68 research outputs found
On a recent proposal of faster than light quantum communication
In a recent paper, A.Y. Shiekh has discussed an experimental set-up which, in
his opinion, should make possible faster-than-light communication using the
collapse of the quantum wave function. Contrary to the many proposals which
have been presented in the past, he does not resort to an entangled state of
two systems but he works with a single particle in a superposition of two
states - corresponding to its propagation in opposite directions - one of which
goes through an appropriate interferometer. The possibility for an observer
near the interferometer to introduce or not, at his free will, a phase shifter
along one of the paths should allow to change instantaneously the probability
of finding the particle in the far-away region corresponding to the other state
of the superposition and, correspondingly, to change the intensity of a beam of
particles reaching a distant observer. In this paper we show a flaw in the
argument: once more, as it has been proved in full generality a long time ago,
the process of wave packet reduction cannot be used for superluminal
communication.Comment: 9 pages, LaTeX. Minor changes mad
Does quantum nonlocality irremediably conflict with Special Relativity?
We reconsider the problem of the compatibility of quantum nonlocality and the
requests for a relativistically invariant theoretical scheme. We begin by
discussing a recent important paper by T. Norsen [arXiv:0808.2178] on this
problem and we enlarge our considerations to give a general picture of the
conceptually relevant issue to which this paper is devoted.Comment: 18 pages, 1 figur
A Kolmogorov Extension Theorem for POVMs
We prove a theorem about positive-operator-valued measures (POVMs) that is an
analog of the Kolmogorov extension theorem, a standard theorem of probability
theory. According to our theorem, if a sequence of POVMs G_n on
satisfies the consistency (or projectivity) condition then there is a POVM G on the space
of infinite sequences that has G_n as its marginal for
the first n entries of the sequence. We also describe an application in quantum
theory.Comment: 6 pages LaTeX, no figure
Cosmogenesis and Collapse
Some possible benefits of dynamical collapse for a quantum theory of
cosmogenesis are discussed. These are a possible long wait before creation
begins, creation of energy and space, and choice of a particular universe out
of a superposition.Comment: For a festschrift in Foundations of Physics in honor of Daniel
Greenberger and Helmut Rauch in Foundations of Physics. This updates the
previous version by adding an appendix (Appendix B) which contains the exact
solution of a partial differential equation of importance in the pape
The effect of spontaneous collapses on neutrino oscillations
We compute the effect of collapse models on neutrino oscillations. The effect
of the collapse is to modify the evolution of the `spatial' part of the wave
function, which indirectly amounts to a change on the flavor components. In
many respects, this phenomenon is similar to neutrino propagation through
matter. For the analysis we use the mass proportional CSL model, and perform
the calculation to second order perturbation theory. As we will show, the CSL
prediction is very small - mainly due to the very small mass of neutrinos - and
practically undetectable.Comment: 24 pages, RevTeX. Updated versio
Quantum measurement in a family of hidden-variable theories
The measurement process for hidden-configuration formulations of quantum
mechanics is analysed. It is shown how a satisfactory description of quantum
measurement can be given in this framework. The unified treatment of
hidden-configuration theories, including Bohmian mechanics and Nelson's
stochastic mechanics, helps in understanding the true reasons why the problem
of quantum measurement can succesfully be solved within such theories.Comment: 16 pages, LaTeX; all special macros are included in the file; a
figure is there, but it is processed by LaTe
Real World Interpretations of Quantum Theory
I propose a new class of interpretations, {\it real world interpretations},
of the quantum theory of closed systems. These interpretations postulate a
preferred factorization of Hilbert space and preferred projective measurements
on one factor. They give a mathematical characterisation of the different
possible worlds arising in an evolving closed quantum system, in which each
possible world corresponds to a (generally mixed) evolving quantum state. In a
realistic model, the states corresponding to different worlds should be
expected to tend towards orthogonality as different possible quasiclassical
structures emerge or as measurement-like interactions produce different
classical outcomes. However, as the worlds have a precise mathematical
definition, real world interpretations need no definition of quasiclassicality,
measurement, or other concepts whose imprecision is problematic in other
interpretational approaches. It is natural to postulate that precisely one
world is chosen randomly, using the natural probability distribution, as the
world realised in Nature, and that this world's mathematical characterisation
is a complete description of reality.Comment: Minor revisions. To appear in Foundations of Physic
Explaining the unobserved: why quantum mechanics is not only about information
A remarkable theorem by Clifton, Bub and Halvorson (2003)(CBH) characterizes
quantum theory in terms of information--theoretic principles. According to Bub
(2004, 2005) the philosophical significance of the theorem is that quantum
theory should be regarded as a ``principle'' theory about (quantum) information
rather than a ``constructive'' theory about the dynamics of quantum systems.
Here we criticize Bub's principle approach arguing that if the mathematical
formalism of quantum mechanics remains intact then there is no escape route
from solving the measurement problem by constructive theories. We further
propose a (Wigner--type) thought experiment that we argue demonstrates that
quantum mechanics on the information--theoretic approach is incomplete.Comment: 34 Page
EPR-Bell Nonlocality, Lorentz Invariance, and Bohmian Quantum Theory
We discuss the problem of finding a Lorentz invariant extension of Bohmian
mechanics. Due to the nonlocality of the theory there is (for systems of more
than one particle) no obvious way to achieve such an extension. We present a
model invariant under a certain limit of Lorentz transformations, a limit
retaining the characteristic feature of relativity, the non-existence of
absolute time resp. simultaneity. The analysis of this model exemplifies an
important property of any Bohmian quantum theory: the quantum equilibrium
distribution cannot simultaneously be realized in all
Lorentz frames of reference.Comment: 24 pages, LaTex, 4 figure
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