Does "rather or not" mean the same thing as "whether or not?"
I am being driven to distraction by someone who writes "rather or not" when I think it should be "whether or not."
You're right. "Rather" is an adverb, meaning with preference for one of two things, or more willingly. "Whether" is a conjunction with several meanings, but the two words are not synonyms.
We might say, "I'd rather not have the seafood platter," meaning we'd prefer not to have it. But we'd never say "Rather OR not."
We can also use it to mean more or less, or somewhat: "He's polite, but rather reserved." The British use it as a strong way to say "yes": "Is she good-looking? Rather!" And we can use it to introduce a contradiction: "He isn't honest; rather, he's a swindler."
"Whether" can be used when we are setting out alternatives: "Whether or not it rains, we're going for a hike." We can also use it as a synonym for "either": "Whether by skill or luck, he reached the top of the mountain." And we also use it in indirect questions when we expect a negative answer: "Please tell me whether you've accept my novel."