I have long been mystified by how particles act as force carriers. Apparently, the exchange of force-carrier particles (or virtual particles?) causes force action. OK, but how does this work? 1) Some forces cause attraction and some repulsion (and others decay). How can an exchange of the same type of particle cause both repulsion (e.g. for particles with a negative charge) and attraction (e.g. for a particle with a negative charge and a particle with a positive charge)? 2) How come photons carry both electric and magnetic forces? 3) If photons are electromagnetic force carriers, why do sources of photons (e.g. a domestic lamp or an X-ray machine) not cause magnetic/electric attraction or repulsion? I mean, shining a torch on an iron bar will not move it closer nor push it further away... 4) What's the deal with virtual particles and do they really exist if they are ' virtual'? How do they relate to real particles?