In relativity there are invariants, things which all observers agree on the value of. Then there are quantities and vectors that transform under SR....
I used this example because they are the simplest proof that a stratified gas can manifest transverse waves. They don't involve surfaces or anything...
The momentum only interactions are neutral current interactions (since neutrinos only really interact via the weak force). I had a quick read around...
At a guess it is simply the surface area of a sphere at the radius the strength of the forces was calculated. I can't find an open access version of...