4,882 research outputs found
Measurements according to Consistent Histories
We critically evaluate the treatment of the notion of measurement in the
Consistent Histories approach to quantum mechanics. We find such a treatment
unsatisfactory because it relies, often implicitly, on elements external to
those provided by the formalism. In particular, we note that, in order for the
formalism to be informative when dealing with measurement scenarios, one needs
to assume that the appropriate choice of framework is such that apparatuses are
always in states of well defined pointer positions after measurements. The
problem is that there is nothing in the formalism to justify this assumption.
We conclude that the Consistent Histories approach, contrary to what is claimed
by its proponents, fails to provide a truly satisfactory resolution for the
measurement problem in quantum theory.Comment: 17 pages. Accepted in Stud. Hist. Phil. Mod. Phy
Losing stuff down a black hole
Over the years, the so-called black hole information loss paradox has
generated an amazingly diverse set of (often radical) proposals. However, forty
years after the introduction of Hawking's radiation, there continues to be a
debate regarding whether the effect does, in fact, lead to an actual problem.
In this paper we try to clarify some aspect of the discussion by describing two
possible perspectives regarding the landscape of the information loss issue.
Moreover, we advance a fairly conservative point of view regarding the relation
between evaporating black holes and the rest of physics, which leads us to
advocate a generalized breakdown of unitarity. We conclude by exploring some
implications of our proposal in relation with conservation laws.Comment: 24 pages, 3 figure
Black Holes, Information Loss and the Measurement Problem
The information loss paradox is often presented as an unavoidable consequence
of well-established physics. However, in order for a genuine paradox to ensue,
not-trivial assumptions about, e.g., quantum effects on spacetime, are
necessary. In this work we will be explicit about these additional, speculative
assumptions required. We will also sketch a map of the available routes to
tackle the issue, highlighting the, often overlooked, commitments demanded of
each alternative. In particular, we will display the strong link between black
holes, the issue of information loss and the measurement problem.Comment: 14 pages, 3 figure
A (not so?) novel explanation for the very special initial state of the universe
We put forward a proposal that combines objective collapse models, developed
in connection with quantum-foundational questions, with the so-called Weyl
curvature hypothesis, introduced by Roger Penrose as an attempt to account for
the very special initial state of the universe. In particular, we explain how a
curvature dependence of the collapse rate in such models, an idea already shown
to help in the context of black holes and information loss, could also offer a
dynamical justification for Penrose's conjecture.Comment: 12 pages; improved and extended versio
Can gravity account for the emergence of classicality?
A recent debate has ensued over the claim by Pikovski et al. [Nat. Phys. 11,
668 (2015)] that systems with internal degrees of freedom undergo a universal,
gravity-induced, type of decoherence that explains their quantum-to-classical
transition. This decoherence is supposed to arise from the different
gravitational redshifts experienced by such systems when placed in a
superposition of two wave packets at different heights in a gravitational
field. Here we investigate some aspects of the discussion with the aid of
simple examples. In particular, we first resolve an apparent conflict between
the reported results and the equivalence principle by noting that the static
and free-fall descriptions focus on states associated with different
hypersurfaces. Next, we emphasize that predictions regarding the observability
of interference become relevant only in the context of concrete experimental
settings. As a result, we caution against hasty claims of universal validity.
Finally, we dispute the claim that, at least in the scenarios discussed by
Pikovski et al., gravitation is responsible for the reported results, and we
question the alleged ability of decoherence to explain the quantum-to-classical
transition. In consequence, we argue against the extraordinary assertion by
Pikovski et al. that gravity can account for the emergence of classicality.Comment: 10 pages, 6 figure
V-QCD: Spectra, the dilaton and the S-parameter
Zero temperature spectra of mesons and glueballs are analyzed in a class of
holographic bottom-up models for QCD (named V-QCD), as a function of x =
N_f/N_c with the full back-reaction included. It is found that spectra are
discrete and gapped (modulo the pions) in the QCD regime, for x below the
critical value x_c where the conformal transition takes place. The masses
uniformly converge to zero in the walking region x -> x_c due to Miransky
scaling. The ratio of masses all asymptote to non-zero constants as x -> x_c
and therefore there is no "dilaton" in the spectrum. The S-parameter is
computed and found to be of O(1) in the walking regime.Comment: 11 pages, 6 figure
The CP-odd sector and dynamics in holographic QCD
The holographic model of V-QCD is used to analyze the physics of QCD in the
Veneziano large-N limit. An unprecedented analysis of the CP-odd physics is
performed going beyond the level of effective field theories. The structure of
holographic saddle-points at finite is determined, as well as its
interplay with chiral symmetry breaking. Many observables (vacuum energy and
higher-order susceptibilities, singlet and non-singlet masses and mixings) are
computed as functions of and the quark mass . Wherever applicable
the results are compared to those of chiral Lagrangians, finding agreement. In
particular, we recover the Witten-Veneziano formula in the small
limit, we compute the -dependence of the pion mass and we derive the
hyperscaling relation for the topological susceptibility in the conformal
window in terms of the quark mass.Comment: 58 pages plus appendices, 19 figures. V2: section 3.1 improved, typos
corrected, published versio
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