18,759 research outputs found
Comment on ``Consistent Sets Yield Contrary Inferences in Quantum Theory''
In a recent paper Kent has pointed out that in consistent histories quantum
theory it is possible, given initial and final states, to construct two
different consistent families of histories, in each of which there is a
proposition that can be inferred with probability one, and such that the
projectors representing these two propositions are mutually orthogonal. In this
note we stress that, according to the rules of consistent history reasoning two
such propositions are not contrary in the usual logical sense namely, that one
can infer that if one is true then the other is false, and both could be false.
No single consistent family contains both propositions, together with the
initial and final states, and hence the propositions cannot be logically
compared. Consistent histories quantum theory is logically consistent,
consistent with experiment as far as is known, consistent with the usual
quantum predictions for measurements, and applicable to the most general
physical systems. It may not be the only theory with these properties, but in
our opinion, it is the most promising among present possibilities.Comment: 2pages, uses REVTEX 3.
The characteristic initial value problem for colliding plane waves: The linear case
The physical situation of the collision and subsequent interaction of plane
gravitational waves in a Minkowski background gives rise to a well-posed
characteristic initial value problem in which initial data are specified on the
two null characteristics that define the wavefronts. In this paper, we analyse
how the Abel transform method can be used in practice to solve this problem for
the linear case in which the polarization of the two gravitational waves is
constant and aligned. We show how the method works for some known solutions,
where problems arise in other cases, and how the problem can always be solved
in terms of an infinite series if the spectral functions for the initial data
can be evaluated explicitly.Comment: 14 pages. To appear in Class. Quantum Gra
On plane gravitational waves in real connection variables
We investigate using plane fronted gravitational wave space-times as model
systems to study loop quantization techniques and dispersion relations. In this
classical analysis, we start with planar symmetric space-times in the real
connection formulation. We reduce via Dirac constraint analysis to a final form
with one canonical pair and one constraint, equivalent to the metric and
Einstein equations of plane-fronted with parallel rays waves. Due to the
symmetries and use of special coordinates general covariance is broken.
However, this allows us to simply express the constraints of the consistent
system. A recursive construction of Dirac brackets results in non-local
brackets, analogous to those of self-dual fields, for the triad variables
chosen in this approach.Comment: v2: Matches published version, up to minor stylistic change
Consistent Quantum Counterfactuals
An analysis using classical stochastic processes is used to construct a
consistent system of quantum counterfactual reasoning. When applied to a
counterfactual version of Hardy's paradox, it shows that the probabilistic
character of quantum reasoning together with the ``one framework'' rule
prevents a logical contradiction, and there is no evidence for any mysterious
nonlocal influences. Counterfactual reasoning can support a realistic
interpretation of standard quantum theory (measurements reveal what is actually
there) under appropriate circumstances.Comment: Minor modifications to make it agree with published version. Latex 8
pages, 2 figure
Atemporal diagrams for quantum circuits
A system of diagrams is introduced that allows the representation of various
elements of a quantum circuit, including measurements, in a form which makes no
reference to time (hence ``atemporal''). It can be used to relate quantum
dynamical properties to those of entangled states (map-state duality), and
suggests useful analogies, such as the inverse of an entangled ket. Diagrams
clarify the role of channel kets, transition operators, dynamical operators
(matrices), and Kraus rank for noisy quantum channels. Positive (semidefinite)
operators are represented by diagrams with a symmetry that aids in
understanding their connection with completely positive maps. The diagrams are
used to analyze standard teleportation and dense coding, and for a careful
study of unambiguous (conclusive) teleportation. A simple diagrammatic argument
shows that a Kraus rank of 3 is impossible for a one-qubit channel modeled
using a one-qubit environment in a mixed state.Comment: Minor changes in references. Latex 32 pages, 13 figures in text using
PSTrick
EPR, Bell, and Quantum Locality
Maudlin has claimed that no local theory can reproduce the predictions of
standard quantum mechanics that violate Bell's inequality for Bohm's version
(two spin-half particles in a singlet state) of the Einstein-Podolsky-Rosen
problem. It is argued that, on the contrary, standard quantum mechanics itself
is a counterexample to Maudlin's claim, because it is local in the appropriate
sense (measurements at one place do not influence what occurs elsewhere there)
when formulated using consistent principles in place of the inconsistent
appeals to "measurement" found in current textbooks. This argument sheds light
on the claim of Blaylock that counterfactual definiteness is an essential
ingredient in derivations of Bell's inequality.Comment: Minor revisions to previous versio
Search For Gravitational Waves Through the Electromagnetic Faraday Rotation
A method is given which renders indirect detection of strong gravitational
waves possible. This is based on the reflection (collision) of a linearly
polarized electromagnetic shock wave from (with) a cross polarized impulsive
and shock gravitational waves in accordance with the general theory of
relativity. This highly non-linear process induces a detectable Faraday
rotation in the polarization vector of the electromagnetic field.Comment: Final version. Minor revision, new figures and references are added.
To appear in Physical Review
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