Lisa Randall’s Quick Study piece in the July 2007 issue of Physics Today (page 80) makes very good reading. It reminded me of a private discussion I had a couple of years ago with a postdoc specializing in string theory, who had already published 10 papers on the subject. He started by explaining the idea of branes and the strings that connect them, and he made a drawing like the one in Randall’s article. I was immediately reminded of dislocations in solid-state physics, about which my colleague had no idea. Shower curtains might be a helpful model, at least for the geometric description. But dislocations offer a situation in the full context of crystal physics. The “gravitybrane” and the “weakbrane” are compared with two surfaces of the crystal, and the “bulk energy,” which is contained in the lattice between the two surfaces, depends on the lattice’s symmetry and elastic properties. The dislocation is a one-dimensional defect that connects the two surfaces and is essential for crystal growth as well as for plastic deformation, depending on its Burgers vector.
Nowadays young physicists don’t have a chance to learn things in unrelated areas. After getting a PhD in quantum field theory, I got a job at the geophysical laboratory of Shell Oil Co, where I worked from 1953 to 1960. My first task there was to learn something about dislocation theory, because the laboratory also did high-pressure experiments on plastic flow for minerals like calcite and dolomite. Such jumps between the specialties of physics do not happen anymore, and the blame belongs equally to university professors and industry leaders. It is up to the young PhDs to ask for such changes in their experience. All three groups might find those jumps helpful in the application of mathematics to physics.
