980 research outputs found
Decoherence: Concepts and Examples
We give a pedagogical introduction to the process of decoherence - the
irreversible emergence of classical properties through interaction with the
environment. After discussing the general concepts, we present the following
examples: Localisation of objects, quantum Zeno effect, classicality of fields
and charges in QED, and decoherence in gravity theory. We finally emphasise the
important interpretational features of decoherence.Comment: 24 pages, LATEX, 9 figures, needs macro lamuphys.sty, to appear in
the Proceedings of the 10th Born Symposiu
Arrow of time in a recollapsing quantum universe
We show that the Wheeler-DeWitt equation with a consistent boundary condition
is only compatible with an arrow of time that formally reverses in a
recollapsing universe. Consistency of these opposite arrows is facilitated by
quantum effects in the region of the classical turning point. Since
gravitational time dilation diverges at horizons, collapsing matter must then
start re-expanding ``anticausally" (controlled by the reversed arrow) before
horizons or singularities can form. We also discuss the meaning of the
time-asymmetric expression used in the definition of ``consistent histories".
We finally emphasize that there is no mass inflation nor any information loss
paradox in this scenario.Comment: Many conceptual clarifications include
On time and the quantum-to-classical transition in Jordan-Brans-Dicke quantum gravity
Any quantum theory of gravity which treats the gravitational constant as a
dynamical variable has to address the issue of superpositions of states
corresponding to different eigenvalues. We show how the unobservability of such
superpositions can be explained through the interaction with other
gravitational degrees of freedom (decoherence). The formal framework is
canonically quantized Jordan-Brans-Dicke theory. We discuss the concepts of
intrinsic time and semiclassical time as well as the possibility of tunneling
into regions corresponding to a negative gravitational constant. We calculate
the reduced density matrix of the Jordan-Brans-Dicke field and show that the
off-diagonal elements can be sufficiently suppressed to be consistent with
experiments. The possible relevance of this mechanism for structure formation
in extended inflation is briefly discussed.Comment: 10 pages, Latex, ZU-TH 15/93, BUTP-93/1
Quantum Theory and Time Asymmetry
The relation between quantum measurement and thermodynamically irreversible
processes is investigated. The reduction of the state vector is fundamentally
asymmetric in time and shows an observer-relatedness which may explain the
double interpretation of the state vector as a representation of physical
states as well as of information about them. The concept of relevance being
used in all statistical theories of irreversible thermodynamics is shown to be
based on the same observer-relatedness. Quantum theories of irreversible
processes implicitly use an objectivized process of state vector reduction. The
conditions for the reduction are discussed, and I speculate that the final
(subjective) observer system might even be carried by a spacetime point.Comment: Latex version of a paper published in 1979 (with minor revisions), 18
page
Viscosity of an ideal relativistic quantum fluid: A perturbative study
We show that a quantized ideal fluid will generally exhibit a small but
non-zero viscosity due to the backreaction of quantum soundwaves on the
background. We use an effective field theory expansion to estimate this
viscosity to first order in perturbation theory. We discuss our results, and
whether this estimate can be used to obtain a more model-independent estimate
of the "quantum bound" on the viscosity of physical systemsComment: Accepted for publication, Phys.Rev.D. Discussion slightly clarified
and extended, references added, error in calculation fixed. COnclusions
unchange
GRB afterglow light curves in the pre-Swift era - a statistical study
We present the results of a systematic analysis of the world sample of
optical/near-infrared afterglow light curves observed in the pre-Swift era by
the end of 2004. After selecting the best observed 16 afterglows with
well-sampled light curves that can be described by a Beuermann equation, we
explore the parameter space of the light curve parameters and physical
quantities related to them. In addition, we search for correlations between
these parameters and the corresponding gamma-ray data, and we use our data set
to look for a fine structure in the light curves.Comment: accepted for publication in ApJ; Version 2: minor changes, one figure
adde
Quantum discreteness is an illusion
I review arguments demonstrating how the concept of "particle" numbers arises
in the form of equidistant energy eigenvalues of coupled harmonic oscillators
representing free fields. Their quantum numbers (numbers of nodes of the wave
functions) can be interpreted as occupation numbers for objects with a formal
mass (defined by the field equation) and spatial wave number ("momentum")
characterizing classical field modes. A superposition of different oscillator
eigenstates, all consisting of n modes having one node, while all others have
none, defines a nondegenerate "n-particle wave function". Other discrete
properties and phenomena (such as particle positions and "events") can be
understood by means of the fast but smooth process of decoherence: the
irreversible dislocalization of superpositions. Any wave-particle dualism thus
becomes obsolete. The observation of individual outcomes of this decoherence
process in measurements requires either a subsequent collapse of the wave
function or a "branching observer" in accordance with the Schr\"odinger
equation - both possibilities applying clearly after the decoherence process.
Any probability interpretation of the wave function in terms of local elements
of reality, such as particles or other classical concepts, would open a
Pandora's box of paradoxes, as is illustrated by various misnomers that have
become popular in quantum theory.Comment: 18 pages. v2: Some text and two references added. v3: Minor changes,
one reference added. v4: 21 pages. Submitted to AmJP (not accepted). v5:
Minor changes (mainly formulations). v6: Accepted by Found.Phys. Final
version is available at http://www.springerlink.co
SHEARING IRRADIATED URANIUM PLATES
Natural U plates that were irradiated to 600 and 1500 Mwd/t were cut under water by a guillotine-iype shear. Irradiation reduced the force required for shearing to 50% of that required for unirradiated U. Measurements were made of radioactivity released to treated and untreated water in which the cut sections were stored. (auth
Following a "Collapsing" Wavefunction
I study the quantum mechanics of a spin interacting with an ``apparatus''.
Although the evolution of the whole system is unitary, the spin evolution is
not. The system is chosen so that the spin exhibits loss of quantum coherence,
or ``wavefunction collapse'', of the sort usually associated with a quantum
measurement. The system is analyzed from the point of view of the spin density
matrix (or ``Schmidt paths''), and also using the consistent histories
approach. These two points of view are contrasted with each other. Connections
between the results and the form of the Hamiltonian are discussed in detail.Comment: 30 pages, plain LaTex, 3 figures in a separate uuencoded fil
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