Where has all the information gone?

The existence of spacetime singularities is irrelevant for the irreversible appearance of black holes. However, confirmation of the latter's unitary dynamics would require the preparation of a coherent superposition of a tremendous number of appropriate ``Everett worlds''.Comment: 10 pages, 1 figure, Latex - Invited paper for a special Einstein issue of Physics Letters

A supersymmetric D-brane Model of Space-Time Foam

We present a supersymmetric model of space-time foam with two stacks of eight D8-branes with equal string tensions, separated by a single bulk dimension containing D0-brane particles that represent quantum fluctuations in the space-time foam. The ground state configuration with static D-branes has zero vacuum energy. However, gravitons and other closed-string states propagating through the bulk may interact with the D0-particles, causing them to recoil and the vacuum energy to become non zero. This provides a possible origin of dark energy. Recoil also distorts the background metric felt by energetic massless string states, which travel at less than the usual (low-energy) velocity of light. On the other hand, the propagation of chiral matter anchored on the D8 branes is not affected by such space-time foam effects.Comment: 33 pages, latex, five figure

Coherent states and the classical-quantum limit considered from the point of view of entanglement

Three paradigms commonly used in classical, pre-quantum physics to describe particles (that is: the material point, the test-particle and the diluted particle (droplet model)) can be identified as limit-cases of a quantum regime in which pairs of particles interact without getting entangled with each other. This entanglement-free regime also provides a simplified model of what is called in the decoherence approach "islands of classicality", that is, preferred bases that would be selected through evolution by a Darwinist mechanism that aims at optimising information. We show how, under very general conditions, coherent states are natural candidates for classical pointer states. This occurs essentially because, when a (supposedly bosonic) system coherently exchanges only one quantum at a time with the (supposedly bosonic) environment, coherent states of the system do not get entangled with the environment, due to the bosonic symmetry.Comment: This is the definitive version of a paper entitled The classical-quantum limit considered from the point of view of entanglement: a survey (author T. Durt). The older version has been replaced by the definitive on

Things of the ground: Children's medicine, motherhood and memory in the Cameroon grassfields

Copyright @ 2011 Cambridge University PressSoon after birth, infants in the Cameroon Grassfields chiefdom of Oku are submitted by their parents to rites known generically as ‘children's medicine’ (k∂fu ∂bwan). Ostensibly performed to protect infants from harm and illness, the rites are in fact fraught with tension: they embrace contradictory perspectives regarding the social role of the mother and belie the normative ideal extolling her as a figure of nurture and protection. The article argues that, beyond their overt purpose and symbolism as rites of passage, the rites evoke collective memories of child abductions and contemporary anxieties regarding the anticipated departure of older children and adolescents into foster care or migrant labour. Going beyond a classic tripartite model, the article takes a long-term view that sees life-crisis rituals as a form of collective memory that bears witness to social tensions that cannot be resolved – in this case the contradictions inherent within the hallowed image of the mother and the compromised nature of parental love