By Rowan Hooper Science fiction thrillers usually send in gun-toting heroes like Will Smith or Tom Cruise to kick invading alien butt. Arrival is completely, wonderfully different: it sends in a linguist, played by Amy Adams. “Language,” one character says, “is the first weapon drawn in a conflict.” The big question to ask the aliens: what is their purpose on Earth? In Contact, the aliens used prime numbers as a Rosetta stone that could be used to decrypt their communication; in Close Encounters of the Third Kind they helpfully used…