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DTSTART;TZID=America/New_York:20230413T093000
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SUMMARY:Resolving the photon ring
DESCRIPTION:General Relativity Seminar \nSpeaker: Shahar Hadar (University of Haifa) \nTitle: Resolving the photon ring \nAbstract: 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 strong lensing by the black hole and thereby conveys information on its spacetime geometry\, potentially providing a new independent avenue for 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 present an upcoming proposal to use spectro-temporal autocorrelations in signals emitted from black hole environs as a probe of strong lensing effects.
URL:https://cmsa.fas.harvard.edu/event/gr_41323/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:General Relativity Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-GR-Seminar-04.13.23.png
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SUMMARY:Control of actin cable length by decelerated growth and network geometry
DESCRIPTION:Active Matter Seminar\n\n\nSpeaker: Shane McInally\, Brandeis \nTitle: Control of actin cable length by decelerated growth and network geometry \nAbstract: The sizes of many subcellular structures are coordinated with cell size to ensure that these structures meet the functional demands of the cell. In eukaryotic cells\, these subcellular structures are often membrane-bound organelles\, whose volume is the physiologically important aspect of their size. Scaling organelle volume with cell volume can be explained by limiting pool mechanisms\, wherein a constant concentration of molecular building blocks enables subcellular structures to increase in size proportionally with cell volume. However\, limiting pool mechanisms cannot explain how the size of linear subcellular structures\, such as cytoskeletal filaments\, scale with the linear dimensions of the cell. Recently\, we discovered that the length of actin cables in budding yeast (used for intracellular transport) precisely matches the length of the cell in which they are assembled. Using mathematical modeling and quantitative imaging of actin cable growth dynamics\, we found that as the actin cables grow longer\, their extension rates slow (or decelerate)\, enabling cable length to match cell length. Importantly\, this deceleration behavior is cell-length dependent\, allowing cables in longer cells to grow faster\, and therefore reach a longer length before growth stops at the back of the cell. In addition\, we have unexpectedly found that cable length is specified by cable shape. Our imaging analysis reveals that cables progressively taper as they extend from the bud neck into the mother cell\, and further\, this tapering scales with cell length. Integrating observations made for tapering actin networks in other systems\, we have developed a novel mathematical model for cable length control that recapitulates our quantitative experimental observations. Unlike other models of size control\, this model does not require length-dependent rates of assembly or disassembly. Instead\, feedback control over the length of the cable is an emergent property due to the cross-linked and bundled architecture of the actin filaments within the cable. This work reveals a new strategy that cells use to coordinate the size of their internal parts with their linear dimensions. Similar design principles may control the size and scaling of other subcellular structures whose physiologically important dimension is their length.
URL:https://cmsa.fas.harvard.edu/event/am-41323/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Active Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Active-Matter-Seminar-04.13.23.png
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