On WGN America’s new TV drama Manhattan, protagonist Frank Winter’s desperate move at the start of the series—turning in scientist Sidney Lao for removing secret documents—continues to reverberate. Meanwhile both Winter and hotshot young scientist Charlie Isaacs struggle with their respective scientific problems: Winter, to try and prove his implosion model for the atomic bomb works, and Isaacs, to solve a difficult task posed to him by Thin Man project boss Reed Akley. Their kind of science isn’t taking place in a world of bubbling beakers and giant cyclotrons: The focal point of Isaacs’ and Winter’s workspaces are the blackboards covered with formulas. But the math you see on the show isn’t going to be famous formulas; it’s the brute work of physicists trying to spin broad theories and principles into hard, engineerable schematics. Familiar equations like E=mc2 “would not really be on a blackboard in a working environment,” says David Saltzberg, Manhattan’s physics consultant (and a professor at the University of California, Los Angeles). A lot of the math seen onscreen in Manhattan is depicting the intermediate calculations for projects in progress. Many of the equations will also be approximations, not perfect solutions. “This was a common technique used by the theorists in the Manhattan project—they would solve the problem approximately to get an answer that is good enough, no need to be exact, and move on quickly.” So what sorts of things would Isaacs and other scientists need to be calculating? Lots of different aspects of the bomb, according to Saltzberg: like the mass of plutonium needed to create the explosion, how the density of the nuclear fuel changes with pressure as two pieces of a Thin Man bomb (the gun-type model that Isaacs is working on in Dr. Akley’s group) come together, and how the fission reaction is likely to proceed depending on various features of the bomb’s design. One specific problem that Akley directs Isaacs to solve, and which seems unsolvable, is figuring out the velocity distribution of neutrons. “The velocity distribution of neutrons is a critical part of understanding if your bomb will work and how big it needs to be,” says Saltzberg. “Every time a neutron induces fission of plutonium it releases 2 or 3 more neutrons. You need at least one of these neutrons to induce another fission to have a chain reaction. Moreover, you actually need even more than one of them to induce another fission in order to have a chain reaction that grows exponentially with time– as in the bomb.” In order to model how the fission chain reaction in the bomb would progress, the scientists had to figure out how probable it was that fission would occur when a neutron passes by the nucleus of a plutonium atom. That probability would be strongly determined by the velocity distribution of the neutrons released by the previous fission event, says Saltzberg. “Knowing the velocity of every neutron, and whether they are near the edges where they can escape, hence the distribution as a function of radius, would be something that they would have to know, but also very difficult to calculate,” Saltzberg says. Think of it like a complicated arrangement of pool balls on a table: you’ve got lots of clusters of balls set up, and you want to hit a cue ball at one cluster such that other balls will fly off and strike other clusters, that strike other clusters, and so on and so forth, continuing for a long time. Knowing the math behind the movements of the billiard balls informs you how best to set up the table, and where and how hard to hit that first ball. The difference is, though, that on a pool table, the speed of the balls drops because energy is conserved; inside a nuclear bomb, each fission reaction (akin to a new cluster of balls breaking up) adds energy to the system. And while the scientists can’t control the arrangement of plutonium atoms they way you can arrange pool balls on a table, they can figure out how much fissile material they need and if that fuel needs to be shaped differently to increase the likelihood of an exponential chain reaction. Another big moment this episode is Isaacs’ talking-to from Akley, who reminds his protégé that the Nazis are working on their own nuclear gadget, headed by one of the greatest minds in physics: Werner Heisenberg. “It was inevitable that theoretical physicists who saw deeply into the atom” would also see the potential for an atomic bomb, says Robert Norris, a nuclear expert with the Federation of American Scientists. And nuclear fission was first discovered in a Berlin laboratory in 1938, when German scientists Otto Hahn and Fritz Strassmann were amazed to discover that bombarding uranium with neutrons left them with much lighter elements than they expected. Manhattan has shown the messy work of taking theory off of the chalkboards and bringing it out into the real world. It’s a process with an astounding array of variables—from producing the fuel, to knowing how much plutonium or uranium to use, to figuring out how to bring the critical mass together—and understanding every facet of those variables proved essential to the greater effort. If the Nazis started with the same understanding of physics, how did America beat them to the bomb? Clearly, the human element—one of the most volatile in the equation, as the show depicts—is a key factor. And Isaacs is one of the characters with the most potential energy in the show, clearly foreshadowed as a future convert to Winter’s implosion design. But what will ultimately break Isaacs off from the Thin Man group? We’re eager to find out. By: Roxanne Palmer
Posted on: Wed, 24 Dec 2014 09:30:46 +0000
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