The Standard Model - What holds it together? - The unseen effect
You can think about forces as being analogous to the following situation:
Two people are standing on an ice pond. One person moves their arm and is pushed backwards;
a moment later the other person grabs at an invisible object
and is driven backwards. Even though you cannot see a basketball, you can assume that one person threw
a basketball to the other person because you see its effect on the people. (Click on the checkmark or cross below the
animation in order to make the basketball appear or disappear.)
It turns out that all interactions which affect matter particles are due to an exchange of
force carrier particles, a different type of particle altogether. These particles are like
basketballs tossed between matter particles (which are like the basketball players).
What we normally think of as "forces" are actually the effects of force carrier particles
on matter particles.
The basketball animation is, of course, a very crude analogy since it can only explain
repulsive forces and gives no hint of how exchanging particles
can result in attractive forces.
We see examples of attractive forces in everyday life (such as magnets and
gravity), and so we generally take it for granted that an
object's presence can just affect another object. It is when we approach
the deeper question, "How can two objects affect one another without
touching?" that we propose that the invisible force could be an
exchange of force carrier particles. Particle physicists have found that we can
explain the force of one particle acting on another to INCREDIBLE
precision by the exchange of these force carrier particles.
One important thing to know about force carriers is that a particular force carrier particle can
only be absorbed or produced by a matter particle which is affected by that particular force. For instance,
electrons and protons have electric charge, so they can produce and absorb the electromagnetic
force carrier, the photon. Neutrinos, on the other hand, have no electric charge, so they cannot
absorb or produce photons.