קולוקוויום

החוג לפיזיקה מארגן סידרה של הרצאות שבועיות בפיזיקה במהלך הסימסטר, שמכסות מגוון נושאים מחקריים בפיזיקה. ההרצאות מתקיימות בימי רביעי בשעה 14:00 (בזום). לפרטים נוספים אנא צרו קשר עם פרופ׳ יהושע פיינברג. 

קולוקוויום החוג לפיזיקה: 8 ביוני, 2022, 14:00

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

קולוקוויום החוג לפיזיקה: 1 ביוני, 2022, 14:00

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

קולוקוויום החוג לפיזיקה: 25 במאי, 2022, 14:00

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 TQM.COURSE@Weizmann.ac.il  

קולוקוויום החוג לפיזיקה: 11 במאי, 2022, 11:00

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. When: May 11, 2022 11:00 AM (Israel Standard Time). Recording Slides 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). 

קולוקוויום החוג לפיזיקה: 30 במרץ, 2022, 14:00

Serge Galam (CEVIPOF-Center for Political Research, Sciences PO & CNRS) | Do humans behave like atoms? An answer from sociophysics | Abstract: This provocative question sounds out of place and indeed is out of place if it concerns a comparison between a human and an atom. However, it takes on its full meaning if being understood as “do human obeys quantitative laws like atoms do? And if yes, could these laws be identified and put into equations?” The subsidiary question which immediately follows this question is “can we predict social and political events, at least some of them?”. Sociophysics answers yes to those three questions, But what is sociophysics? It is a newly established field of research among physicists to address a wide range of social and political phenomena, which result from individual human interactions. The approach is similar to statistical physics, which studies the macroscopic properties of inert matter starting from interactions between its microscopic constituents. I will illustrate sociophysics with the case of opinion dynamics. Indeed, opinion dynamics is today a major driver of policy making, in particular with both the ubiquity of social medias and the scale of contemporary global challenges. In this talk I consider a community of heterogeneous agents, floaters and inflexibles (stubborn), having to choose between two discrete competing choices via open discussion among them. The dynamics is monitored using repeated local updates of individual opinions in small size groups of randomly distributed agents. At each distribution of agents, local majority rules are applied within the discussing groups. However, while floaters update their choices along the local majority, inflexibles keep on their choices. The effect of shared prejudices is also incorporated quite naturally within groups at a tie where floaters select unconsciously a tie breaking local choice, which is in tune with their activated prejudice, while being convinced that it was done at random.  The dynamics is found to be either a tipping point dynamics with

קולוקוויום החוג לפיזיקה: 23 במרץ, 2022, 14:00

Saleem Zaroubi (the Open University, Israel, and the University of Groningen, the Netherlands) | From Darkness unto Light: A Cosmic Odyssey | Abstract: During the first billion years of its history, our Universe transformed from its pristine primordial conditions to the galaxies and stars-filled cosmos we see around us. This period is divided into three eras: the first, known as the Universe’s Dark Ages, and refers to the period before the emergence of any astrophysical sources. The second, known as the Cosmic Dawn, is distinguished by the emergence of the first astrophysical sources of radiation (first stars) that start heating the hydrogen in the intergalactic medium. The third, known as the Epoch of Reionization, is the period in which the hydrogen ionization process, caused by the ultraviolet radiation emanating from the first generations of stars and galaxies, became the main phenomenon, lasting until the Universe is completely ionized. One of the very few probes, if not the only one, of the first billion years, is the 21 cm radiation that emanates from the diffuse atomic hydrogen that permeates the intergalactic medium at these eras. In this talk, I will review the emerging field of "21 cm cosmology" and the worldwide effort that is currently underway to observe it with radio telescopes. Recording Slides When: March 23, 2022 02:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

קולוקוויום החוג לפיזיקה: 9 במרץ, 2022, 14:00

Yosi Avron (Technion) | Shor in Shorts | Abstract: I shall give an elementary introduction to the Peter Shor algorithm for factoring. Quantum magic allows one to find the period of a periodic function, a hard problem, efficiently. This efficiency comes from the exponential gain offered by the quantum Fourier transform compared with the fast Fourier transform. Recording Slides When: March 9, 2022 02:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

קולוקוויום החוג לפיזיקה: 2 במרץ, 2022, 14:00

Naama Brenner (Technion) | Exploratory Learning in Biological Cells | Abstract: The capacity of cells and organisms to respond to challenging conditions is limited by a finite repertoire of repeatable responses. Beyond this capacity, novel and unforeseen challenges may elicit exploratory dynamics, improvisational in nature, potentially providing adaptation to a much broader array of conditions. Exploratory adaptation, its dynamics and convergence properties are not well understood. Such phenomena are naturally described as learning processes: Learning entails self-modification of a system under closed-loop dynamics with its environment. In particular, the interactions between system elements are modified – like synapses during learning in the brain, that alter the connections between neurons. Inspired by classic concepts of Neuroscience, I will describe a theoretic framework for a primitive form of learning that takes place within the single cell. This “exploratory learning” is a random search guided by global feedback; we find this to be a feasible mechanism, but its convergence in high-dimensional gene expression space is non-universal and depends on network properties. Successfully adapting network ensembles are heterogeneous and have outgoing hubs – the analog of “master regulators” in gene regulatory networks. The role of these hubs in guiding the search process is understood by a mapping to a simpler problem that can be analyzed by mean field methods and relates to a chaos-suppression phase transition. Results of this theory connect to several experimental observations in cellular systems. We thus establish a biologically plausible mechanism of adaptation by exploratory learning; this, in turn suggests that learning theory provides a useful theoretical framework to understand cellular dynamics. Recording Slides When: March 2, 2022 02:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

קולוקוויום החוג לפיזיקה: 23 בפברואר, 14:00

Oded Agam (Hebrew University of Jerusalem) |  The dynamics of living cells from the viewpoint of a statistical physicist | Abstract: Deciphering the cellular metabolic network is, perhaps, the most prominent objective of cell biology. It is a formidable enigma because the metabolic network is an extremely complicated system. It comprises several thousand different compounds with intricate interactions among them that are, essentially, unknown. There are two complementary viewpoints of living cells. One is to view the cell as a large computer designed to respond effectively to environmental changes. The second is to consider the cell as a complex physical system obeying physical statistics laws. The first viewpoint encouraged the effort of disentangling the metabolic network structure by identifying specific modules such as metabolic cycles, pathways, and forms of regulations. However, this reductionism  approach can only take you so far because all network components are strongly intertwined. Hence, a holistic approach is required. Recent experimental data, obtained by Balaban’s group, show that the recovery time of E. coli cells, after their growth was arrested by acute stress, exhibits distinctive statistical features typical of complex physical systems. These include a memory of the period that the cell was subjected to the stress and slow power-law decay of the distribution of single-cell recovery time. The same features are common to systems that exhibit physical “aging”, such as amorphous polymers, stretched DNA, paper crumpling, colloidal solutions, spin glasses, supercooled liquids, and coulomb glasses. This observation suggests that a relatively simple statistical model can describe the cell in the acute stress regime. In this talk, I will present a simple spin model that reproduces the experimental observations. In particular, I will show that the statistics of the cells' recovery times are obtained from the statistics of the crossing problem of a random walker in a potential field and present its solution. Recording  Slides When: February 23, 2022 02:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Bldg. & over Zoom

קולוקוויום החוג לפיזיקה: 16 בפברואר, 2022, 16:00

Gabi Zafrir (Stony Brook University) |  Understanding quantum field theory with dimensional reduction | Abstract: The world around us is made of particles that interact through various forces. The behavior of the particles and the forces between them is described by quantum field theory, which as such is of great importance in many branches of physics. Nevertheless, quantum field theory presents many challenges, notably with regard to its behavior at strong coupling, that is when the interactions become strong. In this talk I'll review some recent progress in studying various aspects of quantum field theory at strong coupling using the idea of dimensional reduction. Recording Slides When: February 16, 2022 04:00 PM (Israel Standard Time). Where: over Zoom

קולוקוויום החוג לפיזיקה: 31 בינואר, 2022, 18:00

Shahaf S. Asban (University of California, Irvine) |  Quantum inference: unravelling “which pathway?” information | Abstract: Quantum systems are remarkably sensitive to changes in their environment. This renders them extraordinary probes for sensing applications. In contrast to classical probes, they undergo transitions upon coupling that encode trajectory dependent quantum information in their statistics. Decoding this information requires a new set of inference methodologies, such as the one we introduce here. Entangled photon pairs have inspired a myriad of quantum-enhanced metrology platforms, which outperform their classical counterparts. However, the role of photon exchange-phase and degree of distinguishability have not yet been utilized in quantum-enhanced applications. We show that when a two-photon wave-function is coupled to matter, it is encoded with "which pathway?" information even at a low degree of entanglement. An interferometric exchange-phase-cycling protocol is developed, revealing phase-sensitive information for each interaction history individually. Moreover, we find that quantum-light multimode interferometry introduces a new set of time variables that enable time-resolved signals, unbound by uncertainty to the inverse bandwidth of the wave-packet. We illustrate our findings on an exciton model-system and discuss future applications. Video Presentation When: January 31, 2022 06:00 PM (Israel Standard Time). Where: over Zoom

קולוקוויום החוג לפיזיקה:5 בינואר, 2022, 14:00

Erez Braun (Technion) |  Control of animal morphogenesis by external electric fields | Abstract: The current picture of animal morphogenesis- the emergence of form and function in a developing animal, relies on biochemical patterning and its robustness is typically attributed to the presence of a well-defined hierarchy of forward-driven processes, such as threshold-crossing cellular processes and the development of symmetry-breaking fields. Is it possible to modulate the course of animal morphogenesis and direct its developmental trajectory on demand? We demonstrate that an external electric field can be tuned to drive morphogenesis in Hydra regeneration, backward and forward, around a critical point in a controlled manner. A controlled drive of morphogenesis allows multiple re-initiation of novel developmental trajectories for the same tissue. We further show that calcium spatial fluctuations drive the morphological dynamics. The normalized calcium spatial fluctuations exhibit a universal non-Gaussian shape distribution, across tissue samples and conditions, suggesting a global constraint over these fluctuations. Studying the dynamics at the onset of morphogenesis opens a new vista on this process and paints a picture of development analogous to a dynamical phase transition. Recording Slides When: January 5, 2022 02:00 PM (Israel Standard Time). Where: over Zoom

קולוקוויום החוג לפיזיקה: 29 בדצמבר, 2021, 14:00

Noam Soker (Technion) |  Astrophysical Naturalness | Abstract: I suggest that stars introduce mass and density scales that lead to `naturalness' in the Universe. Namely, two ratios of order unity. The first ratio of order one involves the Planck mass, the mass of the proton, and the mass of stars. The second ratio of order one involves the Planck density and the dynamical density that one derives from the nuclear lifetime of stars. In the pure fundamental particles domain there is no naturalness; either naturalness does not exist or there is a need for a new physics or new particles. The `Astrophysical Naturalness' offers a third possibility: stars introduce the combinations of, or relations among, known fundamental quantities that lead to naturalness. Recording Slides When: December 29, 2021 02:00 PM (Israel Standard Time). Where: Room 223, Multipurpose Building and over Zoom

קולוקוויום החוג לפיזיקה: 22 בדצמבר, 2021, 14:00

Alex Retzker (The Hebrew University of Jerusalem) | Frequency Estimation and Resolution at the Nano-Scale | Abstract: Quantum sensing is an important tool for estimating various parameters at the nano- scale. Of special importance is the estimation of frequencies, i.e., spectroscopy, at small scales. In this talk I will discuss a few surprising aspects of spectroscopy at minute scales, while concentrating at the NV center in diamond. When: December 22, 2021 02:00 PM (Jerusalem). Where: over Zoom Recording Slides

קולוקוויום החוג לפיזיקה: 8 בדצמבר, 2021, 14:00

Shahar Hadar (University of Haifa at Oranim) | Extreme Black Holes | Abstract: The spacetime geometry of a black hole is characterized by a handful of parameters: Its mass, angular momentum, and charge. For a given mass, bounds exist on the maximal possible spin and charge. In the astrophysically relevant neutral case, for example, that bound corresponds to the event horizon spinning at the speed of light. Black holes close to this bound are called (near-)extremal. They display especially remarkable properties, including an emergent conformal symmetry in their near-horizon region. In the talk, I will describe how this symmetry, which was discovered amid attempts to understand quantum black hole properties, can be exploited in order to analytically compute observational gravitational-wave and electromagnetic signatures of near-extremal black holes. Such computations are often practically intractable without an elaborate utilization of the symmetry. The unique near-horizon geometry also explains other peculiar effects occurring in these black holes, such as instabilities developing on their horizons and high-energy particle collisions in their vicinity, which I will describe. Recording Presentation part 1 Presentation part 2 When: December 8, 2021 02:00 PM (Jerusalem). Where: Multipurpose building, seminar room 223 as well as via Zoom (hybrid format).

קולוקוויום החוג לפיזיקה: 1 בדצמבר, 2021, 14:00

Matan Mussel (University of Haifa) | Kinetics of volume transition in polymer gels | Abstract: A gel is a soft material composed of a solid phase interpenetrated by a liquid phase. Gels are abundant in nature. We eat gels, we are made of gels, and we use gels for various applications from contact lenses to vodka jello shots. Many gel systems change their volume in response to changes in environmental conditions, such as temperature, solvent quality, and ionic content. For certain gel characteristics, these volume changes can be abrupt as the control parameter is varied. This nonlinear response has been adopted for various man-made applications, such as sensors, actuators, and drug-delivery carriers, and is also believed to play an important functional role in various biological systems. Understanding the transient changes is important in these cases, but equilibrium swelling models, widely used in polymer physics, do not provide kinetic and dynamic information. I will begin this talk with a broad introduction about basic properties and potential uses of gels, and then turn to discuss a theoretical approach to model transient changes in a gel exposed to a new ionic environment at macroscopic time and length scales. I will demonstrate that a continuum multicomponent approach can be useful for semi-quantitative predictions. Compatibility is demonstrated in three aspects by comparing results with measurements made on sodium polyacrylate gels: (1) dynamics of gel swelling and deswelling, (2) ion partitioning coefficient, and (3) effect of crosslink density. When: December 1, 2021 02:00 PM (Jerusalem)  Where: seminar room 223 as well as via Zoom (hybrid format). Recorded talk Slides

קולוקוויום החוג לפיזיקה: 24 בנובמבר, 2021, 14:00

Ohad Shpielberg (University of Haifa at Oranim) | Power law decay of entanglement quantifiers: Coupling a single agent to a many body system | Abstract: Control over a many-body quantum system can be achieved by entangling the system to a single agent. Here, we study the ground state entanglement properties of two-site lattice models, between a highly occupied subsystem (site A) to a few particles (site B). Both the von Neumann entanglement entropy and Logarithmic negativity show a power law decay in R – the occupancy ratio between the systems. This implies that it is feasible to entangle highly occupied systems to a single atom, as recently reported experimentally. When: November 24, 2021 02:00 PM (Jerusalem) via Zoom. Slides Video

קולוקוויום החוג לפיזיקה: 17 בנובמבר, 2021, 14:00

Jacob Sonnenschein (Tel Aviv University and New York University) | Back to Square One: The HISH (holography inspired stringy hadron) model | Recording Slides Abstract: String theory was born as a theory of Hadrons. I will briefly review that old picture of stringy hadrons and the reasons it has been abandoned.A modern version, Holography inspired stringy hadrons (HISH) model, will then be introduced. Mesons, baryons, and exotic hadrons are described asopen strings with massive endpoint particles and glueballs as closed strings. The model is based on a “map” from stringy hadrons of “holographic confining backgrounds” to strings in four dimensional flat space-time. The HISH model admits “massive modified Regge trajectories”. In particular it includes a novel concept of quark mass, the “ string endpoint mass”. I will  compare the theoretical results with the experimental data and extract the best fits for a universal string tension, quark masses and quantum intercepts. Predictions for yet unobserved higher excited mesons and baryons will be presented. I will suggest a simple method to identify glueballs by searching for flavorless states on trajectories with a slope which is half the one of the stringy mesons and baryons. I will suggest to use the state Y(4630), which decays predominantly to ΛcΛc, as a window to the landscape of tetra-quarks. I will propose a simple criterion to decide whether a state is a stringy exotic hadron – a tetra-quark – or a “molecule”. If it is the former it should be on a (modified) Regge trajectory. I will briefly discuss the zoo of other exotic stringy hadrons. I will further present the predictions for the hadrons strong decay widths and their comparison to the experimental observations. The main decay mechanism is that of a string splitting into two strings. The corresponding total decay width behaves as Γ = πATL/2 where T and L are the tension and length of the string and A

The Physics Department Colloquium: November 10, 2021, 2pm IDT

Nicholas S. Manton (University of Cambridge) | Skyrmions, Topology and Instantons  | Abstract: I will review the Skyrme model, and how Skyrmions are used to model nucleons and nuclei. I will then discuss the relationship, both topological and energetic, between Skyrmions in 3-dimensions and Yang-Mills instantons in 4-dimensions. When: November 10, 2021 02:00 PM (Jerusalem) Recorded talk Presentation file