VIRTUAL Thursday, May 19th 2022 3:45 – 4:45 pm (MT) WEBEX Speaker: Prof. Jonathan I. Katz Washington University, St Louis “Fermi at Trinity” Abstract: Enrico Fermi estimated the yield of the Trinity test to be about 10 kilotons by dropping small pieces of paper and observing their motion in the blast wave. This is about 40\% of the radiochemically derived value of $25i \pm 2$ kilotons that necessarily includes thermal and nuclear radiation that do not contribute to the blast. Although this story is classic, there appears to be no account of how he related his observation to the yield. This note attempts to reconstruct how he might have done so. Bio: https://web.physics.wustl.edu/katz/ Professor Katz's work, mostly in astrophysics, also involves a number of diverse topics in applied physics, biophysics, soft matter, energy and climate. His complete publication list should be consulted for details. His vita can be found here Following the blow-out of the Macondo well in the Gulf of Mexico (April 20, 2010) Prof. Katz was appointed to Secretary of Energy Chu's scientific advisory panel. This experience led him to conceive a novel composition of drilling mud, involving a dilatant polymer that could make the mud viscoelastic, in order to suppress instabilities that would otherwise occur. In collaboration with Richard Garwin (also on the panel) he predicted that attempts at ``top kill'' with conventional muds would only lead to that mud being spat out the well-head with the escaping oil. This prediction was borne out when top kill was attempted and failed. Prof. Katz then organized, in collaboration with Peter Beiersdorfer and his team at the Lawrence Livermore National Laboratory, an experimental effort to test the prediction that a viscoelastic mud would not suffer such instabilities. The prediction was verified (Phys. Rev. Lett. 106, 058301 [2011]). Prof. Katz is studying the recently discovered Fast Radio Bursts, extremely bright but brief (millisecond) sources of radio radiation. Their radiation must be emitted from very small sources, and therefore must have a very high "brightness temperature", like the most intense emission of pulsars. By applying the classical "log number-log flux" test he was able to confirm their origin at cosmological distances (and hence their great power and brightness), and with a new test, using their distribution of dispersion measures (electron column density between us and them) excluded expanding shells, such as supernova remnants, as sources of their dispersion, confirming that most of their dispersion measure results from propagation in the intergalactic medium. Despite this, the most plausible origin of Fast Radio Bursts is in the giant flares of Soft Gamma Repeaters, and he suggested a mechanism involving the excitation of plasma instability by Compton scattering of electron-positron pair annihilation radiation or by two-stream instability in a pair gap. Either may be the consequence of the sudden dissipation of magnetic energy in a neutron star magnetosphere, the most popular model of Soft Gamma Repeaters. Prof. Katz is working on problems in boundary layer hydrodynamics. He analyzed the process of rapid adiabatic blowdown of a pressure vessel, and derived a novel dimensionless number describing the importance of buoyancy-driven circulation, resulting from the competition between conductive heating of the gas near the wall and the adiabatic cooling of the gas in the interior of the vessel. He is now working on double-diffusive boundary layers, such as those between water and glycerin or plasmas of different composition in laser-fusion targets, in which both momentum and mass diffuse, and in which the composition (affected by mass diffusion) affects the viscosity (that determines the diffusion of momentum).