situ generated thiocarbonyl group in the 2-position rearrange upon heating to quinazoline and benzothiazine derivatives, respectively
Gravitational vacuum polarization III: Energy conditions in the (1+1) Schwarzschild spacetime
Building on a pair of earlier papers, I investigate the various point-wise
and averaged energy conditions for the quantum stress-energy tensor
corresponding to a conformally-coupled massless scalar field in the in the
(1+1)-dimensional Schwarzschild spacetime. Because the stress-energy tensors
are analytically known, I can get exact results for the Hartle--Hawking,
Boulware, and Unruh vacua. This exactly solvable model serves as a useful
sanity check on my (3+1)-dimensional investigations wherein I had to resort to
a mixture of analytic approximations and numerical techniques. Key results in
(1+1) dimensions are: (1) NEC is satisfied outside the event horizon for the
Hartle--Hawking vacuum, and violated for the Boulware and Unruh vacua. (2) DEC
is violated everywhere in the spacetime (for any quantum state, not just the
standard vacuum states).Comment: 7 pages, ReV_Te
On the mechanism of the reaction of enamines and dimethyl acetylenedicarboxylate (DMAD) in polar and apolar solvents
[2+2]-Cycloadducts of enamines and DMAD, formed in apolar solvents, isomerize to pyrrolizine derivatives under mild conditions in protic polar solvents like methanol and 1-butanol
van Vleck determinants: traversable wormhole spacetimes
Calculating the van Vleck determinant in traversable wormhole spacetimes is
an important ingredient in understanding the physical basis behind Hawking's
chronology protection conjecture. This paper presents extensive computations of
this object --- at least in the short--throat flat--space approximation. An
important technical trick is to use an extension of the usual junction
condition formalism to probe the full Riemann tensor associated with a thin
shell of matter. Implications with regard to Hawking's chronology protection
conjecture are discussed. Indeed, any attempt to transform a single isolated
wormhole into a time machine results in large vacuum polarization effects
sufficient to disrupt the internal structure of the wormhole before the onset
of Planck scale physics, and before the onset of time travel. On the other
hand, it is possible to set up a putative time machine built out of two or more
wormholes, each of which taken in isolation is not itself a time machine. Such
``Roman configurations'' are much more subtle to analyse. For some particularly
bizarre configurations (not traversable by humans) the vacuum polarization
effects can be arranged to be arbitrarily small at the onset of Planck scale
physics. This indicates that the disruption scale has been pushed down into the
Planck slop. Ultimately, for these configurations, questions regarding the
truth or falsity of Hawking's chronology protection can only be addressed by
entering the uncharted wastelands of full fledged quantum gravity.Comment: 42 pages, ReV_TeX 3.
Closed timelike curves in general relativity
Many solutions of Einstein's field equations contain closed timelike curves
(CTC). Some of these solutions refer to ordinary materials in situations which
might occur in the laboratory, or in astrophysics. It is argued that, in
default of a reasonable interpretation of CTC, general relativity does not give
a satisfactory account of all phenomena within its terms of reference.Comment: 3 pages, PACS: 042
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