92 research outputs found
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'
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
Astrocytic Gq-GPCR-Linked IP3R-Dependent Ca2+ Signaling Does Not Mediate Neurovascular Coupling in Mouse Visual Cortex in vivo
Local blood flow is modulated in response to changing patterns of neuronal activity (Roy and Sherrington, 1890), a process termed neurovascular coupling. It has been proposed that the central cellular pathway driving this process is astrocytic Gq-GPCR-linked IP3R-dependent Ca2+ signaling, though in vivo tests of this hypothesis are largely lacking. We examined the impact of astrocytic Gq-GPCR and IP3R-dependent Ca2+ signaling on cortical blood flow in awake, responsive mice using multiphoton laser-scanning microscopy and novel genetic tools that enable the selective manipulation of astrocytic signaling pathways in vivo. Selective stimulation of astrocytic Gq-GPCR cascades and downstream Ca2+ signaling with the hM3Dq DREADD designer receptor system was insufficient to modulate basal cortical blood flow. We found no evidence of observable astrocyte endfeet Ca2+ elevations following physiological visual stimulation despite robust dilations of adjacent arterioles using cyto-GCaMP3 and Lck-GCaMP6s, the most sensitive Ca2+ indicator available. Astrocytic Ca2+ elevations could be evoked when inducing the startle response with unexpected air puffs. However, startle-induced astrocytic Ca2+ signals did not precede corresponding startle-induced hemodynamic changes. Further, neurovascular coupling was intact in awake, responsive mice genetically lacking astrocytic IP3R-dependent Ca2+ signaling (IP3R2 KO). These data establish that astrocytic Gq-GPCR-linked IP3R-dependent Ca2+ signaling does not mediate neurovascular coupling in visual cortex of awake, responsive mice.Doctor of Philosoph
The best Sobolev trace constant in periodic media for critical and subcritical exponents
In this paper we study homogenisation problems for Sobolev trace embedding H1(Ω) ↪ Lq(∂Ω) in a bounded smooth domain. When q = 2 this leads to a Steklov-like eigenvalue problem. We deal with the best constant of the Sobolev trace embedding in rapidly oscillating periodic media, and we consider H1 and Lq spaces with weights that are periodic in space. We find that extremals for these embeddings converge to a solution of a homogenised limit problem, and the best trace constant converges to a homogenised best trace constant. Our results are in fact more general; we can also consider general operators of the form aÉ›(x, ∇u) with non-linear Neumann boundary conditions. In particular, we can deal with the embedding W1,p(Ω) ↪ Lq(∂Ω).Fil: Fernandez Bonder, Julian. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Investigaciones Matemáticas "Luis A. Santaló". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Matemáticas "Luis A. Santaló"; ArgentinaFil: Orive, Rafael. Universidad Autónoma de Madrid; EspañaFil: Rossi, Julio Daniel. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentin
Operational Geometry on de Sitter Spacetime
Traditional geometry employs idealized concepts like that of a point or a
curve, the operational definition of which relies on the availability of
classical point particles as probes. Real, physical objects are quantum in
nature though, leading us to consider the implications of using realistic
probes in defining an effective spacetime geometry. As an example, we consider
de Sitter spacetime and employ the centroid of various composite probes to
obtain its effective sectional curvature, which is found to depend on the
probe's internal energy, spatial extension, and spin. Possible refinements of
our approach are pointed out and remarks are made on the relevance of our
results to the quest for a quantum theory of gravity.Comment: Replaced to match the published versio
Simultaneous vs. non-simultaneous blow-up in numerical approximations of a parabolic system with non-linear boundary conditions
Space-time variation of the electron-to-proton mass ratio in a Weyl model
Seeking a possible explanation for recent data indicating a space-time variation of the electron-to-proton mass ratio within the Milky Way, we consider a phenomenological model where the effective fermion masses depend on the local value of the Weyl tensor. We contrast the required values of the model's free parameters with bounds obtained from modern tests on the violation of the weak equivalence principle and we find that these quantities are incompatible. This result indicates that the variation of nucleon and electron masses through a coupling with the Weyl tensor is not a viable model.Facultad de Ciencias Astronómicas y GeofÃsica
Space-time variation of the electron-to-proton mass ratio in a Weyl model
Seeking a possible explanation for recent data indicating a space-time variation of the electron-to-proton mass ratio within the Milky Way, we consider a phenomenological model where the effective fermion masses depend on the local value of the Weyl tensor. We contrast the required values of the model's free parameters with bounds obtained from modern tests on the violation of the weak equivalence principle and we find that these quantities are incompatible. This result indicates that the variation of nucleon and electron masses through a coupling with the Weyl tensor is not a viable model.Facultad de Ciencias Astronómicas y GeofÃsica
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