158 research outputs found
The stability of Killing-Cauchy horizons in colliding plane wave space-times
It is confirmed rigorously that the Killing-Cauchy horizons, which sometimes
occur in space-times representing the collision and subsequent interaction of
plane gravitational waves in a Minkowski background, are unstable with respect
to bounded perturbations of the initial waves, at least for the case in which
the initial waves have constant aligned polarizations.Comment: 8 pages. To appear in Gen. Rel. Gra
Averaged Energy Conditions and Evaporating Black Holes
In this paper the averaged weak (AWEC) and averaged null (ANEC) energy
conditions, together with uncertainty principle-type restrictions on negative
energy (``quantum inequalities''), are examined in the context of evaporating
black hole backgrounds in both two and four dimensions. In particular,
integrals over only half-geodesics are studied. We determine the regions of the
spacetime in which the averaged energy conditions are violated. In all cases
where these conditions fail, there appear to be quantum inequalities which
bound the magnitude and extent of the negative energy, and hence the degree of
the violation. The possible relevance of these results for the validity of
singularity theorems in evaporating black hole spacetimes is discussed.Comment: Sections 2.1 and 2.2 have been revised and some erroneous statements
corrected. The main conclusions and the figures are unchanged. 27 pp, plain
Latex, 3 figures available upon reques
From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
The recent interest in ``time machines'' has been largely fueled by the
apparent ease with which such systems may be formed in general relativity,
given relatively benign initial conditions such as the existence of traversable
wormholes or of infinite cosmic strings. This rather disturbing state of
affairs has led Hawking to formulate his Chronology Protection Conjecture,
whereby the formation of ``time machines'' is forbidden. This paper will use
several simple examples to argue that the universe appears to exhibit a
``defense in depth'' strategy in this regard. For appropriate parameter regimes
Casimir effects, wormhole disruption effects, and gravitational back reaction
effects all contribute to the fight against time travel. Particular attention
is paid to the role of the quantum gravity cutoff. For the class of model
problems considered it is shown that the gravitational back reaction becomes
large before the Planck scale quantum gravity cutoff is reached, thus
supporting Hawking's conjecture.Comment: 43 pages,ReV_TeX,major revision
Particle creation in a colliding plane wave spacetime: wave packet quantization
We use wave packet mode quantization to compute the creation of massless
scalar quantum particles in a colliding plane wave spacetime. The background
spacetime represents the collision of two gravitational shock waves followed by
trailing gravitational radiation which focus into a Killing-Cauchy horizon. The
use of wave packet modes simplifies the problem of mode propagation through the
different spacetime regions which was previously studied with the use of
monocromatic modes. It is found that the number of particles created in a given
wave packet mode has a thermal spectrum with a temperature which is inversely
proportional to the focusing time of the plane waves and which depends on the
mode trajectory.Comment: 23, latex, figures available by fa
Stress-energy tensor in the Bel-Szekeres space-time
In a recent work an approximation procedure was introduced to calculate the
vacuum expectation value of the stress-energy tensor for a conformal massless
scalar field in the classical background determined by a particular colliding
plane wave space-time. This approximation procedure consists in appropriately
modifying the space-time geometry throughout the causal past of the collision
center. This modification in the geometry allows to simplify the boundary
conditions involved in the calculation of the Hadamard function for the quantum
state which represents the vacuum in the flat region before the arrival of the
waves. In the present work this approximation procedure is applied to the
non-singular Bel-Szekeres solution, which describes the head on collision of
two electromagnetic plane waves. It is shown that the stress-energy tensor is
unbounded as the killing-Cauchy horizon of the interaction is approached and
its behavior coincides with a previous calculation in another example of
non-singular colliding plane wave space-time.Comment: 17 pages, LaTex file, 2 PostScript figure
Structural phase transitions in multipole traps
A small number of laser-cooled ions trapped in a linear radiofrequency
multipole trap forms a hollow tube structure. We have studied, by means of
molecular dynamics simulations, the structural transition from a double ring to
a single ring of ions. We show that the single-ring configuration has the
advantage to inhibit the thermal transfer from the rf-excited radial components
of the motion to the axial component, allowing to reach the Doppler limit
temperature along the direction of the trap axis. Once cooled in this
particular configuration, the ions experience an angular dependency of the
confinement if the local adiabaticity parameter exceeds the empirical limit.
Bunching of the ion structures can then be observed and an analytic expression
is proposed to take into account for this behaviour
Quantum Complex Scalar Field in the Two-Dimensional Spacetime with Closed Timelike Curves and a Time-Machine Problem
It is considered the quantum complex scalar field which obeys the
authomorphic condition in the two-dimensional spacetime with closed timelike
curves and the chronology horizon. The renormalized stress-energy tensor is
obtained. It is shown that the value of the stress-energy tensor is regular at
the chronology horizon for specific authomorphic parameters. Thus the
particular example of field configuration is given for which the Hawking's
chronology protection conjecture is violated.Comment: 21(Latex)Postscript file of the figure may be received from author,
CQG-94-100
Lorentz invariant intrinsic decoherence
Quantum decoherence can arise due to classical fluctuations in the parameters
which define the dynamics of the system. In this case decoherence, and
complementary noise, is manifest when data from repeated measurement trials are
combined. Recently a number of authors have suggested that fluctuations in the
space-time metric arising from quantum gravity effects would correspond to a
source of intrinsic noise, which would necessarily be accompanied by intrinsic
decoherence. This work extends a previous heuristic modification of
Schr\"{o}dinger dynamics based on discrete time intervals with an intrinsic
uncertainty. The extension uses unital semigroup representations of space and
time translations rather than the more usual unitary representation, and does
the least violence to physically important invariance principles. Physical
consequences include a modification of the uncertainty principle and a
modification of field dispersion relations, in a way consistent with other
modifications suggested by quantum gravity and string theory .Comment: This paper generalises an earlier model published as Phys. Rev. A
vol44, 5401 (1991
Quantum Field Theory Constrains Traversable Wormhole Geometries
Recently a bound on negative energy densities in four-dimensional Minkowski
spacetime was derived for a minimally coupled, quantized, massless, scalar
field in an arbitrary quantum state. The bound has the form of an uncertainty
principle-type constraint on the magnitude and duration of the negative energy
density seen by a timelike geodesic observer. When spacetime is curved and/or
has boundaries, we argue that the bound should hold in regions small compared
to the minimum local characteristic radius of curvature or the distance to any
boundaries, since spacetime can be considered approximately Minkowski on these
scales. We apply the bound to the stress-energy of static traversable wormhole
spacetimes. Our analysis implies that either the wormhole must be only a little
larger than Planck size or that there is a large discrepancy in the length
scales which characterize the wormhole. In the latter case, the negative energy
must typically be concentrated in a thin band many orders of magnitude smaller
than the throat size. These results would seem to make the existence of
macroscopic traversable wormholes very improbable.Comment: 26 pages, plain LaTe
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