1,699 research outputs found
Proof of a New Area Law in General Relativity
A future holographic screen is a hypersurface of indefinite signature,
foliated by marginally trapped surfaces with area . We prove that
grows strictly monotonically. Future holographic screens arise in gravitational
collapse. Past holographic screens exist in our own universe; they obey an
analogous area law. Both exist more broadly than event horizons or dynamical
horizons. Working within classical General Relativity, we assume the null
curvature condition and certain generiticity conditions. We establish several
nontrivial intermediate results. If a surface divides a Cauchy surface
into two disjoint regions, then a null hypersurface that contains
splits the entire spacetime into two disjoint portions: the
future-and-interior, ; and the past-and-exterior, . If a family of
surfaces foliate a hypersurface, while flowing everywhere to the
past or exterior, then the future-and-interior grows monotonically
under inclusion. If the surfaces are marginally trapped, we prove
that the evolution must be everywhere to the past or exterior, and the area
theorem follows. A thermodynamic interpretation as a Second Law is suggested by
the Bousso bound, which relates to the entropy on the null slices
foliating the spacetime. In a companion letter, we summarize the proof and
discuss further implications.Comment: 15 pages, 10 figures; v4: conclusion of Theorem IV.2 strengthene
Locality from Quantum Gravity: All or Nothing
In a full theory of quantum gravity, local physics is expected to be
approximate rather than innate. It is therefore important to understand how
approximate locality emerges in the semiclassical limit. Here we show that any
notion of locality emergent from a holographic theory of quantum gravity is
"all or nothing": local data is not obtained gradually from subregions of the
boundary, but is rather obtained all at once when enough of the boundary is
accessed. Our assumptions are mild and thus this feature is quite general; in
the special case of AdS/CFT, a slightly different manifestation follows from
well-known and familiar properties.Comment: 7 pages; 4 figures. v2: added references, minor edit
A New Area Law in General Relativity
We report a new area law in General Relativity. A future holographic screen
is a hypersurface foliated by marginally trapped surfaces. We show that their
area increases monotonically along the foliation. Future holographic screens
can easily be found in collapsing stars and near a big crunch. Past holographic
screens exist in any expanding universe and obey a similar theorem, yielding
the first rigorous area law in big bang cosmology. Unlike event horizons, these
objects can be identified at finite time and without reference to an asymptotic
boundary. The Bousso bound is not used, but it naturally suggests a
thermodynamic interpretation of our result.Comment: 4 pages, 2 figures; v3: typos fixe
Evidence for Dynamic Excitation-Inhibition Ratio in Networks of Cortical Neurons
In this report trial-to-trial variations in the synchronized responses of
neural networks are offered as evidence for excitation-inhibition ratio being a
dynamic variable over time scales of minutes. Synchronized network responses to
stimuli were studied in ex-vivo large scale cortical networks. We show that
sub-second measures of the individual synchronous response, namely -- its
latency and decay duration, are related to minutes-scale network response
dynamics. Network responsiveness is reflected as residency in, or shifting
amongst, areas of the latency-decay plane. The different sensitivities of
latency and decay durations to synaptic blockers imply that these two measures
reflect the effective impacts of inhibitory and excitatory neuronal populations
on response dynamics. Taken together, the data suggest that network response
variations under repeated stimuli result from excitation-inhibition ratio being
a dynamic variable rather than a parameter
Generalized Second Law for Cosmology
We conjecture a novel Generalized Second Law that can be applied in
cosmology, regardless of whether an event horizon is present: the generalized
entropy increases monotonically outside of certain hypersurfaces we call past
Q-screens. A past Q-screen is foliated by surfaces whose generalized entropy
(sum of area and entanglement entropy) is stationary along one future null
direction and increasing along the other. We prove that our Generalized Second
Law holds in spacetimes obeying the Quantum Focussing Conjecture. An analogous
law applies to future Q-screens, which appear inside evaporating black holes
and in collapsing regions.Comment: 14 pages, 5 figure
Causal Density Matrices
We define a new construct in quantum field theory - the causal density matrix
- obtained from the singularity structure of correlators of local operators.
This object provides a necessary and sufficient condition for a quantum field
theory state to have a holographic semiclassical dual causal geometry. By
exploiting the causal density matrix, we find that these dual causal geometries
quite generally (even away from AdS/CFT) exhibit features of quantum error
correction. Within AdS/CFT, we argue that the "reduced" causal density matrix
is the natural dual to the causal wedge. Our formalism is very well-suited to
generalizations of holography beyond AdS/CFT or even gravity/QFT.Comment: 34+4 pages, 10 figures. v2: minor edits, references added. v3: minor
typos correcte
Covariant Constraints on Hole-ography
Hole-ography is a prescription relating the areas of surfaces in an AdS bulk
to the differential entropy of a family of intervals in the dual CFT. In (2+1)
bulk dimensions, or in higher dimensions when the bulk features a sufficient
degree of symmetry, we prove that there are surfaces in the bulk that cannot be
completely reconstructed using known hole-ographic approaches, even if extremal
surfaces reach them. Such surfaces lie in easily identifiable regions: the
interiors of holographic screens. These screens admit a holographic
interpretation in terms of the Bousso bound. We speculate that this
incompleteness of the reconstruction is a form of coarse-graining, with the
missing information associated to the holographic screen. We comment on
perturbative quantum extensions of our classical results.Comment: 26+4 pages, 16 figures; v2: references added, typos fixe
The Gravity Dual of Boundary Causality
In gauge/gravity duality, points which are not causally related on the
boundary cannot be causally related through the bulk; this is the statement of
boundary causality. By the Gao-Wald theorem, the averaged null energy condition
in the bulk is sufficient to ensure this property. Here we proceed in the
converse direction: we derive a necessary as well as sufficient condition for
the preservation of boundary causality under perturbative (quantum or stringy)
corrections to the bulk. The condition that we find is a (background-dependent)
constraint on the amount by which light cones can "open" over all null bulk
geodesics. We show that this constraint is weaker than the averaged null energy
condition.Comment: 14+5 pages, 3 figures. v2: addressed referee comments; amended
figure
Surface Theory: the Classical, the Quantum, and the Holographic
Motivated by the power of subregion/subregion duality for constraining the
bulk geometry in gauge/gravity duality, we pursue a comprehensive and
systematic approach to the behavior of extremal surfaces under perturbations.
Specifically, we consider modifications to their boundary conditions, to the
bulk metric, and to bulk quantum matter fields. We present a unified framework
for treating such perturbations for classical extremal surfaces, classify some
of their stability properties, and develop new technology to extend our
treatment to quantum extremal surfaces, culminating in an "equation of quantum
extremal deviation". The power of this formalism stems from its ability to map
geometric statements into the language of elliptic operators; to illustrate, we
show that various a priori disparate bulk constraints all follow from basic
consistency of subregion/subregion duality. These include familiar properties
such as (smeared) versions of the quantum focusing conjecture and the
generalized second law, as well as new constraints on (i) metric and matter
perturbations in spacetimes close to vacuum and (ii) the bulk stress tensor in
generic (not necessary close to vacuum) spacetimes. This latter constraint is
highly reminiscent of a quantum energy inequality.Comment: 75 (60+9+6) pages, 13 figures. v2: fixed typos, minor clarification
Inhibition in Random Neuronal Networks Enhances Response Variability and Disrupts Stimulus Discrimination
Inhibition is considered to shape neural activity, and broaden its pattern
repertoire. In the sensory organs, where the anatomy of neural circuits is
highly structured, lateral inhibition sharpens contrast among stimulus
properties. The impact of inhibition on stimulus processing and the involvement
of lateral inhibition is less clear when activity propagates to the
less-structured relay stations. Here we take a synthetic approach to
disentangle the impacts of inhibition from that of specialized anatomy on the
repertoire of evoked activity patterns, and as a result, the network capacity
to uniquely represent different stimuli. To this aim, we blocked inhibition in
randomly rewired networks of cortical neurons in-vitro, and quantified response
variability and stimulus discrimination among stimuli provided at different
spatial loci, before and after the blockade. We show that blocking inhibition
quenches variability of responses evoked by repeated stimuli through any
spatial source; for all tested response features. Despite the sharpening role
of inhibition in the highly structured sensory organs, in these random networks
we find that blocking inhibition enhances stimulus discrimination between
spatial sources of stimulation, when based on response features that emphasize
the relation among spike times recorded through different electrodes. We
further show that under intact inhibition, responses to a given stimulus are a
noisy version of those revealed by blocking inhibition; such that intact
inhibition disrupts an otherwise coherent, wave propagation of activity
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