Speaker
Alvaro Corral

Description Power-law distributions have been a paradigm of complex systems for several decades. But it has been only recently that attention has been devoted to the proper fitting and goodness-of-fit testing of these distributions; in particular, Clauset et al.’s method is widely cited in this regard [1]. In the first part of the talk we present evidence that this method fails in some concrete examples of continuous distributions, rejecting the power-law hypothesis even for simulated data with a power-law tail [2]. We propose an alternative more reliable procedure that, additionally, can be generalized to upper truncated power-law distributions [3]. The second part is devoted to one of the classic examples of power-law distributions: the so-called Zipf’s law, in the context of word frequencies in texts. This is considered one of the key statistical regularities of human language. We extend the fitting method to cover this discrete case, showing that, in general, Zipf’s law does not hold for the whole domain of word frequencies, but only for the upper tail. In addition, we show how the distribution of word frequencies scales with the size of the text and the size of the vocabulary, providing a recipe for the proper comparison of texts of different size [4]. The distinction between power law and scaling law is fundamental here. The third and last part is devoted to the extension of Zipf’s law to music, drawing parallelisms and differences with texts. The construction of music code-words from the chords defining the pitch in modern popular music reveals the validity of Zipf’s law in this case. This law has kept stability for the last 50 years, although other characteristics of the musical complex-network have shown an evolution that seems to indicate a decrease of the complexity of music with time [5]. References [1] A. Clauset, C. R. Shalizi, and M. E. J. Newman (2009) Power-law distributions in empirical data. SIAM Rev., 51, 661–703. [2] A. Corral, F. Font, and J. Camacho (2011) Non-characteristic half-lives in radioactive decay. Phys. Rev. E, 83, 066103. [3] A. Deluca and A. Corral (2013) Fitting and goodness-of-fit test of non-truncated and truncated power-law distributions. Acta Geophys., 61, 1351–1394. [4] F. Font-Clos, G. Boleda, and A. Corral (2013) A scaling law beyond Zipf ’s law and its relation to Heaps’ law. New J. Phys., 15, 093033. [5] J. Serra, A. Corral, M. Boguna, M. Haro, and J. Ll. Arcos (2012) Measuring the evolution of contemporary western popular music. Sci. Rep., 2, 521

Speaker
Édgar Roldán

Description A symmetry breaking (SB) involves an abrupt change in the set of microstates that a system can explore. This change has unavoidable thermodynamic implications. According to Boltzmann's microscopic interpretation of entropy, a shrinkage of the set of compatible states implies a decrease of entropy, which eventually needs to be compensated by dissipation of heat and consequently requires work. Examples are the compression of a gas and the erasure of information. On the other hand, in a spontaneous SB, the available phase space volume changes without the need for work, yielding an apparent decrease of entropy. Here we show that this decrease of entropy is a key ingredient in the Szilard engine and Landauer's principle and report on a direct measurement of the entropy change along SB transitions in a Brownian particle. The SB is induced by a bistable potential created with two optical traps. edThe experiment confirms theoretical results based on fluctuation theorems, allows us to reproduce the Szilard engine extracting energy from a single thermal bath, and shows that the signature of a SB in the energetics is measurable, providing new methods to detect, for example, the coexistence of metastable states in macromolecules.

Speaker
Jan O. Haerter

Description An important property of communication in professional environments are time constraints. Time constraints force a prioritization of tasks and necessitate particularly tough decisions when one is confronted with the choice of either helping others complete their tasks or completing one’s own. Each subject in “The Expert Game” has one task and one expertise. The goal for each subject is to complete his task by receiving help from the expert matching this task. The only means of communication is through minimal standardized electronic messages. The game mimics electronic communication and associated information spreading under time-limitations, by allowing each subject to send a limited number of messages. Messages are either inquiries or responses, never both. The time-constraint forces the players to choose between either helping others, which consumes valuable time, or focusing on their own task. Our experiment, carried out with 16 random subjects, shows, that even in this completely impersonal environment, with emotion-free messages and anonymized subject identities, iterations of the game with randomly re-assigned expertise and tasks lead to the emergence of a trust network. Our experiment demonstrates that collaboration and trust can evolve in a community, even in the absence of emotional content of communication, face-to-face encounters or opinion alignment. Neither is policing or indirect reciprocity a requirement for cooperation. In our experiment cooperation emerges based on the long term expectation of mutual benefit in a trust based relationship in the absence of a direct incentive to cooperate.

Speaker
Anke Lindner

Description The flow of complex suspensions is ubiquitous in nature and industrial applications. Their non-Newtonian character is due to flow-induced orientation, rearrangement, or deformation of microscopic objects suspended in simple fluids. Linking the microstructure on the particle level to the macroscopic response under flow is one of the fundamental scientific challenges of soft matter physics. Here, we present two examples of flows of complex suspensions in chosen microfluidic geometries which allow this link to be established. First, we use a solution of flexible polymers, where normal stresses are known to arise when the polymers undergo a coil-stretch transition under flow and we characterize the onset of elastic flow instability in a serpentine channel as a function of its curvature. The calibrated serpentine channel can then be used as a sensitive rheometer to detect even small normal stresses in unknown suspensions. Second, we employ a Y-channel, a powerful rheometer for measuring shear viscosities, to study the viscosity of active suspensions of e-coli bacteria. In this way we link the activity of the bacteria to the measured non-Newtonian effective viscosity for the first time.

Speaker
Marta Ibañes

Description During embryonic development equivalent cells self-organize and form ordered and robust patterns of gene expression. One example is the periodic pattern of hair cells surrounded by supporting cells that arises during vertebrate inner ear development. A minimal gene regulatory circuit that involves nonlinear dynamics has been proposed to drive this patterning process. Yet, experimental data indicate that another gene regulatory circuit, with an antagonistic function, participates as well. In this talk we will address how the periodic pattern can arise from these two antagonistic gene regulatory circuits. Our theoretical results show that patterning is the result of cooperation between the antagonistic circuits and that this cooperation stems from competition for common resources. We support these conclusions with experimental data.