Farther vs. further…which is right?
Short Answer
Farther describes physical distance, while further describes figurative distance. In some cases, when the distinction between physical and figurative is unclear, both are correct and can be used interchangeably.
Long Answer
Farther is used to describe physical distance, such as from New York to Boston or from the couch to the door.
For example: Jake’s mailbox is farther away from his house than my mailbox is from mine.
Further is used to describe figurative distance, such as from one time period to another or from the current amount of information one has to a different, greater amount.
For example: We hoped to look into the matter further. The professor further reviewed his thesis and deemed it worthy.
In cases where the distinction between physical and figurative distances is not clear, there is no right or wrong answer (good to know, right? We thought so!).
For example: We grew further/farther apart with each passing day.
Just can’t get enough? 
