Radar and the Manhattan Project (MIT, L14)

STS.042J / 8.225J — Einstein, Oppenheimer, Feynman: Physics in the 20th Century MIT OpenCourseWare · Fall 2020 · Prof. David Kaiser · Undergraduate · CC BY-NC-SA 4.0 Lecture 14: Radar and the Manhattan Project This lecture examines the conceptual, engineering, and institutional dimensions of two major American wartime physics projects, deliberately reserving the ethical and moral questions about the use of nuclear weapons for a later optional discussion and the film The Day After Trinity. The first half focuses on radar, a project often overshadowed by the atomic bomb but, by many participants' reckoning, the one that "won the war." Radar works by emitting Maxwellian electromagnetic waves, collecting the reflected echo, and using the constant speed of light to determine a target's distance; more sophisticated wartime units added Doppler analysis to measure speed. The central research challenge was shrinking wavelengths from meters to centimeters in order to detect small targets, above all the periscopes of the devastating German U-boats. British researchers developed the handheld cavity magnetron, which could emit powerful short-wavelength beams, but constant German bombing made large-scale production in Britain impossible. In autumn 1940, more than a year before the U.S. entered the war, the Tizard delegation brought blueprints and a single cavity magnetron to the United States. A heavily MIT-connected meeting led to the establishment of the MIT Radiation Laboratory (the "Rad Lab") in the hastily built Building 20. Kaiser uses this to introduce Vannevar Bush's institutional innovations, the NDRC and then the OSRD, which deliberately used business-style contracts rather than grants to preserve the appearance of universities as independent partners. The Rad Lab grew explosively from 30 staff to thousands, designing dozens of radar systems and the Applied Physics Laboratory's proximity fuse. Theoretical physicists, initially dismissive of radar as "just Maxwell's equations," learned from engineers a new input-output, effective-circuit approach to problem-solving, a lesson Julian Schwinger and others carried back into postwar quantum theory. The second half turns to the Manhattan Project (officially the Manhattan Engineer District), one of thousands of OSRD projects but the largest, employing over 125,000 people across more than 30 sites, most of whom had no idea what they were building. Kaiser tours the four principal sites. At Chicago's Metallurgical Laboratory, Enrico Fermi built Chicago Pile-1, the first working nuclear reactor, beneath the Stagg Field stands, achieving the first controlled self-sustaining chain reaction on December 2, 1942, using graphite to moderate neutrons and cadmium control rods to absorb them. At Berkeley, Glenn Seaborg's team synthesized neptunium and then plutonium, and (with Emilio Segrè) established plutonium's fissionability. General Leslie Groves, fresh from building the Pentagon, was placed in overall command and made the surprising choice of theoretical physicist Robert Oppenheimer as scientific director of the new Los Alamos laboratory. Robert Serber's initiation lectures became the Los Alamos Primer (Report One), which stated plainly that the goal was a fast-neutron-chain-reaction bomb and laid out the energy estimates, the contrast with conventional explosives, and the coded references to fissionable materials (U-235 as "25," plutonium as "49"). Kaiser explains the materials problem: U-235 is readily fissionable but under 1% of natural uranium, while plutonium exists only in micrograms. Oak Ridge, Tennessee, a secret "classified city," housed the mile-long gaseous-diffusion plant that separated U-235 by exploiting the tiny mass difference of uranium hexafluoride molecules across thousands of stages. Hanford, Washington, built enormous reactors (with DuPont) to mass-produce plutonium. The final section covers weapon design. Drawing on Lillian Hoddeson's scholarship, Kaiser explains critical mass, the use of a heavy inert "tamper" to reflect neutrons and reduce the required mass, and the role of hybrid human-machine "computer" teams (largely women, often staff spouses) in the numerical estimates. The uranium "gun" method, firing two subcritical pieces together with an initiator, was deemed so reliable it was never tested before being used on Hiroshima. Plutonium's high spontaneous-fission rate made the gun method too slow, requiring the far more complex implosion method, using precisely shaped charges of different burn rates to crush a plutonium core into criticality. This was tested at the Trinity test on July 16, 1945, and the implosion design ("Fat Man") was dropped on Nagasaki three days after Hiroshima Source:    • Lecture 14: Radar and the Manhattan Project