Interesting example of word evolution...they are of some origin and are used interchangeably (comparative of the word 'far'). In the US farther is used often to denote physical distance as you mentioned (no so in other areas) . There are some specific cases only further can be used, such as meaning 'more, extra, additional'. Good reference here: http://dictionary.cambridge.org/us/grammar/british-grammar/farther-farthest-or-further-furthest
Great post and looking forward to more!