20,852 research outputs found
Decoherence of Macroscopic Closed Systems within Newtonian Quantum Gravity
A theory recently proposed by the author aims to explain decoherence and the
thermodynamical behaviour of closed systems within a conservative, unitary,
framework for quantum gravity by assuming that the operators tied to the
gravitational degrees of freedom are unobservable and equating physical entropy
with matter-gravity entanglement entropy. Here we obtain preliminary results on
the extent of decoherence this theory predicts. We treat first a static state
which, if one were to ignore quantum gravitational effects, would be a quantum
superposition of two spatially displaced states of a single classically well
describable ball of uniform mass density in empty space. Estimating the quantum
gravitational effects on this system within a simple Newtonian approximation,
we obtain formulae which predict e.g. that as long as the mass of the ball is
considerably larger than the Planck mass, such a would-be-coherent static
superposition will actually be decohered whenever the separation of the centres
of mass of the two ball-states excedes a small fraction (which decreases as the
mass of the ball increases) of the ball radius. We then obtain a formula for
the quantum gravitational correction to the would-be-pure density matrix of a
non-relativistic many-body Schroedinger wave function and argue that this
formula predicts decoherence between configurations which differ (at least) in
the "relocation" of a cluster of particles of Planck mass. We estimate the
entropy of some simple model closed systems, finding a tendency for it to
increase with "matter-clumping" suggestive of a link with existing
phenomenological discussions of cosmological entropy increase.Comment: 11 pages, plain TeX, no figures. Accepted for publication as a
"Letter to the Editor" in "Classical and Quantum Gravity
Quenching of Cross Sections in Nucleon Transfer Reactions
Cross sections for proton knockout observed in (e,e'p) reactions are
apparently quenched by a factor of ~0.5, an effect attributed to short-range
correlations between nucleons. Here we demonstrate that such quenching is not
restricted to proton knockout, but a more general phenomenon associated with
any nucleon transfer. Measurements of absolute cross sections on a number of
targets between 16O and 208Pb were analyzed in a consistent way, with the cross
sections reduced to spectroscopic factors through the distorted-wave Born
approximation with global optical potentials. Across the 124 cases analyzed
here, induced by various proton- and neutron-transfer reactions and with
angular momentum transfer l=0-7, the results are consistent with a quenching
factor of 0.55. This is an apparently uniform quenching of single-particle
motion in the nuclear medium. The effect is seen not only in (d,p) reactions
but also in reactions with A=3 and 4 projectiles, when realistic wave functions
are used for the projectiles.Comment: 5 pages, 3 figures, accepted to Physical Review Letter
Quantum Field Theory on Spacetimes with a Compactly Generated Cauchy Horizon
We prove two theorems which concern difficulties in the formulation of the
quantum theory of a linear scalar field on a spacetime, (M,g_{ab}), with a
compactly generated Cauchy horizon. These theorems demonstrate the breakdown of
the theory at certain `base points' of the Cauchy horizon, which are defined as
`past terminal accumulation points' of the horizon generators. Thus, the
theorems may be interpreted as giving support to Hawking's `Chronology
Protection Conjecture', according to which the laws of physics prevent one from
manufacturing a `time machine'. Specifically, we prove: Theorem 1: There is no
extension to (M,g_{ab}) of the usual field algebra on the initial globally
hyperbolic region which satisfies the condition of F-locality at any base
point. In other words, any extension of the field algebra must, in any globally
hyperbolic neighbourhood of any base point, differ from the algebra one would
define on that neighbourhood according to the rules for globally hyperbolic
spacetimes. Theorem 2: The two-point distribution for any Hadamard state
defined on the initial globally hyperbolic region must (when extended to a
distributional bisolution of the covariant Klein-Gordon equation on the full
spacetime) be singular at every base point x in the sense that the difference
between this two point distribution and a local Hadamard distribution cannot be
given by a bounded function in any neighbourhood (in MXM) of (x,x). Theorem 2
implies quantities such as the renormalized expectation value of \phi^2 or of
the stress-energy tensor are necessarily ill-defined or singular at any base
point. The proofs rely on the `Propagation of Singularities' theorems of
Duistermaat and H\"ormander.Comment: 37 pages, LaTeX, uses latexsym and amsbsy, no figures; updated
version now published in Commun. Math. Phys.; no major revisions from
original versio
Active Galactic Nuclei and Massive Galaxy Cores
Self-consistent N-body simulations have been performed in order to study the effects of a central active galactic nucleus (AGN) on the dark matter profile of a typical giant elliptical galaxy. In our analysis, we assume that periodic bipolar outbursts from a central AGN can induce harmonic oscillatory motions on both sides of the gas core. Using realistic AGN properties, we find that the motions of the gas core, driven by such feedback processes, can flatten the dark matter and/or stellar profiles after 4-5 Gyr. Such results are consistent with observational studies such as those of Kormendy et al. (2006) which suggest that most giant elliptical galaxies have cores or ``missing light'' in their inner part. Since stars behave as a ``collisionless'' fluid similar to dark matter, the density profile both of stars and dark matter should be affected in a similar way, leading to an effective reduction in the central brightness
Optimal Cloning and Singlet Monogamy
The inability to produce two perfect copies of an unknown state is inherently
linked with the inability to produce maximal entanglement between multiple
spins. Despite this, there is no quantitative link between how much
entanglement can be generated between spins (known as monogamy), and how well
an unknown state can be cloned. This situation is remedied by giving a set of
sufficient conditions such that the optimal Completely Positive map can be
implemented as a teleportation operation into a standard, reference, state. The
case of arbitrary 1 to N asymmetric cloning of d-dimensional spins can then be
solved exactly, yielding the concept of `singlet monogamy'. The utility of this
relation is demonstrated by calculating properties of Heisenberg systems, and
contrasting them with the results from standard monogamy arguments.Comment: 4 pages, 1 figure. v2: conjecture upgraded to proof and generalized
to arbitrary local hilbert space dimensions. v3: published versio
Distributional Modes for Scalar Field Quantization
We propose a mode-sum formalism for the quantization of the scalar field
based on distributional modes, which are naturally associated with a slight
modification of the standard plane-wave modes. We show that this formalism
leads to the standard Rindler temperature result, and that these modes can be
canonically defined on any Cauchy surface.Comment: 15 pages, RevTe
Quantum Communication in Spin Systems With Long-Range Interactions
We calculate the fidelity of transmission of a single qubit between distant
sites on semi-infinite and finite chains of spins coupled via the magnetic
dipole interaction. We show that such systems often perform better than their
Heisenberg nearest-neighbour coupled counterparts, and that fidelities closely
approaching unity can be attained between the ends of finite chains without any
special engineering of the system, although state transfer becomes slow in long
chains. We discuss possible optimization methods, and find that, for any
length, the best compromise between the quality and the speed of the
communication is obtained in a nearly uniform chain of 4 spins.Comment: 15 pages, 8 eps figures, updated references, corrected text and
corrected figs. 1, 4 and
Size distributions of shocks and static avalanches from the Functional Renormalization Group
Interfaces pinned by quenched disorder are often used to model jerky
self-organized critical motion. We study static avalanches, or shocks, defined
here as jumps between distinct global minima upon changing an external field.
We show how the full statistics of these jumps is encoded in the
functional-renormalization-group fixed-point functions. This allows us to
obtain the size distribution P(S) of static avalanches in an expansion in the
internal dimension d of the interface. Near and above d=4 this yields the
mean-field distribution P(S) ~ S^(-3/2) exp(-S/[4 S_m]) where S_m is a
large-scale cutoff, in some cases calculable. Resumming all 1-loop
contributions, we find P(S) ~ S^(-tau) exp(C (S/S_m)^(1/2) -B/4 (S/S_m)^delta)
where B, C, delta, tau are obtained to first order in epsilon=4-d. Our result
is consistent to O(epsilon) with the relation tau = 2-2/(d+zeta), where zeta is
the static roughness exponent, often conjectured to hold at depinning. Our
calculation applies to all static universality classes, including random-bond,
random-field and random-periodic disorder. Extended to long-range elastic
systems, it yields a different size distribution for the case of contact-line
elasticity, with an exponent compatible with tau=2-1/(d+zeta) to
O(epsilon=2-d). We discuss consequences for avalanches at depinning and for
sandpile models, relations to Burgers turbulence and the possibility that the
above relations for tau be violated to higher loop order. Finally, we show that
the avalanche-size distribution on a hyper-plane of co-dimension one is in
mean-field (valid close to and above d=4) given by P(S) ~ K_{1/3}(S)/S, where K
is the Bessel-K function, thus tau=4/3 for the hyper plane.Comment: 34 pages, 30 figure
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