9,252 research outputs found
Black holes as mirrors: quantum information in random subsystems
We study information retrieval from evaporating black holes, assuming that
the internal dynamics of a black hole is unitary and rapidly mixing, and
assuming that the retriever has unlimited control over the emitted Hawking
radiation. If the evaporation of the black hole has already proceeded past the
"half-way" point, where half of the initial entropy has been radiated away,
then additional quantum information deposited in the black hole is revealed in
the Hawking radiation very rapidly. Information deposited prior to the half-way
point remains concealed until the half-way point, and then emerges quickly.
These conclusions hold because typical local quantum circuits are efficient
encoders for quantum error-correcting codes that nearly achieve the capacity of
the quantum erasure channel. Our estimate of a black hole's information
retention time, based on speculative dynamical assumptions, is just barely
compatible with the black hole complementarity hypothesis.Comment: 18 pages, 2 figures. (v2): discussion of decoding complexity
clarifie
Unitarity of black hole evaporation in final-state projection models
Almheiri et al. have emphasized that otherwise reasonable beliefs about black
hole evaporation are incompatible with the monogamy of quantum entanglement, a
general property of quantum mechanics. We investigate the final-state
projection model of black hole evaporation proposed by Horowitz and Maldacena,
pointing out that this model admits cloning of quantum states and polygamous
entanglement, allowing unitarity of the evaporation process to be reconciled
with smoothness of the black hole event horizon. Though the model seems to
require carefully tuned dynamics to ensure exact unitarity of the black hole
S-matrix, for a generic final-state boundary condition the deviations from
unitarity are exponentially small in the black hole entropy; furthermore
observers inside black holes need not detect any deviations from standard
quantum mechanics. Though measurements performed inside old black holes could
potentially produce causality-violating phenomena, the computational complexity
of decoding the Hawking radiation may render the causality violation
unobservable. Final-state projection models illustrate how inviolable
principles of standard quantum mechanics might be circumvented in a theory of
quantum gravity.Comment: (v3) 27 pages, 16 figures. Expanded discussion of measurements inside
black hole
Primordial Black Holes and -Process Nucleosynthesis
We show that some or all of the inventory of -process nucleosynthesis can
be produced in interactions of primordial black holes (PBHs) with neutron stars
(NSs) if PBHs with masses make up a few percent or more of the dark matter. A
PBH captured by a neutron star (NS) sinks to the center of the NS and consumes
it from the inside. When this occurs in a rotating millisecond-period NS, the
resulting spin-up ejects of relatively cold
neutron-rich material. This ejection process and the accompanying decompression
and decay of nuclear matter can produce electromagnetic transients, such as a
kilonova-type afterglow and fast radio bursts. These transients are not
accompanied by significant gravitational radiation or neutrinos, allowing such
events to be differentiated from compact object mergers occurring within the
distance sensitivity limits of gravitational wave observatories. The PBH-NS
destruction scenario is consistent with pulsar and NS statistics, the dark
matter content and spatial distributions in the Galaxy and Ultra Faint Dwarfs
(UFD), as well as with the -process content and evolution histories in these
sites. Ejected matter is heated by beta decay, which leads to emission of
positrons in an amount consistent with the observed 511-keV line from the
Galactic Center.Comment: 6 pages + 3 page supplement, 3 figures; matches published versio
Observational Properties of Simulated Galaxies in Overdense and Average Regions at High Redshifts z= 6-12
We use high-resolution zoom-in cosmological simulations of galaxies of
Romano-Diaz et al., post-processing them with a panchromatic three-dimensional
radiation transfer code to obtain the galaxy UV luminosity function (LF) at z ~
6-12. The galaxies are followed in a rare, heavily overdense region within a ~
5-sigma density peak, which can host high-z quasars, and in an average density
region, down to the stellar mass of M_star ~ 4* 10^7 Msun. We find that the
overdense regions evolve at a substantially accelerated pace --- the most
massive galaxy has grown to M_star ~ 8.4*10^10 Msun by z = 6.3, contains dust
of M_dust~ 4.1*10^8 Msun, and is associated with a very high star formation
rate, SFR ~ 745 Msun/yr.The attained SFR-M_star correlation results in the
specific SFR slowly increasing with M_star. Most of the UV radiation in massive
galaxies is absorbed by the dust, its escape fraction f_esc is low, increasing
slowly with time. Galaxies in the average region have less dust, and agree with
the observed UV LF. The LF of the overdense region is substantially higher, and
contains much brighter galaxies. The massive galaxies are bright in the
infrared (IR) due to the dust thermal emission, with L_IR~ 3.7*10^12 Lsun at z
= 6.3, while L_IR < 10^11 Lsun for the low-mass galaxies. Therefore, ALMA can
probe massive galaxies in the overdense region up to z ~ 10 with a reasonable
integration time. The UV spectral properties of disky galaxies depend
significantly upon the viewing angle.The stellar and dust masses of the most
massive galaxy in the overdense region are comparable to those of the
sub-millimetre galaxy (SMG) found by Riechers et al. at z = 6.3, while the
modelled SFR and the sub-millimetre flux fall slightly below the observed one.
Statistical significance of these similarities and differences will only become
clear with the upcoming ALMA observations.Comment: 17 pages, 13 figures, accepted for publication in MNRA
The strategic role of agriculture in a development system of exploitation of territorial resources
The great difficulties Italy is facing with in order to overcame the crisis that began in 2008, lead us to reconsider the development models which have determined it. In the present context of increasing competitiveness, inducted by the globalization process, to avoid being marginalized or given up, Italian socio-economic systems need to think out models of differentiated development on a local level and based on the exploitation of territorial resources. In this context, taking into consideration that 92% of national surface is classified as rural, it is indispensable to think up models of development whose main reference point is agriculture. Agriculture is indeed the fulcrum of agribusiness system that counts for 15% of national wealth and it is the productive activity which more than others set the characteristics of a territory. It takes on both the role of producer of goods and promoter of a socio, cultural and environmental context able to put together the different territorial entities, making them operating as a system. It is therefore evident that agriculture is not a separate sector but an entity strictly integrated with the other components of the socio-economic system and thanks to this it is able to play a fundamental role for the territorial development. In this paper it is highlighted how the creation of an economic model revolving around the concept of “identity”, through the exploitation of territorial resources, if well organized and supported by incisive political actions can be an occasion of economic recovery. Identity and uniqueness can be the right answer to the homologation and standardization of globalization. The aim of the paper is to underline how agriculture operating in an integrated way with tourism, handcraft and the other territorial activities is strategic to develop and carry out a development model where the territory is the landmark
The Quantum PCP Conjecture
The classical PCP theorem is arguably the most important achievement of
classical complexity theory in the past quarter century. In recent years,
researchers in quantum computational complexity have tried to identify
approaches and develop tools that address the question: does a quantum version
of the PCP theorem hold? The story of this study starts with classical
complexity and takes unexpected turns providing fascinating vistas on the
foundations of quantum mechanics, the global nature of entanglement and its
topological properties, quantum error correction, information theory, and much
more; it raises questions that touch upon some of the most fundamental issues
at the heart of our understanding of quantum mechanics. At this point, the jury
is still out as to whether or not such a theorem holds. This survey aims to
provide a snapshot of the status in this ongoing story, tailored to a general
theory-of-CS audience.Comment: 45 pages, 4 figures, an enhanced version of the SIGACT guest column
from Volume 44 Issue 2, June 201
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