The Primer tagline: “What happens if it actually works?” You’ll notice the “it” isn’t specified. More appropriate question: Wouldn’t it be cool to make a time-machine movie that never utters the words “time machine”? Shane Carruth’s feature debut, about two young scientists who inadvertently invent the aforementioned “it” in a garage, and run afoul of causality and morality, gives absolutely none of itself away–and wants congratulations for its expository asceticism. OK, Carruth is impressively frugal: He wrote, directed, starred in, shot, designed, produced, edited and soundtracked the thing (and did a commentary track for the DVD) for about $7,000. The filmmaker, formerly an engineer, has an ear for scientists’ chatter — without, mercifully, a jargon dependency. Whether it pays off depends on what sort of fanboy you are. Primer has only one special effect: It messes with your head.