  The energy for protein folding arises from multiple sources and is not large
in total. In spite of the many specific successes of energy landscape and other
approaches, there still seems to be some missing guiding factor that explains
how energy from diverse small sources can drive a complex molecule to a unique
state. We explore the possibility that the missing factor is in the geometry.
  A comparison of folding with other physical phenomena, together with analytic
modeling of a molecule, led us to analyze the physics of optical caustic
formation and of folding behavior side-by-side. The physics of folding and
caustics is ostensibly very different but there are several strong parallels.
This comparison emphasizes the mathematical similarity and also identifies
differences. Since the 1970's, the physics of optical caustics has been
developed to a very high degree of mathematical sophistication using
catastrophe theory. That kind of quantitative application of catastrophe theory
has not previously been applied to folding nor have the points of similarity
with optics been identified or exploited.
  A putative underlying physical link between caustics and folding is a torsion
wave of non-constant wave speed, propagating on the dihedral angles and $\Psi$
found in an analytical model of the molecule.
  Regardless of whether we have correctly identified an underlying link, the
analogy between caustic formation and folding is strong and the parallels (and
differences) in the physics are useful.
