The department hosts a series of weekly Physics Colloquia during the semester, which covers a wide range of topics, and takes place on Wednesdays at 14:00 (IST), nowadays on zoom. For additional details please contact Joshua Feinberg.

The Physics Department Colloquium: March 29, 2023, 2pm IDT

Boris Shapiro (Technion) | Fluctuation-induced forces: from van Der-Waals to quantum friction | Abstract: All bodies are surrounded by a fluctuating electromagnetic field, due to the random motion of charges inside a body. These fluctuating fields give rise to forces between nearby bodies. Such fluctuation-induced forces go by the names of van der-Waals, Casimir or Lifshitz, depending on the circumstances. The purpose of this talk is to review the subject, with emphasis on the recent developments related to systems out of equilibrium. Slides When: March 29, 2023 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: March 22, 2023, 2pm IDT

Eran Sharon (Hebrew University of Jerusalem) | Self-Morphing of frustrated sheets – from the lab to the real world and from statics to locomotion | Abstract: Humanity spends huge amounts of time, energy and resources in the shaping of solids into desired three-dimensional shapes. Unlike (nearly) all manmade structures, which are shaped by external constraints, most natural structures shape themselves via the distribution of non-uniform active growth. Apparently, this mode of shaping can be implemented with synthetic solid structures. Solids are not necessarily passive, they can be “programmed” to shape themselves upon induction. I will briefly present the principles and status of the field of “Self-Morphing”. Then I will focus on two different projects. In the first we attempt to “export” self-morphing from the scientific community and lab scale, to the real world of architecture and design. In the second project we study the locomotion of active gel sheets on a curved fluid interface. The sheets metabolize chemical energy (“food”) from their surroundings and convert it into periodic change of their curvature and to their periodic motion along the fluid interface. We suggest that the difference between the curvature of the sheet and that of the substrate leads to forces and torques on the sheet and to its locomotion. This mechanism is likely to be relevant to the motion of cells on curved surfaces (curvotaxis) When: March 22, 2023 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: March 15, 2023, 2pm IDT

Haim Diamant (Tel Aviv University) | Using entropy to study systems out of thermal equilibrium | Abstract: Thermodynamic variables such as temperature and pressure are ill-defined out of thermal equilibrium. However, the relation between entropy and the information contained in the statistical distribution of the system’s microstates is assumed to hold regardless of whether or not the system is at equilibrium. Therefore, entropy should be a useful global property for characterizing non-equilibrium behaviors. We have obtained, based on first principles, a universal inequality relating the entropy of a system at steady state and the diffusion coefficient of its constituents. The relation can be used to obtain useful bounds for the diffusion coefficient (normal or anomalous) from the calculated thermodynamic entropy or, conversely, to estimate the entropy based on measured diffusion coefficients. We demonstrate the validity and applicability of the relation in several examples. We have derived a functional which takes as input measurable pair-correlations (such as the structure factor) and gives a useful upper bound for the entropy. We use it to pin-point and characterize dynamic transitions in several experimental and computational systems, including driven and active particles. When: March 15, 2023 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: March 8, 2023, 2pm IDT

Yuval Garini (Technion) | The multi-scale structure of chromatin in the nucleus | Abstract: The DNA in a human cell which is ~2 meters long is packed in a ~10 μm radius nucleus. It is immersed in a condensed soup of proteins, RNA and enzymes and it is highly dynamic, while it must stay organized to prevent chromosome entanglement and for ensuring proper genome expression. Studying this nanometer – micrometer scale structure requires to use both high spatial and temporal resolutions and we combine comprehensive live-cell and molecular methods. Diffusion-based analysis allowed us to identify complex normal and sub-diffusion modes. The results allowed us to identify lamin A as the main player in the chromatin organization mechanism. It forms chromatin loops in the whole nuclear volume thereby restricting the chromatin dynamics increasing its elasticity and rigidity. Together with other mechanisms, it takes part in controlling gene expression and provides the nucleus its rigidity that is important even for preventing cell metastasis. When: March 8, 2023 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: January 11, 2023, 2pm IDT

Barak Kol (the Hebrew University of Jerusalem) | The flux-based statistical theory for the three- body problem | Abstract: The Newtonian three-body problem is one of the richest, deepest and longest-standing open problems in physics. It is the fertile soil that brought about the paradigm change from the clockwork universe to chaos, and it grew numerous scientific theories including perturbation theory, the symplectic formulation of mechanics, and the mathematical field of topology. The generic, non-hierarchical, three-body system is known to be chaotic. In fact, it is so chaotic that one expects that a statistical solution is the optimal solution. Yet, despite considerable progress, all extant statistical approaches displayed two flaws. First, probability was equated with phase space volume, thereby ignoring the fact that significant regions of phase space describe regular motion, including post-decay motion. Secondly and relatedly, an adjustable parameter, the strong interaction region, which is a sort of cutoff, was a central ingredient of the theory. The talk will describe a theory that is based on phase-space flux, rather than phase-space volume, which remedies these flaws. Statistical predictions for the identity of the escaper, and other measurable quantities, will be shown to agree with computerized simulations considerably better than previous theories. Moreover, the flux-based theory enables to predict the distribution of decay times. This prediction relies on the definition and determination of a regularized phase-volume for the system, and the latter led us to a second aspect of the problem, namely a decomposed formulation of it. Basically, this decomposition separates the motion of the instantanoues plane defined by the three bodies, from the motion of the bodies within the plane. Recording Slides Youtube movie of a 3 body movement Based on: Barak Kol, “Flux-based statistical prediction of three-body outcomes”, Celest. Mech. Dyn. Astron. 133 17 (2021). Viraj Manwadkar, Barak Kol,

The Physics Department Colloquium: January 4, 2023, 2pm IDT

Matan Mussel (University of Haifa) | On spikes and sound: debating the physical nature of action potentials | Abstract: Excitable cells generate a characteristic transient change in transmembrane potential that propagates along the cell membrane in response to suitable stimuli. These signals are called action potentials (APs), and are principally associated with behavioral activities of many organisms. Thus, an understanding of the mechanism of APs, as well as their actions and interactions, constitutes one of the fundamental aspects of biology. The mechanism that underlies an AP is widely considered to be electrical, and is typically interpreted through a representation of the cell membrane as an equivalent electric circuit. However, the theory relies on phenomenological equations that require many fit parameters. In addition, several experimental facts are neither readily explained nor predicted by the electrical theory. On the other hand, solitary sound waves that propagate within lipid monolayers show remarkable similarities to APs and the theory of sound can bridge some of the gaps while using zero fit parameters. An important prediction of this approach is that electricity is merely one aspect of the signal, and it is, therefore, possible that valuable information is overlooked. The talk will conclude by proposing falsifiable predictions and discussing experimental and theoretical challenges. Recording Slides When: January 4, 2023 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: December 28, 2022, 2pm IDT

Erez Zohar (the Hebrew University of Jerusalem) | Quantum Information and Technology Methods for Lattice Gauge Theories | Abstract: While the road to a full scale universal quantum computer is still long, in the last decades an enormous progress has been made along it. In particular, new techniques for complex many-body quantum systems based on concepts from quantum technology and information are currently available, allowing one to study non-perturbative, strongly correlated physics, using current technologies. In my talk I will focus on two such approaches – quantum simulation and tensor network states. The first suggests building non-universal quantum devices which mimic the dynamics of quantum models, and the latter to use the power of entanglement to significantly simplify the study of quantum models on classical computers. In particular, I will demonstrate how these tools can be applied for studying lattice gauge theories, aiming (in the long term) at open questions in particle physics. Recording Slides When: December 28, 2022 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: December 14, 2022, 2pm IDT

Doron Cohen (Ben-Gurion University of the Negev) | Quantum Irreversibility of Quasistatic Protocols | Abstract: In quantum mechanics, a driving process is expected to be reversible in the quasistatic limit, aka adiabatic theorem. This statement stands in opposition to classical mechanics, where mixed chaotic dynamics implies irreversibility in this limit. A paradigm for demonstrating the signatures of chaos in quantum irreversibility, is a sweep protocol whose objective is to transfer condensed bosons from a source orbital. As the sweep rate is lowered, a crossover to a chaos-assisted-depletion regime that is dominated by universal quantum fluctuations is encountered, featuring a breakdown of quantum-to-classical correspondence. Recording Slides When: December 14, 2022 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: November 28, 2022, 3pm IDT

Dan Gorbonos (Max Planck Institute for Animal Behavior) | Mean Field trajectories in a spin model for decision making on the move | Abstract: How animals navigate and perform directional decision making while migrating and foraging, is an open puzzle. We have recently proposed a spin-based model for this process, where each optional target that is presented to the animal is represented by a group of Ising spins, that have all-to-all connectivity, with ferromagnetic intra-group interactions. The inter-group interactions are in the form of a vector dot product, depending on the instantaneous relative, and deformed, angle between the targets. The deformation of the angle in these interactions enhances the effective angular differences for small angles, as was found by fitting data from several animal species. We expose here the rich variety of trajectories predicted by the mean-field solutions of the model, for systems of three and four targets. We find that depending on the arrangement of the targets the trajectories may have an infinite series of self-similar bifurcations, or have a space-filling property. The bifurcations along the trajectories occur on “bifurcation curves”, that determine the overall nature of the trajectories. The angular deformation that was found to fit experimental data, is shown to greatly simplify the trajectories. This work demonstrates the rich space of trajectories that emerge from the model. Slides When: November 28, 2022 3:00 PM (Israel Standard Time). Where: Meeting room in the Dean’s office complex, 2nd floor, the Multipurpose Building Note the unusual room for this talk!

The Physics Department Colloquium: November 30, 2022, 2pm IDT

Alexander Migdal (New York University Abu Dhabi) | Quantum Geometry of Classical Turbulence | Abstract: We argue that in the strong turbulence phase, as opposed to the weak one, the Clebsch variables compactify to the sphere S2 and are not observable as wave excitations. Various topologically nontrivial configurations of this confined Clebsch field are responsible for vortex sheets. Stability equations (CVS) for closed vortex surfaces (bubbles of Clebschfield) are derived and investigated. The exact non-compact solution for the stable vortex sheet family is presented. Compact solutions are proven not to exist by De Lellis and Bru. Asymptotic conservation of anomalous dissipation on stable vortex surfaces in the turbulent limit is discovered. We derive an exact formula for this anomalous dissipation as a surface integral of the square of velocity gap times the square root of minus local normal strain. Topologically stable time-dependent solutions, which we call Kelvinons, are introduced. They have a conserved velocity circulation around stationary loop; this makes them responsible for asymptotic PDF tails of velocity circulation, perfectly matching numerical simulations. The loop equation for circulation PDF as functional of the loop shape is derived and studied. This equation is exactly equivalent to the Schrödinger equation in loop space, with viscosity ν playing the role of Planck’s constant. This equivalence opens the way for direct numerical simulation of turbulence on quantum computers. Kelvinons are fixed points of the loop equation at WKB limit ν→ 0. Area law and the asymptotic scaling law for mean circulation at a large area are derived. The representation of the solution of the loop equation in terms of a singular stochastic equation for momentum loop trajectory is presented.  Recording Slides Review on Turbulence as Clebsch Confinement When: November 30, 2022 2:00 PM (Israel Standard Time). Where:  Over Zoom

The Physics Department Colloquium: November 16, 2022, 2pm IDT

Eugene Kanzieper (Holon Institute of Technology) | Random Matrix Theory of the Power Spectrum in Quantum Chaotic Systems | Abstract: The power spectrum analysis of stochastic spectra has emerged as a powerful tool for studying both system-specific and universal properties of complex systems. In the context of complex quantum systems, it reveals whether the corresponding classical dynamics is regular or chaotic, or a mixture of both, and encodes a ‘degree of chaoticity’. In combination with other long- and short-range spectral fluctuation measures, it provides an effective way to identify system symmetries, determine a degree of incompleteness of experimentally measured spectra, and get the clues about systems’ internal structure. In this talk, I shall formulate a random-matrix-theory approach to the power spectrum of energy level fluctuations in fully chaotic quantum structures. In the particular case of broken time-reversal symmetry, our theory produces a parameter-free prediction for the power spectrum expressed– in the domain of its universality – in terms of a fifth Painlevé transcendent. Finally, I shall present fair evidence that a universal Painlevé V curve can be observed in the power spectrum of nontrivial zeros of the Riemann zeta function. Deviations from universality will also be briefly discussed. Slides When: November 16, 2022 2:00 PM (Israel Standard Time). Where: Room 108, Multipurpose Bldg. & over Zoom Note the unusual room for this talk!

The Physics Department Colloquium: November 9, 2022, 2pm IDT

Yossi Nir (Weizmann Institute of Science) | The Jewels in the Crown of the LHC | Abstract: The ATLAS and CMS experiments have made several major discoveries: The discovery of an elementary spin-zero particle, the discovery of the mechanism that makes the weak interactions short-range, the discovery of the mechanism that gives the third generation fermions their masses, and more. I explain how this progress in our understanding of the basic laws of Nature was achieved. Slides When: November 9, 2022 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: Fall 2022 Schedule

Fall 2022 Colloquium Schedule  | 2.11.2022 Dr. Eva Maria Graefe, Department of Mathematics, Imperial College London.| 9.11.2022 Prof. Yossi Nir, Department of Particle Physics & Astrophysics, The Weizmann Institute| 16.11.2022 Prof. Eugene Kanzieper, Department of Mathematics, Holon Institute of Technology| 23.11.2022 Prof. Elisabetta Boaretto, Scientific Archaeology Unit, The Weizmann Institute| 30.11.2022 Prof. Sasha Migdal, Department of Physics, New York University, Abu Dhabi| 28.12.2022 Dr. Erez Zohar, The Racah Institute of Physics, The Hebrew University of Jerusalem| 4.1.2023   Dr. Matan Mussel, The Physics Department, The University of Haifa.| 11.1.2023  Prof. Barak Kol, The Racah Institute of Physics, The Hebrew University of Jerusalem| 18.1.2023  Dr. Shahar Hadar, The Physics Department, University of Haifa at Oranim

The Physics Department Colloquium: November 2, 2022, 2pm IDT

Eva-Maria Graefe (Imperial College London (ICL), The UK) | Husimi Dynamics Generated by Non-Hermitian Hamiltonians | Abstract: While traditional quantum mechanics focusses on systems conserving energy and probability, described by Hermitian Hamiltonians, in recent years there has been ever growing interest in the use of non-Hermitian Hamiltonians. These can effectively describe loss and gain in a quantum system. In particular systems with a certain balance of loss and gain, so-called PT-symmetric systems, have attracted considerable attention. The realisation of PT-symmetric quantum dynamics in optical systems has opened up a whole new field of investigations.  The dynamics generated by non-Hermitian Hamiltonians are often less intuitive than those of conventional Hermitian systems. Even for models as simple as a complexified harmonic oscillator, the dynamics beyond coherent states shows surprising features. Here we analyse the flow of the Husimi phase-space distribution in a semiclassical limit, leading to a first order partial differential equation, that helps illuminate the foundations of the full quantum evolution. We discuss instructive examples, demonstrating how the full quantum dynamics unfolds on top of the classical Husimi flow. Recording Slides When: November 2, 2022 2:00 PM (Israel Standard Time). Where: Over Zoom

The Physics Department Colloquium: June 8, 2022, 2pm IDT

Ray Rivers (Imperial College London) | Entropology: an information-theoretic approach to understanding archaeological data | In collaboration with: Paula Gheorghiade (U of Helsinki), Vaiva Vasiliauskate (ETH Zurich), Henry Price (ICL), Sasha Diachenko (U of Kyiv), Tim Evans (ICL) Abstract: In this talk we shall look for ways in which archaeological datasets can give information about the society that produces them, as exemplified by Late Bronze Age Cretan ceramic assemblages catalogued by Dr Paula Gheorghiade from what might loosely be termed the ‘Minoan-Mycenaean transition’. Such archaeological data is highly multivariate and to proceed it is necessary to implement a dramatic reduction in the dimension of artefact space. Identifying the artefacts with the labels in this reduced space lets us think of assemblages as ‘word-heaps’ whose information content can provide clues about Cretan society. We analyse our assemblages using syntactic tools taken from ecology (think of our word-heaps as the labels of insects in bug-traps), using different entropies and their induced diversities as a proxy to explore cultural change and separation (e.g. see [1] by one of us). This phenomenological approach is complemented with semantic tools taken from theoretical economics that use utility and which allow us to incorporate ‘value’ in our analysis (think of our word-heaps as the clothes labels of shopping trips). Because of limited data our results are incomplete but promising.  Reference: [1] A Diachenko, I Sobkowiak-Tabaka, S Ryzhov. Approaching unification and diversity of pottery assemblages: The case of Western Tripolye culture (WTC) ceramics in the Southern Bug and Dnieper interfluve, 4100 – 3600 BCE, Academica Praeistorica 47 (2020), 522-535 Recording Slides When: June 8, 2022 2:00 PM (Israel Standard Time). Where: Over Zoom

The Physics Department Colloquium: June 1, 2022, 2pm IDT

Ehud Meron (Ben-Gurion University) | Ecosystem responses to climate change across different levels of organization | Abstract: Understanding the response of dryland vegetation to climate extremes, such as severe droughts, is of utmost importance in light of the crucial ecosystem services that dryland vegetation provides to humans and the projections for more frequent and intense events in the future. The response is likely to involve mechanisms operating at different levels of ecological organization, including phenotypic changes (organism level), spatial self-organization (population level) and community reassembly (community level). In this talk I will describe the modeling approach that we use to study multi-level responses, and new dynamical behaviors that emerge from such responses. These include multi-scale spatial patterning induced by phenotypic changes from shallow-roots to deep-roots plants along with spatial self-organization, and a homeostatic-like effect of spatial patterning on community composition and species diversity. Relations of some of the results to fairy-circle patterns in Namibia will be discussed. Recording Slides When: June 1, 2022 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

The Physics Department Colloquium: May 25, 2022, 2pm IDT

Ady Stern (Weizmann Institute of Science) | How low can electronic resistance go? | Abstract: Electronic resistance is a fundamental notion both in condensed matter physics and in everyday life, where it is a source of heating caused by electronic currents. Typically, resistance originates from electrons scattering off impurities. However, even a perfectly clean system harbors a resistance, inversely proportional to the number of its conduction channels. Recent theories have shown that scattering of the flowing electrons off one another reduces this resistance, raising the question of its lower bound. Here we show that for a fixed number of channels the resistance may be practically eliminated, and give a transparent physical picture of this elimination. Recording Slides When: May 25, 2022 2:00 PM (Israel Standard Time). Where: over Zoom In addition, you might be also interested in a new online course, titled “Topological Quantum Matter”. The teachers in the course are a group of Weizmann Institute PIs and alumni – Ady Stern, Yuval Oreg, Yuval Ronen, Raquel Queiroz, Erez Berg, Shahal Ilani, Haim Beidenkopf, Binghai Yan, Roni Ilan, Nurit Avraham and Moty Heiblum. The course is available on two online education platforms, EDX and CampusIL. Click here or here to view a trailer of the course.  The course covers a broad spectrum of subjects, including the Quantum Hall effects; Topological superconductors, insulators, and semi-metals; Graphene, including twisted bi-layers; Schemes for topological classification; Techniques of predicting material properties; Experimental tools; And states of topological order. The instructors of the course welcome every response. Please address any response to

The Physics Department Colloquium: May 18, 2022, 2pm IDT

Yariv Kafri (Technion) | The long-ranged influence of disorder on active systems | Abstract: Active systems are a class of non-equilibrium systems in which each particle consumes and dissipates energy to self-propel or exert forces on its environment.Examples range from bird flocks, through swarms of bacteria, to man-made colloidal systems. They have attracted enormous amount of attention in recent years due to the novel phases they exhibit, many without a counterpart in equilibrium system. The talk will give an overview of active systems and then focus on the impact of quenched random potentials on active matter. For dilute systems it will be shown that bulk disorder leads to generic long-range correlations, decaying as a power-law, and steady-state currents. Disorder localized along a wall confining the system leads to long-range density modulations and eddies whose amplitude decays as a power law with the distance from the wall, but whose extent grows with it. It will also be shown that disorder, even when localized on the boundary, can destroy bulk phase transitions in active systems. Recording Slides When: May 18, 2022 2:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom4

The Physics Department Colloquium: May 11, 2022, 11am IDT

Ofek Birnholtz (Bar-Ilan University) | Gravitational Wave Astrophysics – The LVK Catalog GWTC-3 | Abstract: Gravitational Waves were originally predicted in the 1910’s by Einstein, and measured directly for the first time in 2015. In the 6 years since, they have become a household acquaintance and a cornerstone of astrophysical observations and discoveries. I will present their background, history, and how they have begun to shape current day astrophysics, including the recent O3b results and the prospects for the future. Recording Slides When: May 11, 2022 11:00 AM (Israel Standard Time). Where: Over Zoom

The Physics Department Colloquium: Spring 2022 Schedule

Spring 2022 Colloquium Schedule |  May 11: Ofek Birnholtz, Bar-Ilan University (Ramat Gan). May 18: Yariv Kafri, Technion (Haifa). May 25: Ady Stern, The Weizmann Institute of Science (Rehovot). June 1: Ehud Meron, Ben-Gurion University (Beer Sheva). June 8: Ray Rivers, Imperial College London (UK).