686 research outputs found
Quantum Time and Spatial Localization: An Analysis of the Hegerfeldt Paradox
Two related problems in relativistic quantum mechanics, the apparent
superluminal propagation of initially localized particles and dependence of
spatial localization on the motion of the observer, are analyzed in the context
of Dirac's theory of constraints. A parametrization invariant formulation is
obtained by introducing time and energy operators for the relativistic particle
and then treating the Klein-Gordon equation as a constraint. The standard,
physical Hilbert space is recovered, via integration over proper time, from an
augmented Hilbert space wherein time and energy are dynamical variables. It is
shown that the Newton-Wigner position operator, being in this description a
constant of motion, acts on states in the augmented space. States with strictly
positive energy are non-local in time; consequently, position measurements
receive contributions from states representing the particle's position at many
times. Apparent superluminal propagation is explained by noting that, as the
particle is potentially in the past (or future) of the assumed initial place
and time of localization, it has time to propagate to distant regions without
exceeding the speed of light. An inequality is proven showing the Hegerfeldt
paradox to be completely accounted for by the hypotheses of subluminal
propagation from a set of initial space-time points determined by the quantum
time distribution arising from the positivity of the system's energy. Spatial
localization can nevertheless occur through quantum interference between states
representing the particle at different times. The non-locality of the same
system to a moving observer is due to Lorentz rotation of spatial axes out of
the interference minimum.Comment: This paper is identical to the version appearing in J. Math. Phys.
41; 6093 (Sept. 2000). The published version will be found at
http://ojps.aip.org/jmp/. The paper (40 page PDF file) has been completely
revised since the last posting to this archiv
Laser-induced forces between carbon nanotubes
Carbon nanotubes are the focus of intense research interest because of their unique properties and applications potential. We present a study based on quantum electrodynamics concerning the optical force between a pair of nanotubes under laser irradiance. To identify separate effects associated with the pair orientation and laser beam geometry, two different systems are analyzed, For each, an analytical expression for the laser-induced optical force is determined, and the corresponding magnitude is estimated. © 2005 Optical Society of America
Quantum times of arrival for multiparticle states
Using the concept of crossing state and the formalism of second quantization,
we propose a prescription for computing the density of arrivals of particles
for multiparticle states, both in the free and the interacting case. The
densities thus computed are positive, covariant in time for time independent
hamiltonians, normalized to the total number of arrivals, and related to the
flux. We investigate the behaviour of this prescriptions for bosons and
fermions, finding boson enhancement and fermion depletion of arrivals.Comment: 10 a4 pages, 5 inlined figure
Examining Neolithic Building and Activity Areas through Historic Cultural Heritage in Jordan: A Combined Ethnographic, Phytolith and Geochemical Investigation
The INEA project (Identifying activity areas in Neolithic
sites through Ethnographic Analysis of phytoliths and
geochemical residues, https://research.bournemouth.
ac.uk/2014/07/inea-project-2/) develops and applies
a method that combines the analysis of plant remains
(silica phytoliths) and geochemical residues to inform
on construction methods and the use of space in recently
abandoned historical villages and Neolithic settlements. It
is a collaborative project based at Bournemouth University,
in partnership with the Council for British Research in the
Levant
Macroscopically local correlations can violate information causality
Although quantum mechanics is a very successful theory, its foundations are
still a subject of intense debate. One of the main problems is the fact that
quantum mechanics is based on abstract mathematical axioms, rather than on
physical principles. Quantum information theory has recently provided new ideas
from which one could obtain physical axioms constraining the resulting
statistics one can obtain in experiments. Information causality and macroscopic
locality are two principles recently proposed to solve this problem. However
none of them were proven to define the set of correlations one can observe. In
this paper, we present an extension of information causality and study its
consequences. It is shown that the two above-mentioned principles are
inequivalent: if the correlations allowed by nature were the ones satisfying
macroscopic locality, information causality would be violated. This gives more
confidence in information causality as a physical principle defining the
possible correlation allowed by nature.Comment: are welcome. 6 pages, 4 figs. This is the originally submitted
version. The published version contains some bounds on quantum realizations
of d2dd isotropic boxes (table 1), found by T. Vertesi, who kindly shared
them with u
A measurement-based approach to quantum arrival times
For a quantum-mechanically spread-out particle we investigate a method for
determining its arrival time at a specific location. The procedure is based on
the emission of a first photon from a two-level system moving into a
laser-illuminated region. The resulting temporal distribution is explicitly
calculated for the one-dimensional case and compared with axiomatically
proposed expressions. As a main result we show that by means of a deconvolution
one obtains the well known quantum mechanical probability flux of the particle
at the location as a limiting distribution.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
Heat Kernel for Spin-3/2 Rarita-Schwinger Field in General Covariant Gauge
The heat kernel for the spin-3/2 Rarita-Schwinger gauge field on an arbitrary
Ricci flat space-time () is investigated in a family of covariant gauges
with one gauge parameter . The -dependent term of the kernel is
expressed by the spin-1/2 heat kernel. It is shown that the axial anomaly and
the one-loop divegence of the action are -independent, and that the
conformal anomaly has an -dependent total derivative term in
dimensions.Comment: 11 pages, latex, ITP-SB-94-3
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