That's not fixed at all, that's an opinion; the dictionary (several) says either is fine.
There is no final, undisputed arbiter of what is right and proper in English, no Academy of English, no written law. Maybe there is consensus within a social group, nation, or profession, but a whole lot of these things are not provable.
Less/fewer is another one where people pipe up and claim one is "correct" (again, according to whom?), when in actual fact, either is fine and no-one cares and it's not provable either way.
"Farther and further have been used more or less interchangeably throughout most of their history,"
"""
A polarizing process appears to be taking place in their adjective use. Farther is taking over the meaning of distance
the farther shore
and further the meaning of addition.
needed no further invitation
"""
That, of course, means all of this is new and in no way "correct" by any current modern standard. It's just something quasi-literate internauts like to quip in the comments. Oh, and university style guide authors and editors who consider themselves authoritative, but to whom no-one except community college English teachers defer.
Maybe I'm a little bit of a linguistic anarchist, I don't know.
There is no final, undisputed arbiter of what is right and proper in English, no Academy of English, no written law. Maybe there is consensus within a social group, nation, or profession, but a whole lot of these things are not provable.
Less/fewer is another one where people pipe up and claim one is "correct" (again, according to whom?), when in actual fact, either is fine and no-one cares and it's not provable either way.