Still in Los Alamos trying to work out the calculation of critical mass for the atomic bomb -- which appears to be nothing more than a classic diffusion calculation.
For a spherical bomb, the mathematics resemble the diffusion problem of the sun pertaining to limb darkening. The sun doesn't have a crisp edge... no boundary marks a division between sun and empty space. So why do we see a boundary when we look up at the sun?
I think I'm going to go for a brisk hike down in Omega Canyon gorge later today. It's been a long week.