An amazing post lifted from this great Atom Hoax thread
The Spherical Cow
A spherical cow is a metaphor for highly simplified scientific models of complex real life phenomena.
The phrase comes from a joke about theoretical physicists:
Milk production at a dairy farm was low, so the farmer wrote to the local university, asking for help from academia. A multidisciplinary team of professors was assembled, headed by a theoretical physicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader. Shortly thereafter the physicist returned to the farm, saying to the farmer “I have the solution, but it only works in the case of spherical cows in a vacuum.”
The point of the joke is that physicists will often reduce a problem to the simplest form they can imagine in order to make calculations more feasible, even though such simplification may hinder the model’s application to reality. The concept is well enough known that it can be referred to in scientific discourse without explanation.
The atom bomb hoax is a spherical cow. The physics was oversimplified to accomodate it’s application to reality.