Yasmine Meroz (Tel Aviv University) | Plant Tropisms as a Window on Plant Computational Processes |

Plants survive in a harsh and fluctuating environment, optimising their search for fluctuating nutrients, and predicting danger. They achieve this through complex response processes, such as decision-making, based on memory, or the capability to accumulate and compare past stimuli. For example, a plant shoot accumulates sensory information from various fluctuating light sources, decides which direction yields consistently most light for photosynthesis, and grows in that direction. Here we propose a reverse-engineering approach to investigating the underlying rules for the accumulation and integration of sensory inputs. Our theoretical model, based on response theory, predicts that plants respond to the sum of stimuli at short timescales, and to the difference in stimuli at longer timescales. We confirm this experimentally, and suggest that this process may be essential for navigational problem-solving capabilities of plants.

I will also briefly talk about the role of art in science, giving two particular examples of collaborations with artist Liat Segal: “Tropism” and “Impossible Object”

When: May 31, 2023 2:00 PM (Israel Standard Time).

Where: Room 223, Multipurpose Bldg. & over Zoom

David Andelman (Tel Aviv University) | One Hundred Years of Electrified Interfaces: What’s New with the Theories of Debye and Onsager? |

The Poisson-Boltzmann theory stems from the pioneering works of Debye and Onsager and is considered even today as the benchmark of ionic solutions and electrified interfaces. It has been instrumental during the last century in predicting charge distributions and interactions between charged surfaces, membranes, electrodes, macromolecules, and colloids. The electrostatic model of charged fluids, on which the Poisson-Boltzmann description rests and its statistical mechanical consequences have been scrutinized in great detail. Much less, however, is understood about its probable shortcomings when dealing with various aspects of real physical, chemical, and biological systems. After reviewing the Poisson-Boltzmann theory, I will discuss several extensions and modifications to the seminal works of Debye and Onsager as applied to ions and macromolecules in confined geometries. These novel ideas include the effect of dipolar solvent molecules, finite size of ions, ionic specificity, surface tension, and conductivity of concentrated ionic solutions.

When: May 17, 2023 2:00 PM (Israel Standard Time).

Where: Room 223, Multipurpose Bldg. & over Zoom

Shahar Hadar (The University of Haifa at Oranim) | Resolving the Photon Ring |

In the past few years, the Event Horizon Telescope has released the first close-up interferometric images of two supermassive black holes, M87* and SgrA*. It is believed that within these images is embedded a fine, yet-unresolved brightness enhancement called the photon ring. The ring is a universal consequence of extreme lensing by the black hole and thereby conveys information on its spacetime geometry, potentially providing a new independent avenue for future tests of general relativity in the strong-field regime. In the talk I will briefly review the theory of the photon ring and its corresponding spacetime region, the photon shell, which governs the universal lensing structure. I will then describe some current efforts and future prospects for resolving the ring, which include both the construction of transformative new instruments and the development of novel analysis methods. Focusing on the latter, I will discuss how source variability may be harnessed to detect the ring. In particular I will review a recently proposed observable, the 2-point correlation function of intensity fluctuations around the ring, and present its recent generalization to frequency-dependent sources. Using a simple toy model of line- emitting, orbiting sources and integrating over the image, I will argue that extreme lensing effects induce particular spectro-temporal correlations in specific flux fluctuations that could be relevant for unresolved spectrometric observations. 

When: May 24, 2023 2:00 PM (Israel Standard Time).

Where: Room 223, Multipurpose Bldg. & over Zoom

Hillel Aharoni (Weizmann Institute of Science) | Tunable Architecture of Nematic Disinclination Lines |

In this talk, I will introduce a theorethical framework to tailor three-dimensional defect line architecture in nematic liquid crystals. By drawing an analogy between nematic liquid crystals and magnetostatics, I will show that: i) disclination lines connect defects with the same topological charge on opposite surfaces and ii) disclination lines are attracted to regions of maximal twist. Using these principles, I will show quantitative predictions for the connectivity and shape of defect lines in a nematic confined between two thinly spaced glass substrates. I will demonstrate experimental and numerical verification of these predictions, and identify critical parameters that tune the disclination lines' curvature within an experimental setup, as well as non- dimensional parameters that allow matching experiments and simulations at different length scales. Our system provides both physical insight and powerful tools to induce desired shape changes of defect lines, opening opportunities to design new types of smart materials.

When: May 10, 2023 2:00 PM (Israel Standard Time).

Where: Room 223, Multipurpose Bldg. & over Zoom

Elisabetta Boaretto (Weizmann Institute of Science) | Plants-Climate-Atmosphere: Understanding Rapid Fluctuations in Radiocarbon Recorded in Archaeology |

When: May 3, 2023 2:00 PM (Israel Standard Time).

Where: Room 223, Multipurpose Bldg. & over Zoom

Mikhail Shifman (University of Minnesota) | 50 Years of Supersymmetry |

Abstract:

I present a broad non-technical review of supersymmetry in our world from its inception to the current status. Related concepts of naturalness and strongly coupled field theories will be discusses too. This colloquium will be structured in a conversational manner; questions are most welcome.

When: April 19, 2023 2:00 PM (Israel Standard Time).

Where: Room 223, Multipurpose Bldg. & over Zoom