Actually specifying the "or not" is more correct. "Whether" is a conjunction expressing a choice. The meaning needs to be clear, whether to "go to the circus" or..... "go to the movies", "stay at home", or, most simply, "not go to the circus". It's only a freak of evolved usage that we insert the "or not" where we do anyway. Dropping it is, of course, proper, but, again, it is an evolved construction.