867 research outputs found
Quantum Histories and Their Implications
Classical mechanics and standard Copenhagen quantum mechanics respect
subspace implications. For example, if a particle is confined in a particular
region of space, then in these theories we can deduce that it is confined
in regions containing . However, subspace implications are generally
violated by versions of quantum theory that assign probabilities to histories,
such as the consistent histories approach. I define here a new criterion,
ordered consistency, which refines the criterion of consistency and has the
property that inferences made by ordered consistent sets do not violate
subspace relations. This raises the question: do the operators defining our
observations form an ordered consistent history? If so, ordered consistency
defines a version of quantum theory with greater predictive power than the
consistent histories formalism. If not, and our observations are defined by a
non-ordered consistent quantum history, then subspace implications are not
generally valid.Comment: To appear in ``Relativistic Quantum Measurement and Decoherence'', F.
Petruccione (ed.), Springer-Verla
Quantum nonlocal correlations are not dominated
We show that no probability distribution of spin measurement outcomes on
pairs of spin 1/2 particles is unambiguously more nonlocal than the quantum
correlations. That is, any distribution that produces a CHSH violation larger
than the quantum violation for some axis choices also produces a smaller CHSH
violation for some other axis choices. In this sense, it is not possible for
nature to be strictly more nonlocal than quantum theory allows
Testing Causal Quantum Theory
Causal quantum theory assumes that measurements or collapses are well-defined
physical processes, localised in space-time, and never give perfectly reliable
outcomes and that the outcome of one measurement only influences the outcomes
of others within its future light cone. Although the theory has unusual
properties, it is not immediately evident that it is inconsistent with
experiment to date. I discuss its implications and experimental tests.Comment: Accepted manuscrip
A critical look at risk assessments for global catastrophes
Recent papers by Busza et al. (BJSW) and Dar et al. (DDH) argue that
astrophysical data can be used to establish small bounds on the risk of a
"killer strangelet" catastrophe scenario in the RHIC and ALICE collider
experiments. DDH and other commentators (initially including BJSW) suggested
that these empirical bounds alone do give sufficient reassurance. This seems
unsupportable when the bounds are expressed in terms of expected cost -- a good
measure, according to standard risk analysis arguments. For example, DDH's main
bound, , implies only that the
expectation value of the number of deaths is bounded by 120. This paper
reappraises the DDH and BJSW risk bounds by comparing risk policy in other
areas. For example, it is noted that, even if highly risk tolerant assumptions
are made and no value is placed on the lives of future generations, a
catastrophe risk no higher than per year would be required
for consistency with established policy for radiation hazard risk minimization.
It is concluded that the costs of small risks of catastrophe have been
significantly underestimated by BJSW (initially), by DDH and by other
commentators. Lessons for future policy are proposed.Comment: Minor corrections and note added July 2015. Previous arxiv version
corresponds to 2004 journal published versio
A Proposed Test of the Local Causality of Spacetime
A theory governing the metric and matter fields in spacetime is {\it locally
causal} if the probability distribution for the fields in any region is
determined solely by physical data in the region's past, i.e. it is independent
of events at space-like separated points. General relativity is manifestly
locally causal, since the fields in a region are completely determined by
physical data in its past. It is natural to ask whether other possible theories
in which the fundamental description of space-time is classical and geometric
-- for instance, hypothetical theories which stochastically couple a classical
spacetime geometry to a quantum field theory of matter -- might also be locally
causal.
A quantum theory of gravity, on the other hand, should allow the creation of
spacetimes which violate local causality at the macroscopic level. This paper
describes an experiment to test the local causality of spacetime, and hence to
test whether or not gravity behaves as quantum theories of gravity suggest, in
this respect. The experiment will either produce direct evidence that the
gravitational field is not locally causal, and thus weak confirmation of
quantum gravity, or else identify a definite limit to the domain of validity of
quantum theory.Comment: Further clarifications and addition
Consistent Sets Yield Contrary Inferences in Quantum Theory
In the consistent histories formulation of quantum theory, the probabilistic
predictions and retrodictions made from observed data depend on the choice of a
consistent set. We show that this freedom allows the formalism to retrodict
contrary propositions which correspond to orthogonal commuting projections and
which each have probability one. We also show that the formalism makes contrary
probability one predictions when applied to Gell-Mann and Hartle's generalised
time-neutral quantum mechanics.Comment: 10 pages, TeX with harvmac. Revised version, with extended discussion
and references added. To appear in Phys. Rev. Let
Unconstrained Summoning for relativistic quantum information processing
We define a summoning task to require propagating an unknown quantum state to
a point in space-time belonging to a set determined by classical inputs at
points in space-time. We consider the classical analogue, in which a known
classical state must be returned at precisely one allowed point. We show that,
when the inputs are unconstrained, any summoning task that is possible in the
classical case is also possible in the quantum case.Comment: Explanatory comments added. Minor typos corrected. Title expanded.
Accepted versio
Semi-quantum Gravity and Testing Gravitational Bell Non-locality
Semi-classical gravity attempts to define a hybrid theory in which a
classical gravitational field is coupled to a unitarily evolving quantum state.
Although semi-classical gravity is inconsistent with observation, a viable
theory of this type might be appealing, since it potentially might preserve the
basic features of our two most successful theories while unifying them. It
might also offer a natural solution to the quantum measurement problem. I
explore the scope for such "semi-quantum" hybrid theories, and note some
interesting, though daunting, constraints. Consistency with observation
generally requires pyschophysical parallelism with the classical gravitational
field rather than the quantum matter. Solvability suggests the gravitational
field at a point should be determined by physics in its past light cone, which
requires local hidden variables and predicts anomalously non-Newtonian
gravitational fields. These predictions could be tested by low energy, although
technologically challenging, experiments in which the Bell non-locality of the
gravitational field is verified by direct measurement.Comment: Draft circulated for comment. Not for the foundationally
faint-hearte
Quantum Theory's Reality Problem
This review, intended for a popular audience, was originally published in the
online magazine Aeon on 28 January 2014. It is reproduced on the arxiv with
permission. The online version (without references) can be found at
https://aeon.co/essays/what-really-happens-in-schrodinger-s-box.Comment: Typo correcte
Quantum imaging: Scattered observations on "Copenhagen"
Remarks on Michael Frayn's play "Copenhagen"
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