1,794 research outputs found
Collective motion of organisms in three dimensions
We study a model of flocking in order to describe the transitions during the
collective motion of organisms in three dimensions (e.g., birds). In this model
the particles representing the organisms are self-propelled, i.e., they move
with the same absolute velocity. In addition, the particles locally interact by
choosing at each time step the average direction of motion of their neighbors
and the effects of fluctuations are taken into account as well. We present the
first results for large scale flocking in the presence of noise in three
dimensions. We show that depending on the control parameters both disordered
and long-range ordered phases can be observed. The corresponding phase diagram
has a number of features which are qualitatively different from those typical
for the analogous equilibrium models.Comment: 3 pages, 4 figure
Complexity: The bigger picture
If a concept is not well defined, there are grounds for its abuse. This is
particularly true of complexity, an inherently interdisciplinary concept that
has penetrated very different fields of intellectual activity from physics to
linguistics, but with no underlying, unified theory. Complexity has become a
popular buzzword used in the hope of gaining attention or funding -- institutes
and research networks associated with complex systems grow like mushrooms. Why
and how did it happen that this vague notion has become a central motif in
modern science? Is it only a fashion, a kind of sociological phenomenon, or is
it a sign of a changing paradigm of our perception of the laws of nature and of
the approaches required to understand them? Because virtually every real system
is inherently extremely complicated, to say that a system is complex is almost
an empty statement - couldn't an Institute of Complex Systems just as well be
called an Institute for Almost Everything? Despite these valid concerns, the
world is indeed made of many highly interconnected parts over many scales,
whose interactions result in a complex behaviour needing separate
interpretation for each level. This realization forces us to appreciate that
new features emerge as one goes from one scale to another, so it follows that
the science of complexity is about revealing the principles governing the ways
by which these new properties appear.Comment: Concepts essay, published in Nature
http://www.nature.com/nature/journal/v418/n6894/full/418131a.htm
A question of scale
If you search for 'collective behaviour' with your web browser most of the
texts popping up will be about group activities of humans, including riots,
fashion and mass panic. Nevertheless, collective behaviour is also considered
to be an important aspect of observed phenomena in atoms and molecules, for
example, during spontaneous magnetization. In your web search, you might also
find articles on collectively migrating bacteria, insects or birds; or
phenomena where groups of organisms or non- living objects synchronize their
signals or motion (think of fireflies flashing in unison or people clapping in
phase during rhythmic applause).Comment: Concepts essay, published in Nature
http://www.nature.com/nature/journal/v411/n6836/full/411421a0.htm
The kinesin walk: a dynamic model with elastically coupled heads
Recently individual two-headed kinesin molecules have been studied in in
vitro motility assays revealing a number of their peculiar transport
properties. In this paper we propose a simple and robust model for the kinesin
stepping process with elastically coupled Brownian heads showing all of these
properties. The analytic and numerical treatment of our model results in a very
good fit to the experimental data and practically has no free parameters.
Changing the values of the parameters in the restricted range allowed by the
related experimental estimates has almost no effect on the shape of the curves
and results mainly in a variation of the zero load velocity which can be
directly fitted to the measured data. In addition, the model is consistent with
the measured pathway of the kinesin ATPase.Comment: 6 pages, 3 figure
Controlling edge dynamics in complex networks
The interaction of distinct units in physical, social, biological and
technological systems naturally gives rise to complex network structures.
Networks have constantly been in the focus of research for the last decade,
with considerable advances in the description of their structural and dynamical
properties. However, much less effort has been devoted to studying the
controllability of the dynamics taking place on them. Here we introduce and
evaluate a dynamical process defined on the edges of a network, and demonstrate
that the controllability properties of this process significantly differ from
simple nodal dynamics. Evaluation of real-world networks indicates that most of
them are more controllable than their randomized counterparts. We also find
that transcriptional regulatory networks are particularly easy to control.
Analytic calculations show that networks with scale-free degree distributions
have better controllability properties than uncorrelated networks, and
positively correlated in- and out-degrees enhance the controllability of the
proposed dynamics.Comment: Preprint. 24 pages, 4 figures, 2 tables. Source code available at
http://github.com/ntamas/netctr
Collective motion of cells: from experiments to models
Swarming or collective motion of living entities is one of the most common
and spectacular manifestations of living systems having been extensively
studied in recent years. A number of general principles have been established.
The interactions at the level of cells are quite different from those among
individual animals therefore the study of collective motion of cells is likely
to reveal some specific important features which are overviewed in this paper.
In addition to presenting the most appealing results from the quickly growing
related literature we also deliver a critical discussion of the emerging
picture and summarize our present understanding of collective motion at the
cellular level. Collective motion of cells plays an essential role in a number
of experimental and real-life situations. In most cases the coordinated motion
is a helpful aspect of the given phenomenon and results in making a related
process more efficient (e.g., embryogenesis or wound healing), while in the
case of tumor cell invasion it appears to speed up the progression of the
disease. In these mechanisms cells both have to be motile and adhere to one
another, the adherence feature being the most specific to this sort of
collective behavior. One of the central aims of this review is both presenting
the related experimental observations and treating them in the light of a few
basic computational models so as to make an interpretation of the phenomena at
a quantitative level as well.Comment: 24 pages, 25 figures, 13 reference video link
Hierarchical self-organization of non-cooperating individuals
Hierarchy is one of the most conspicuous features of numerous natural,
technological and social systems. The underlying structures are typically
complex and their most relevant organizational principle is the ordering of the
ties among the units they are made of according to a network displaying
hierarchical features. In spite of the abundant presence of hierarchy no
quantitative theoretical interpretation of the origins of a multi-level,
knowledge-based social network exists. Here we introduce an approach which is
capable of reproducing the emergence of a multi-levelled network structure
based on the plausible assumption that the individuals (representing the nodes
of the network) can make the right estimate about the state of their changing
environment to a varying degree. Our model accounts for a fundamental feature
of knowledge-based organizations: the less capable individuals tend to follow
those who are better at solving the problems they all face. We find that
relatively simple rules lead to hierarchical self-organization and the specific
structures we obtain possess the two, perhaps most important features of
complex systems: a simultaneous presence of adaptability and stability. In
addition, the performance (success score) of the emerging networks is
significantly higher than the average expected score of the individuals without
letting them copy the decisions of the others. The results of our calculations
are in agreement with a related experiment and can be useful from the point of
designing the optimal conditions for constructing a given complex social
structure as well as understanding the hierarchical organization of such
biological structures of major importance as the regulatory pathways or the
dynamics of neural networks.Comment: Supplementary videos are to be found at
http://hal.elte.hu/~nepusz/research/supplementary/hierarchy
- …