WEISBUCH Gérard
Laboratoire de Physique Statistique de
l'ENS
Département de Physique - Ecole Normale Supérieure
24, rue Lhomond
75230 Paris Cedex 05 - France
Office: Dc16
For info regarding books, conferences and other activities please
refer
to reseaux complexes
homepage. To access a selection of
notes
and transparencies. To access
CV
and
publications
.
Research interests: Complex Systems Dynamics.
Living systems or social systems can be viewed as complex
adaptive systems: They are composed of a large number of elements
with
intricate interactions which are not identical between several
instanciations
of the system (say between individual organisms of the same species for
instance). Still, a number of their properties remain identical between
individuals or equivalent systems. Concepts from statistical physics
and
non-linear dynamics accounts for these properties. The basic assumption
of our approach is the interpretation of organization in real complex
systems
in terms of the attractors of the dynamics of a formal model system.
Bounded Rationality and Socio-Economic Institutions
Classical economics, especially micro-economics, is based on the
hypothesis
of agents full rationality and does try to describe organisation
phenomena
which structure agents interactions. By contrast, social sciences, and
some economists try to characterise institutions in connection with
cooperation
and risk aversion issues. The following
note describes an interpretation of institutions in terms of
dynamical
attractors of social interactions: this is the framework of our
work in economics and environmental sciences.
Market organization
In some sense standard economical theory ignores dynamics and is mostly
based on equilibrium. How prices are adjusted or how agents decide to
have
transactions are supposed to be the output of some mysterious and fast
process, refered to as "Walrasian tatonnement". In fact real markets,
wether
financial markets or everyday life markets where ordinary goods are
exchanged
display a lot of organisation in relation to information
available
to the agents. We worked on market of perishable goods when only private
information is available (PostScript
file).
The case when information is exchanged
among agents is more classical among economics
studying herd behaviour: we studied several problems associated which
this case, including new technologies adoption.
Information Contagion
The simplest models of cultural diffusions, such as new technology
adoption
or commercial products are inspired from epidemiology. Diffusion is
based
on a contagion process occuring on the occasion of random encounters.
Random
encounters result in an S shaped dynamics for the adoption rate. Things
get more intricate when one takes into account the existence of social
networks which support influence propagation and the fact that some
innovation
does not benefit equally to all agents. For binary choices, the
dynamics
is described by the use of inhomogeneous automata
networks. When only one neighbour is enough to transmit
information,
percolation phenomena control the dynamics.
When
economic adjustments are introduced, self-organised
criticality and non gaussian fluctuations are
observed.
In the case of non-binary opinions, either
scalar
or vectors, one observes social segregation when information contagion
is restricted to close enough opinions.
Sustainable development.
Sustainable
development
is a field of research concerning the possible transitions from a
consumption
society to types of societies which do not evolve towards the
destruction
of their own environment by depleting its resources in the case of
fisheries
for instance or increasing its
pollution.
Our group participates to a related EEC funded collaboration on
extensive
agriculture (project IMAGES).
A first manuscript about the dynamics of agri-environmental contracts
adoption
by farmers is
available.
Theoretical Immunology
Jerne networks.
The basic assumption of Jerne networks is that lymphocytes and
immunoglobulines interact via the same mechanisms, namely molecular
recognition,
as they interact with antigens. The immune response to antigen
presentation
can then be interpreted as an attractor in the dynamics of a network of
a large number of connected entities. The analogy with memories and
neural
nets brings to the paradigm of an immune network proposed by N.Jerne in
the early seventies. (Immunology
for physicists)
Mutualism.
Mutualistic systems are known to occur in nature , e.g. lichens made of
algae and fungus, corals made of polypes and algae, insects and
bacteria,
rumen and bacteria, legumes and bacteria... Exchanges among partners
are
beneficial for the species involved, which live together in close
association.
The benefits can be food, energy, protection and transport
(pollination).
The emergence and stability of mutualism constitute a paradox in
terms
of individual selection of the fittest: since giving food to the other
symbiont should be costly for the donor, one expects the donor to be
disadvantaged
in terms of fitness with respect to a more selfish species, which would
not give anything back. We contributed a couple of papers to the
hydra-algae
symbiosis to study the role of molecular recognition in favoring the
emergence
of mutualism.
weisbuch@lps.ens.fr