168 research outputs found
Unitarity Restoration in the Presence of Closed Timelike Curves
A proposal is made for a mathematically unambiguous treatment of evolution in
the presence of closed timelike curves. In constrast to other proposals for
handling the naively nonunitary evolution that is often present in such
situations, this proposal is causal, linear in the initial density matrix and
preserves probability. It provides a physically reasonable interpretation of
invertible nonunitary evolution by redefining the final Hilbert space so that
the evolution is unitary or equivalently by removing the nonunitary part of the
evolution operator using a polar decomposition.Comment: LaTeX, 17pp, Revisions: Title change, expanded and clarified
presentation of original proposal, esp. with regard to Heisenberg picture and
remaining in original Hilbert spac
Unitarity of Quantum Theory and Closed Time-Like Curves
Interacting quantum fields on spacetimes containing regions of closed
timelike curves (CTCs) are subject to a non-unitary evolution . Recently, a
prescription has been proposed, which restores unitarity of the evolution by
modifying the inner product on the final Hilbert space. We give a rigorous
description of this proposal and note an operational problem which arises when
one considers the composition of two or more non-unitary evolutions. We propose
an alternative method by which unitarity of the evolution may be regained, by
extending to a unitary evolution on a larger (possibly indefinite) inner
product space. The proposal removes the ambiguity noted by Jacobson in
assigning expectation values to observables localised in regions spacelike
separated from the CTC region. We comment on the physical significance of the
possible indefiniteness of the inner product introduced in our proposal.Comment: 13 pages, LaTeX. Final revised paper to be published in Phys Rev D.
Some changes are made to expand our discussion of Anderson's Proposal for
restoring unitarit
Processor Allocation for Optimistic Parallelization of Irregular Programs
Optimistic parallelization is a promising approach for the parallelization of
irregular algorithms: potentially interfering tasks are launched dynamically,
and the runtime system detects conflicts between concurrent activities,
aborting and rolling back conflicting tasks. However, parallelism in irregular
algorithms is very complex. In a regular algorithm like dense matrix
multiplication, the amount of parallelism can usually be expressed as a
function of the problem size, so it is reasonably straightforward to determine
how many processors should be allocated to execute a regular algorithm of a
certain size (this is called the processor allocation problem). In contrast,
parallelism in irregular algorithms can be a function of input parameters, and
the amount of parallelism can vary dramatically during the execution of the
irregular algorithm. Therefore, the processor allocation problem for irregular
algorithms is very difficult.
In this paper, we describe the first systematic strategy for addressing this
problem. Our approach is based on a construct called the conflict graph, which
(i) provides insight into the amount of parallelism that can be extracted from
an irregular algorithm, and (ii) can be used to address the processor
allocation problem for irregular algorithms. We show that this problem is
related to a generalization of the unfriendly seating problem and, by extending
Tur\'an's theorem, we obtain a worst-case class of problems for optimistic
parallelization, which we use to derive a lower bound on the exploitable
parallelism. Finally, using some theoretically derived properties and some
experimental facts, we design a quick and stable control strategy for solving
the processor allocation problem heuristically.Comment: 12 pages, 3 figures, extended version of SPAA 2011 brief announcemen
Unitarity and Causality in Generalized Quantum Mechanics for Non-Chronal Spacetimes
Spacetime must be foliable by spacelike surfaces for the quantum mechanics of
matter fields to be formulated in terms of a unitarily evolving state vector
defined on spacelike surfaces. When a spacetime cannot be foliated by spacelike
surfaces, as in the case of spacetimes with closed timelike curves, a more
general formulation of quantum mechanics is required. In such generalizations
the transition matrix between alternatives in regions of spacetime where states
{\it can} be defined may be non-unitary. This paper describes a generalized
quantum mechanics whose probabilities consistently obey the rules of
probability theory even in the presence of such non-unitarity. The usual notion
of state on a spacelike surface is lost in this generalization and familiar
notions of causality are modified. There is no signaling outside the light
cone, no non-conservation of energy, no ``Everett phones'', and probabilities
of present events do not depend on particular alternatives of the future.
However, the generalization is acausal in the sense that the existence of
non-chronal regions of spacetime in the future can affect the probabilities of
alternatives today. The detectability of non-unitary evolution and violations
of causality in measurement situations are briefly considered. The evolution of
information in non-chronal spacetimes is described.Comment: 40pages, UCSBTH92-0
Prophetic Reading: Sisterhood and Psychoanalysis in H.D.âs HERmione
This article offers a comparative reading of H.D.âs 1927 kunstlerroman Ă clef, HERmione, and Freudâs Dora alongside an intertextual close reading of its dense web of literary allusions in order to argue that it offers a sustained critique of Freudian psychoanalysis and an alternative origin story for the condition of hysteria. Drawing on the notion of prophecy as it is thematised in the novel, the article demonstrates H.D.âs prefiguring of Juliet Mitchellâs recent reconfiguration of hysteria as a response to, replacement by, or failure of identification with a sibling
Antiflow of kaons in relativistic heavy ion collisions
We compare relativistic transport model calculations to recent data on the
sideward flow of neutral strange K^0_s mesons for Au+Au collisions at 6 AGeV. A
soft nuclear equation of state is found to describe very well the positive
proton flow data measured in the same experiment. In the absence of kaon
potential, the K^0 flow pattern is similar to that of protons. The kaon flow
becomes negative if a repulsive kaon potential determined from the impulse
approximation is introduced. However, this potential underestimates the data
which exhibits larger antiflow. An excellent agreement with the data is
obtained when a relativistic scalar-vector kaon potential, that has stronger
density dependence, is used. We further find that the transverse momentum
dependence of directed and elliptic flow is quite sensitive to the kaon
potential in dense matter.Comment: 5 pages, Revtex, 4 figure
On the EPR-type Entanglement in the Experiments of Scully et Al. I. The Micromaser Case and Delayed-choice Quantum Erasure
Delayed-choice erasure is investigated in two-photon two-slit experiments
that are generalizations of the micromaser experiment of Scully et al. [Scully,
M. O. et al. Nature 351, 111-116 (1991)]. Applying quantum mechanics to the
localization detector, it is shown that erasure with delayed choice in the
sense of Scully, has an analogous structure as simple erasure. The description
goes beyond probabilities. The EPR-type disentanglement, consisting in two
mutually incompatible distant measurements, is used as a general framework in
both parts of this study. Two simple coherence cases are shown to emerge
naturally, and they are precisely the two experiments of Scully et al. The
treatment seems to require the relative-reality-of-unitarily-evolving-state
(RRUES) approach. Besides insight in the exoeriments, this study has also the
goal of insight in quantum mechanics. The question is if it can be more than
just a "book-keeping device" for calculating probabilities as Scully et al.
modestly and cautiously claim.Comment: Latex2e, no figures, this manuscript is the first part of a study in
two part
Model-independent measurement of -channel single top quark production in collisions at TeV
We present a model-independent measurement of -channel electroweak
production of single top quarks in \ppbar collisions at . Using of integrated luminosity collected by the D0
detector at the Fermilab Tevatron Collider, and selecting events containing an
isolated electron or muon, missing transverse energy and one or two jets
originating from the fragmentation of quarks, we measure a cross section
\sigma({\ppbar}{\rargap}tqb+X) = 2.90 \pm 0.59\;\rm (stat+syst)\; pb for a
top quark mass of . The probability of the background to
fluctuate and produce a signal as large as the one observed is
, corresponding to a significance of 5.5 standard deviations.Comment: 8 pages, 4 figures, submitted to Phys. Lett.
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