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> Arrival is arguably science fantasy since it's central conceit isn't very sciencey

Read the original short story: the actual premise is actually very science-y, and completely "true sci-fi." Don't want to spoil it here though.

> though everything _except_ that is very hard sci fi.

Yeah, personally I think sci-fi (rather than sci-fantasy) is allowed to break a single physical law and still be considered sci-fi, as long as it's done consistently and the consequences properly explored.

> basically a magic substance

Is it though? Sufficiently advanced technology, after all, is indistinguishable from magic...

Personally I like James Nicoll's definition of hard science fiction: it's hard SF if the author gives enough details that the reader can be _certain_ it wouldn't work.