Editing Stanisław Ulam (section)

==Manhattan Project==
[[File:Ulam-stanislaw m.jpg|thumb|Ulam's ID badge photo from [[Los Alamos National Laboratory]] |alt=A mug shot style ID photo, with the serial H 0]]
In early 1943, Ulam asked von Neumann to find him a war job. In October, he received an invitation to join an unidentified project near [[Santa Fe, New Mexico]].<ref name='VITA'/> The letter was signed by [[Hans Bethe]], who had been appointed as leader of the theoretical division of [[Los Alamos National Laboratory]] by [[Robert Oppenheimer]], its scientific director.<ref name="ULAM 143-147"/> Knowing nothing of the area, he borrowed a New Mexico guide book. On the checkout card, he found the names of his Wisconsin colleagues, [[Joan Hinton]], [[David H. Frisch|David Frisch]], and Joseph McKibben, all of whom had mysteriously disappeared.<ref name='VITA'/> This was Ulam's introduction to the [[Manhattan Project]], which was the US's wartime effort to create the atomic bomb.<ref name='LANLBIO'/>

===Hydrodynamical calculations of implosion===
A few weeks after Ulam reached [[Los Alamos, New Mexico|Los Alamos]] in February 1944, the project experienced a crisis. In April, [[Emilio G. Segrè|Emilio Segrè]] discovered that [[plutonium]] made in [[Nuclear reactors#Early reactors|reactors]] would not work in a [[gun-type fission weapon|gun-type]] plutonium weapon like the "[[Thin Man nuclear bomb|Thin Man]]", which was being developed in parallel with a uranium weapon, the "[[Little Boy]]" that was dropped on [[Atomic bombings of Hiroshima and Nagasaki#Hiroshima|Hiroshima]]. This problem threatened to waste an enormous investment in new reactors at the [[Hanford site]] and to make slow uranium [[isotope separation]] the only way to prepare [[fissile]] material suitable for use in bombs. To respond, Oppenheimer implemented, in August, a sweeping reorganization of the laboratory to focus on development of an [[Nuclear weapon design#Implosion-type weapon|implosion-type weapon]] and appointed [[George Kistiakowsky]] head of the implosion department. He was a professor at Harvard and an expert on precise use of explosives.<ref name="HODDESON 130-137"/>

The basic concept of [[Implosion (mechanical process)|implosion]] is to use chemical explosives to crush a chunk of fissile material into a [[critical mass]], where [[neutron]] multiplication leads to a [[nuclear chain reaction]], releasing a large amount of energy. Cylindrical implosive configurations had been studied by [[Seth Neddermeyer]], but von Neumann, who had experience with [[shaped charge]]s used in [[High-explosive anti-tank|armor-piercing ammunition]], was a [[John von Neumann#Manhattan Project|vocal advocate of spherical implosion]] driven by [[explosive lens]]es. He realized that the symmetry and speed with which implosion compressed the plutonium were critical issues,<ref name="HODDESON 130-137"/> and enlisted Ulam to help design lens configurations that would provide nearly spherical implosion.  Within an implosion, because of enormous pressures and high temperatures, solid materials behave much like fluids. This meant that [[Fluid dynamics|hydrodynamical]] calculations were needed to predict and minimize asymmetries that would spoil a nuclear detonation. Of these calculations, Ulam said:{{quote|The hydrodynamical problem was simply stated, but very difficult to calculate&nbsp;– not only in detail, but even in order of magnitude. In this discussion, I stressed pure pragmatism and the necessity to get a heuristic survey of the problem by simple-minded brute force, rather than by massive numerical work.<ref name='VITA'/>}}

Nevertheless, with the primitive facilities available at the time, Ulam and von Neumann did carry out numerical computations that led to a satisfactory design. This motivated their advocacy of a powerful computational capability at Los Alamos, which began during the war years,<ref name='COMPEARLY'/> continued through the cold war, and still exists.<ref name='COMPLATE'/> [[Otto Frisch]] remembered Ulam as "a brilliant Polish topologist with a charming French wife. At once he told me that he was a pure mathematician who had sunk so low that his latest paper actually contained numbers with decimal points!"<ref name='FRISCH'/>

===Statistics of branching and multiplicative processes===
Even the inherent statistical fluctuations of [[neutron]] multiplication within a [[chain reaction]] have implications with regard to implosion speed and symmetry. In November 1944, [[David Hawkins (philosopher)|David Hawkins]]<ref name='HAWKINS'/> and Ulam addressed this problem in a report entitled "Theory of Multiplicative Processes".<ref name='LA171'/> This report, which invokes [[probability-generating function]]s, is also an early entry in the extensive literature on statistics of [[Branching process|branching]] and multiplicative processes. In 1948, its scope was extended by Ulam and Everett.<ref name='MULT123'/>

Early in the Manhattan project, [[Enrico Fermi]]'s attention was focused on the use of reactors to produce plutonium. In September 1944, he arrived at Los Alamos, shortly after breathing life into the [[Manhattan Project#Hanford reactors|first Hanford reactor]], which had been [[Nuclear poison|poisoned]] by a [[Xenon-135|xenon isotope]].<ref name="HEWLETT"/> Soon after Fermi's arrival, [[Teller's "Super" bomb]] group, of which Ulam was a part, was transferred to a new division headed by Fermi.<ref name="ULAM 152-153"/> Fermi and Ulam formed a relationship that became very fruitful after the war.<ref name="ULAM 162-167"/>