we often come across the phrase superselection sector. I have couple of questions from that.
First of all, I learned from some references about it (as a overview) and how is it related to superselection rules etc. There is of course this mathematical definition (from C* algebra, unfortunately I haven't gone through that), but I wanted to understand it physically. It seemed to me that basically if I construct two sectors (two hilbert spaces?) whose basis vectors don't cohere, then they belong to two different superselection sectors. My first question will be : Is that it or there are more into it than meets the eye! Please.
Secondly, I encountered it in String theory. I will probably mention the place (I have an unrelated question here too!) where I encountered it in string theory. Its the argument using which we can guess the presence of D branes from SUGRA action. We see that in the low energy ST action, there can be a term (some p+1 form potential, properly coupled etc.) Now I saw this argument that "this term has finite tension and hence its not finite energy excitation above the vacuum" (why? and how did we guess about finite tension?- Is it because of the very fact that this term exists in the ST action?) and also its suggested that these objects (multi form potentials) are in different superselection sector. So again, please let me know if there is any general comment on superselection sector that you want to make in this context and also about the arguments.