TV’s highest-rated scripted show, which is all about science, has proven a crucial law of nature.

Opposites attract.

The stoic Gil Grissom (William Peterson) and the more emotional Sara Sidle (Jorja Fox) have become a couple on “CSI: Crime Scene Investigation” (9 p.m. Thursdays on CBS, Channel 62 - currently in reruns). But it’s no surprise to executive producer Carol Mendelsohn, who has seen this coming for a long time.

Mendelsohn says the writers have debated since the first season whether Grissom and Sidle, two forensic investigators, were ever together. “I happen to think that they were intimate in San Francisco (where they worked together), but not intimate once they got to Las Vegas, at least not in the beginning. And since I’m the showrunner, you know, I won the debate.” The romance comes as “CSI” faces some competition. “Grey’s Anatomy,” a popular medical drama with its share of love, will move this fall to 9 p.m. Thursdays on ABC.

Fox says the beauty of a popular long-running series is the plots that the actors can’t predict. “I think Gary (Dourdan) might not have had any idea that (his character, Warrick Brown) would be married. Or I don’t think George (Eads) had any idea that he would spend 48 hours in a box (in the episode in which his character, Nick Stokes, is buried alive). You know, you just get this little description and you take this journey.”

Fox says she liked how the writers ended last season by showing Grissom and Sidle suddenly together in a romantic relationship. “And we don’t know how long this has been going on. And I think that was a really brilliant way to do it.

“I think the idea right now going into season seven is that Grissom and Sara and the beautiful audience are the only people who know this is actually taking place,” Fox says.

Fox says viewers are split 50-50 about whether “CSI” should focus strictly on its murder stories or allow some character-driven tales, such as the new love story. “And I think the writers did something bold and brilliant by trying to just really follow their hearts, which is what I think we try to do.”