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Part 3 of 3 Parts (Please read Part 1 and Part 2 first)
      The evolution of nuclear weapons development can be divided into generations based on the personal experience of researchers. Some designers were familiar with all physical testing of nuclear weapons, some were familiar only with underground testing, some had only a passing acquaintance with physical nuclear detonations and some had only experienced simulations of nuclear explosions. Some designers were trained to program their own simulation code from scratch and others just borrowed code from other programmers. In addition to experience with physical nuclear detonations and writing simulation code there is also the question of the cultures of the individual weapons laboratories that must be taken into account.
     At first, the senior scientists at Lawrence Livermore were very hostile to simulations of nuclear weapons. Then the laboratory adopted a new metaphor for weapons design that compared weapons design to the design for a bridge. According to this approach, designers do not actually have to physically test all the parts of a bridge before they build the bridge. The Lawrence Livermore designers became confident that successful simulations of the weapons they were working on were all that was needed to construct the actual weapons.
     At Los Alamos there are young scientists who are critics of immersive virtual reality displays. One of them said, “I was so attuned to making plots on my computer screen. I was surprised at how little new I learned from [the RAVE]. (The RAVE is the nickname for Los Alamos’s virtual CAVE technology.) He went on to say that he was not able to work analytically in the RAVE. Other young designers say that they get disoriented in the RAVE. Inside the RAVE, the designers work in a closed system that has very rigorous internal consistency, but it is not always so easy to decide what aspect of the simulation is most relevant to the real world.
     Across scientific and engineering disciplines, scientists and engineers have widely praised the advantages of simulations. Devices, buildings, drugs and other technological marvels would never have existed at all if it weren’t for powerful simulation systems. However, many researchers who have access to these simulation systems also talk about the anxiety of “reality blur” which is described as the “breaking point” where the user loses a sense of anchors with no real-world referents and precedence. Another problem is that the incredible complexity of today’s simulations can make it very difficult to test their veracity. Luft said, “You just can’t check every differential equation. You just can’t, there are just too many. In nuclear weapons design you can make sure that you have solved equations correctly and that your system has internal consistency. In other words, you can “verify.” Validation is the hard part. That is, are you solving the right equations? Proof is not an option.”
     There is an old saying that while there is no difference between theory and practice in theory, in practice, there is. The real world is more complex that any possible simulation. Developing systems to be constructed and used in the real world from simulation will only work if all the relevant information about the environment and the materials is known during the design process. Failure to take into account all that must be included in the design process might not become apparent until the catastrophic failure of the real-world artifact.