>>10681442there are many classes of equivalence theorems that allow you to transform one problem into a potentially easier problem to model or solve, often at the expense of introducing something unphysical. these kind of theorems are good for both theoretical and practical applications.
for instance, in antenna engineering, one way to experimentally characterize an antenna's radiation pattern and predict it's field somewhere in space is to measure it's electric field on a plane, invoke Love's surface equivalence which introduces magnetic sources, apply image theory to get rid of electric sources because they don't radiate, and then apply green's function propagators to the (equivalent) magnetic sources floating in a homogeneous background, and derive the fields from those.
this idea isn't so weird. you are probably aware of image theory. to solve for the field of a charge over an infinite conducting plane, you can solve an equivalent problem (in the region of interest above the plane that is) by assuming there is a charge of opposite sign an equal distance below the plane.
