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.
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