Hello! Im hoping people can answer this for me, as ive not been able to find the answer anywhere else: As far as I understand it (and please correct me if im wrong), the strong force is caused by the transfer of a gluon from one quark to another, electromagnetism by the transfer of photons and so. My problem is that I can't find anywhere why this transfer actually causes them to move closer together / attract. And with electromagnetism sometimes repel. Help!