Hanna Salman, University of Pittsburgh
Title: Collective bacterial navigation through obstacle-laden terrains
Abstract: The mechanisms of bacterial chemotaxis have been extensively studied, but how the physical environment influences the collective migration of bacterial cells remains less understood. Based on previous models of bacterial chemotaxis the migrating of bacteria across obstacle-laden terrains should slow down as the obstacle coverage increases. Experimentally, however, we find that the size or density of obstacles do not alter the average migration rate of Escherichia coli cells following an external attractant. We show, both by analyzing a revised theoretical model and by experimentally following cell populations and single-cells, that the accelerated migration in the presence of obstacles is a consequence of two effects: 1) cell-cell communication in response to the presence of an external gradient, and 2) reduced tumbling frequency that is adjusted by the E. coli cells in response to the topology of their environment.
Roi Holtzman, WIS
Title: Acceleration from a clustering environment
Abstract: The dynamics of a system coupled to an environment are strongly influenced by the correlations within that environment. We investigate this effect in a one-dimensional random walker interacting with an environment governed by Ising model statistics. Our results show that the walker’s asymptotic speed is nontrivially affected by the environment’s temperature and magnetization. Notably, when the environment undergoes slow cooling and clustering increases, the walker’s speed exhibits complex behaviors, including threshold effects and non-monotonic dependencies on environmental parameters. These findings highlight the crucial role of environmental correlations in transport phenomena and demonstrate that a homogenized (mean-field) description of the environment fails to capture these effects.
Amir Bashan, BIU
Title: Complexity–stability trade-off in empirical microbial ecosystems
Abstract: May’s stability theory, which holds that large ecosystems can be stable up to a critical level of complexity, a product of the number of resident species and the intensity of their interactions, has been a central paradigm in theoretical ecology. So far, however, empirically demonstrating this theory in real ecological systems has been a long-standing challenge with inconsistent results. Especially, it is unknown whether this theory is pertinent in the rich and complex communities of natural microbiomes, mainly due to the challenge of reliably reconstructing such large ecological interaction networks. Here we introduce a computational framework for estimating an ecosystem’s complexity without relying on a priori knowledge of its underlying interaction network. By applying this method to human-associated microbial communities from different body sites and sponge-associated microbial communities from different geographical locations, we found that in both cases the communities display a pronounced trade-off between the number of species and their effective connectance. These results suggest that natural microbiomes are shaped by stability constraints, which limit their complexity.
Shlomi Reuveni, TAU
Title: Sokoban: a model for percolation with obstacle-pushing.
Abstract: Alfréd Rényi once humorously remarked that a mathematician functions as a contraption designed to convert coffee into theorems. Though the precise ritual of brewing one's morning cup may vary, the essence lies in the process of extraction. This requires water to find its way through the ground coffee beans and into the cup. However, can water do so effectively? And is there always a path that will allow them to percolate from top to bottom? Percolation theory emerged as an attempt to tackle this and similar questions mathematically. It can thus be ironically described as a concerted effort to convert coffee into theorems about coffee.
Observe an espresso puck before (left) and after (right) the extraction process. As hot pressurized water is pushed through the puck, it does not leave the original arrangements of coffee grains intact. Rather, it paves its way through the grains, displacing them from their original positions (see the holes formed in the used puck on the right). However, canonical models for percolation assume that obstacles (coffee grains) are static, completely ignoring any interaction between them and the percolating liquid.
In this talk, I will introduce Sokoban percolation, a model that challenges the conventional assumptions of percolation theory by allowing for weak and localized obstacle-pushing (see figure). By exploring how such interactions affect transport across different network topologies, e.g., the Bethe lattice and the 2D lattice, I will show that they have a profound effect that cannot be neglected when coming to understand spreading phenomena in disordered media.
Daniel Podolsky, Technion
Title: Lattice melting in two-dimensional antiferromagnets
Abstract: In this talk, I will discuss the role of antiferromagnetic interactions in two-dimensional crystal melting. For strong enough magnetic interactions, fundamental lattice dislocations become prohibitive due to magnetic frustration. This gives rise to a novel tetratic phase that is seemingly antiferromagnetically ordered. This phase will be demonstrated numerically in an experimentally realizable system of colloids confined between parallel plates. Although the antiferromagnetic tetratic is not a distinct thermodynamic phase, there is nonetheless a "computational phase transition" separating the antiferromagnetic tetratic from a regular tetratic.
Doron Cohen, BGU
Title: Broken quantum classical correspondence in quasi-static protocols.
Abstract: Quantum mechanically, a driving process is expected to be reversible in the quasistatic limit, aka adiabatic theorem. This statement stands in opposition to classical mechanics, where mix of regular and chaotic dynamics implies irreversibility. We discuss a prototype protocol where this observation has a practical implications: many-body adiabatic passage along Bose-Hubbard chains [1]. The conditions for breakdown of quantum-to-classical correspondence are highlighted.
[1] A.V. Varma, A. Vardi, D. Cohen, Phys Rev Lett (2025).
David Gelbwaser, Technion
Title: Thermalization of open quantum systems that violate detailed balance
Abstract: Open quantum systems that comply with detailed balance decay in a non-oscillatory manner to thermal equilibrium. Beyond the weak coupling limit, systems that break microreversibility (e.g., in the presence of magnetic fields) violate detailed balance but still thermalize. We study the thermalization of these systems and show that a temperature rise produces novel exceptional points that indicate a sharp transition in the thermalization dynamics. A further temperature increase fuels oscillations of the energy level populations even without quantum coherences. Moreover, the violation of detailed balance introduces an energy scale that characterizes the oscillatory regime at high temperatures.
Yevgeny Bar-Lev, BGU
Title: Absence of localization in interacting spin chains with a discrete symmetry
Abstract: In this talk, I will present a proof of delocalization in spin chains symmetric under a combination of mirror and spin-flip symmetries and with a nondegenerate spectrum. The proof applies to two prominent examples: the Stark many-body localization system (Stark-MBL) and the symmetrized many-body localization system (symmetrized–MBL). I will also provide numerical evidence of delocalization at all energy densities in these models and show that the delocalization mechanism appears robust to weak symmetry breaking.
Efi Efrati, WIS
Title: Cumulative geometric frustration in materials with discrete degrees of freedom
Abstract: Geometric frustration arises whenever the constituents in a material are associated with mutually contradicting local tendencies that could not be simultaneously realized in the bulk.
Geometric frustration is said to be cumulative if the compromise the system finds to resolve the frustration is non-uniform and cooperative. This is, however, not always the case. In many natural systems the frustration is locally resolved. Frustrated spin systems also tend to not display cumulative frustration. I will present a spin system capable of displaying cumulative frustration. I will then examine its discrete analog and discuss how discrete degrees of freedom modify the manifestations of the geometric frustration.
Michal Moshe, HUJI
Title: Analytic Framework for Predicting Curved Crack Trajectories.
Abstract:. TBA
Noam Levi, EPFL
Title: Unraveling Compositionality in Data Through the Lens of Generative Models
Abstract: In this talk, I will discuss how fundamental properties of high-dimensional natural data - its hierarchical and compositional structure - can be quantitatively measured and understood using pre-trained generative machine learning models. While the hierarchical nature of real world data has long been hypothesized as key to its learnability by neural networks, direct evidence has remained elusive. I will first motivate compositionality as an underlying principle in data structure, and introduce the concept of generative diffusion models. I will then explain how forward-backward denoising experiments reveal a surprisingly universal pattern: changes in data occur in correlated chunks, with a characteristic length scale that diverges at a critical noise level. This observation holds across different data modalities, from text to images, suggesting a unique statistical signature of hierarchical structure that persists regardless of the specific data generation process.
Furthermore, I will demonstrate how this finding provides a new window into understanding the organization of natural data purely from its statistical properties, by appearing to a simplified model of hierarchical data generation. This approach allows us to bridge the gap between theoretical predictions in simple hierarchical models and empirical observations in complex real-world datasets. Finally, I will discuss the potential implications of these results for our understanding of natural language and how its hidden structure may manifest in measurable statistical patterns.
Shmuel Rubinstein, HUJI
Title: The Physics of Fracking Hydrogels: Ballistic Annihilation and the Rules of Roughness
Abstract: Fracture roughness increases the surface area and is an important component of the energy budget required to advance a crack. Indeed, most fractured surfaces are rough. Rough fracture surfaces are the remnant of the tortuous path taken by the singular crack line, distorted by dynamic instabilities or heterogeneities. Despite its importance, the connection between material heterogeneity and roughness is still not well known. Here we demonstrate that fracture roughness and heterogeneity are indeed inextricably linked through the production of step lines, a prominent constituent of the roughness present in brittle fractures, which arise from mixed (I+III) mode loading. We show that step lines are fundamental building blocks of large-scale roughness in brittle fractures. Studying the three-dimensional morphology of steps and their interactions in brittle hydrogels using confocal microscopy, the rules that govern step formation and interactions, and thus, roughness formation are uncovered. Steps nucleate at a constant rate during crack propagation and are annihilated through step interactions. The annihilation rate grows with step density, eventually leading to a steady state number of steps. We propose a simple 1D, modified ballistic annihilation model which qualitatively captures the evolution of surface roughness
Eran Sharon, HUJI
Title: Confirming the Prediction of Wave-Turbulence Theory in Weak Rotating Turbulence
Abstract: Rotating turbulent flows are common in geophysical systems and are not fully understood. The linear modes of rotating flows are Coriolis-driven helical inertial waves. The existence of these modes in turbulent rotating flows was documented in experiments and simulations. The possibility that rotating turbulence can be described as a wave turbulence of inertial waves has been studied for several decades. More than 20 years ago, Galtier derived explicit predictions for the weak inertial wave turbulence spectrum. However, this spectrum has not been detected and its relevance to real physical flows was questionable.
I will present a set of experiments in turbulence within a rapidly rotating water tank. We separate the velocity field into is 2D and 3D components and show that the 3D part consists of inertial waves. We then analyze the statistics of the 3D field, confirming Galtier’s predictions, for the full energy spectrum.
Based on these observations we suggest a co-existence of a 3D wave turbulence field and a quasi 2D field. The relations between these two components are yet to be studied.
