# Distinction between forces by contact and forces in distance?

1. Jan 11, 2005

### ramollari

Is there any real distinction between forces by contact and forces in distance?

2. Jan 11, 2005

### marlon

Well, the way the are described. I mean, the mathematical tool that is used in order to construct physical theories. You are talking about action at a distance and local action right. Well, for example, Coulomb's law can be seen as an action at a distance, meaning that two charges interact with one and other when there is a certain distance between them AND the interaction actually depends on this interparticle distance. If you look at the Lorentz-law, the EM field ONLY interacts with some charge just right at the position of the charge. This is local-action. The interaction takes place at the very position of the charge and nowhere else. The EM-field is spread over the entire space if you will and you just plug in the charges that will interact with this field. The latter example will be described in terms of FIELDS used to express the fact that the EM-radiation is everywhere AND the fact of the local action. All known field theories (QED, QHD, QCD) work like this...

regards
marlon