6,999 research outputs found

### Initial states and decoherence of histories

We study decoherence properties of arbitrarily long histories constructed
from a fixed projective partition of a finite dimensional Hilbert space. We
show that decoherence of such histories for all initial states that are
naturally induced by the projective partition implies decoherence for arbitrary
initial states. In addition we generalize the simple necessary decoherence
condition [Scherer et al., Phys. Lett. A (2004)] for such histories to the case
of arbitrary coarse-graining.Comment: 10 page

### Causality in Time-Neutral Cosmologies

Gell-Mann and Hartle (GMH) have recently considered time-neutral cosmological
models in which the initial and final conditions are independently specified,
and several authors have investigated experimental tests of such models.
We point out here that GMH time-neutral models can allow superluminal
signalling, in the sense that it can be possible for observers in those
cosmologies, by detecting and exploiting regularities in the final state, to
construct devices which send and receive signals between space-like separated
points. In suitable cosmologies, any single superluminal message can be
transmitted with probability arbitrarily close to one by the use of redundant
signals. However, the outcome probabilities of quantum measurements generally
depend on precisely which past {\it and future} measurements take place. As the
transmission of any signal relies on quantum measurements, its transmission
probability is similarly context-dependent. As a result, the standard
superluminal signalling paradoxes do not apply. Despite their unusual features,
the models are internally consistent.
These results illustrate an interesting conceptual point. The standard view
of Minkowski causality is not an absolutely indispensable part of the
mathematical formalism of relativistic quantum theory. It is contingent on the
empirical observation that naturally occurring ensembles can be naturally
pre-selected but not post-selected.Comment: 5 pages, RevTeX. Published version -- minor typos correcte

### Current Algebra: Quarks and What Else?

After receiving many requests for reprints of this article, describing the
original ideas on the quark gluon gauge theory, which we later named QCD, we
decided to place the article in the e-Print archive

### A simple necessary decoherence condition for a set of histories

Within the decoherent histories formulation of quantum mechanics, we
investigate necessary conditions for decoherence of arbitrarily long histories.
We prove that fine-grained histories of arbitrary length decohere for all
classical initial states if and only if the unitary evolution preserves
classicality of states (using a natural formal definition of classicality). We
give a counterexample showing that this equivalence does not hold for
coarse-grained histories.Comment: 11 pages,LaTe

### Quasiclassical Coarse Graining and Thermodynamic Entropy

Our everyday descriptions of the universe are highly coarse-grained,
following only a tiny fraction of the variables necessary for a perfectly
fine-grained description. Coarse graining in classical physics is made natural
by our limited powers of observation and computation. But in the modern quantum
mechanics of closed systems, some measure of coarse graining is inescapable
because there are no non-trivial, probabilistic, fine-grained descriptions.
This essay explores the consequences of that fact. Quantum theory allows for
various coarse-grained descriptions some of which are mutually incompatible.
For most purposes, however, we are interested in the small subset of
``quasiclassical descriptions'' defined by ranges of values of averages over
small volumes of densities of conserved quantities such as energy and momentum
and approximately conserved quantities such as baryon number. The
near-conservation of these quasiclassical quantities results in approximate
decoherence, predictability, and local equilibrium, leading to closed sets of
equations of motion. In any description, information is sacrificed through the
coarse graining that yields decoherence and gives rise to probabilities for
histories. In quasiclassical descriptions, further information is sacrificed in
exhibiting the emergent regularities summarized by classical equations of
motion. An appropriate entropy measures the loss of information. For a
``quasiclassical realm'' this is connected with the usual thermodynamic entropy
as obtained from statistical mechanics. It was low for the initial state of our
universe and has been increasing since.Comment: 17 pages, 0 figures, revtex4, Dedicated to Rafael Sorkin on his 60th
birthday, minor correction

### Adaptive Coarse Graining, Environment, Strong Decoherence, and Quasiclassical Realms

Three ideas are introduced that when brought together characterize the
realistic quasiclassical realms of our quantum universe as particular kinds of
sets of alternative coarse-grained histories defined by quasiclassical
variables: (1) Branch dependent adaptive coarse grainings that can be close to
maximally refined and can simplify calculation. (2) Narrative coarse grainings
that describe how features of the universe change over time and allow the
construction of an environment. (3) A notion of strong decoherence that
characterizes realistic mechanisms of decoherence.Comment: 11 pages, revtex

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