Invisible Forces
================

Since the first time someone saw a lodestone stick to an iron sword as if by magic, we humans have marveled at "invisible forces".  What we usually mean by that, of course, is UNFAMILIAR forces.  Gravity is every bit as "invisible" as magnetism, but we are used to it and think we understand it.  In truth, gravity is no longer considered a "force" at all by those who know it best.  Perhaps this will someday be true of ALL the "forces" we teach our Physics students about -- that would be elegant!  But for the time being we still use the term "force" to describe most known interactions.  

IMNERHO, there is no such thing as a "visible" force.  We imagine that our feet pushing on the ground is a "direct" force; but that is only because we cannot see the electrons and nuclei attracting or repelling each other at a distance, much as the magnet under the table causes the iron ball bearing on top of the table to move mysteriously.  That microscopic (nay, nanoscopic) world is "out of sight" by virtue of being too small to see, and therefore pragmatically "invisible".  Physics shows us that the electrons are moved by electromagnetism: "action at a distance" (albeit no longer "spooky") just like Newton's gravity.  The same is true (albeit on an even smaller scale) of the weak and strong nuclear "forces", which are "carried" by virtual vector bosons and gluons, respectively, much as electromagnetic forces are "carried" by virtual photons of light. 

One can argue equally well that, because virtual "force carriers" are emitted and absorbed by the particles they bind or repel, ALL such "forces" are truly "direct", in which case there is no such thing as an "invisible force" except in the sense that the "vertices" where the creation and destruction take place are too small to ever "see" directly, although we keep trying with more and more powerful accelerators.  

Let us then be content to use the term "invisible"in the colloquial sense: we can't see what's going on with our naked eyes.  

This still allows us to put some pretty weird stuff into that category: Higgs "forces", the undiscovered interactions of Dark Matter, the pressure of Dark Energy and the theoretical actions of 26-dimensional "strings", to name a few.