208 research outputs found
Comments on Challenges for Quantum Gravity
We examine radiative corrections arising from Lorentz violating dimension
five operators presumably associated with Planck scale physics as recently
considered by Myers and Pospelov. We find that observational data result in
bounds on the dimensionless parameters of the order . These represent
the most stringent bounds on Lorentz violation to date
Searching for spacetime granularity: analyzing a concrete experimental setup
In this work we show that the spin pendulum techniques developed by the
E\:{o}t-Wash group could be used to put very stringent bounds on the free
parameters of a Lorentz invariant phenomenological model of quantum gravity.
The model is briefly described as well as the experimental setup that we have
in mind.Comment: 9 pages. For the proceedings of the VIII School of the Gravitation
and Mathematical Physics Division of the Mexican Physical Society 'Speakable
and unspeakable in gravitational physics: testing gravity from submillimeter
to cosmic scale'
Dark energy from quantum gravity discreteness
We argue that discreteness at the Planck scale (naturally expected to arise
from quantum gravity) might manifest in the form of minute violations of
energy-momentum conservation of the matter degrees of freedom when described in
terms of (idealized) smooth fields on a smooth spacetime. In the context of
applications to cosmology such `energy diffusion' from the low energy matter
degrees of freedom to the discrete structures underlying spacetime leads to the
emergence of an effective dark energy term in Einstein's equations. We estimate
this effect using a (relational) hypothesis about the materialization of
discreteness in quantum gravity which is motivated by the strict observational
constraints supporting the validity of Lorentz invariance at low energies. The
predictions coming from simple dimensional analysis yield a cosmological
constant of the order of magnitude of the observed value without fine tuning.Comment: Typos corrected, closer to published versio
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
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
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