In late 2009, a writer, a producer, a director , and three scientists sat in a Los Angeles conference room. They were discussing Marvel's Thor—a film based on a comic book that was in turn inspired by the Norse god of thunder—about an arrogant warrior who, at the start of the film, violates a truce by attacking the Frost Giants. As the film team described their vision of the fight, Sean Carroll, a theoretical physicist at the California Institute of Technology, knew the filmmakers had a problem. "They wanted the Frost Giants to fall off the edge of a disc-shaped planet," he says. "That makes no sense. Where does the gravity to pull them down come from? Enough people know how gravity works it would throw them out of the movie. You'd get a lot of giggles." Carroll and the other scientists argued their point, even though, Carroll says, "it was clear some people thought we were being uptight killjoys."



But producer Kevin Feige sided with the scientists, and in the final cut, the Frost Giants' planet was spherical. That was just one way that Carroll, a clean-cut 45-year-old who has advised on films such as TRON: Legacy and the TV show Bones, helped the production. As punishment for breaking the truce, Thor is exiled to earth. When Feige complained that using the term wormhole for Thor's passageway to our planet was "too '90s," Carroll suggested the scientific name for the phenomenon, the Einstein-Rosen bridge. That explanation is given by Natalie Portman's character, astrophysicist Jane Foster, whose motivations Carroll helped shape.



Scientists have been helping Hollywood since the start of cinema. But as science-fiction movies account for more revenue—in the '90s, an average of six a year were in the top 50 moneymakers; that number increased by nearly 50 percent in the first decade of the 2000s—filmmakers are turning more frequently to experts for ideas. "The more you ground your film in the real thing, the better it plays," says D.J. Gugenheim, VP of production at Inferno Entertainment. Scientists are willing to help Hollywood because they see a chance to expose a broader audience to science and humanize their profession. "People get images of what science is from movies," Carroll says. "I want to help get that image right."



To improve the information flow between the science community and Hollywood, the National Academy of Sciences launched the Los Angeles–based Science & Entertainment Exchange in 2008. The organization connects film- makers with scientists in biology, chemistry, and other fields. In its first year the Exchange's scientists consulted (for free) on 70 projects; by September 2011, on 350. Creators of TV shows such as Fringe and The Big Bang Theory, and films like Green Lantern and 2012's Battleship, have all talked to scientists with the Exchange.



But science and entertainment don't always mix. "Story creators might think scientists are geeks, but there's a sense of respect," says Malcolm MacIver, associate professor of biomedical engineering at Northwestern University and adviser on TRON: Legacy. "That respect is not always there in the other direction. Scientists feel that filmmakers dumb down everything to make a buck." Despite holding scientists in high esteem, some filmmakers find them hard to work with. "Scientists say, ‘No, you can't do that!'" Carroll says. "And the moviemaker finds that unhelpful."



These days, audiences are savvier than ever. And thanks to the Internet, there's little they haven't seen—so filmmakers look to what's happening in cutting-edge research. "Scientists are more imaginative than we are in Hollywood," says Jeffrey Silver, producer of Terminator Salvation and 300. "I used to say, that only happens in the movies, but now I say, that only happens in science."



The average moviegoer is also less willing to suspend disbelief. "If people see a movie and sense a disconnect between the logic of the movie and the science that governs the world of the film," Gugenheim says, "you risk turning off the audience." Viewers take their complaints to the Inter- net, where they spread faster than a zombie virus. "Advisers help you construct the movie with rules that keep you in the realm of what is theoretically plausible," Gugenheim says. That's what makes films feel real—and prevents bad word-of- mouth that could cripple box office.



Scientists are more concerned that inaccuracies will harm scientific literacy. In The Day After Tomorrow, a man-made ice age occurred in just a week. It would actually take at least a decade for the real thing to set in. And when scientists in K-19: The Widowmaker worried that a nuclear reactor would explode, it spread a dangerous notion: Damaged reactors don't explode, they melt.



Most scientists are willing to advise not only because it allows them to be gate- keepers of their disciplines, but because they want to be portrayed accurately on-screen. "It's rare that you have a relatable character," says Sheril Kirshenbaum, a research associate at the Center for International Energy and Environmental Policy at the University of Texas at Austin. That's why James Cameron created Avatar's xenobotanist, Grace. "Scientists are usually shown as geeks or losers or evil," he says. "I wanted to celebrate the mind and the passion of a scientist."



Working in Hollywood can be an educational experience for novice advisers, as Carroll discovered during his first consulting gig, on Ron Howard's Angels & Demons. In the film, Professor Robert Langdon tries to find antimatter stolen from CERN's Large Hadron Collider. It's a fact that when antimatter and matter come into contact, they annihilate each other in a violent explosion. What, Howard wondered, would it look like if that explosion occurred in the sky? Carroll suggested a series of rapid booms caused by air rushing into the vacuum created by the explosion.



But then the 2007 Writers Guild strike derailed both the production and his consultations. "They were over budget and behind schedule, and we didn't talk anymore," Carroll says. "That's Hollywood. I was pleasantly surprised by the intellectual curiosity of those involved, but disillusioned that you can't always do it right." Angels & Demons eventually hit theaters in 2009. (Carroll's contribution, he says, looked "more or less" as he advised.)



Often filmmakers ignore a scientist's advice. When paleontologist Robert T. Bakker worked on Jurassic Park, he found the dinosaur artists to be "better animal morphologists than most tenured professors." But when he sent the film team diagrams of the T. rex's banana-shaped crowns, "the powers that be didn't like the real tooth shape," he says. "The CGI rex and the robot had their fangs sharpened."



Filmmakers defend their creative license; their first responsibility, they say, is to entertain. For 2012, director Roland Emmerich wanted an impossible global flood. "There isn't enough water on earth for that," he admits, "so you have to figure out something." Emmerich asked a geologist to work from the 1950s theory of earth-crust displacement. "He said, ‘This could never happen.' And we said, ‘Well, if it did happen, how would it work?'" Silver often talks to advisers, but even he says that "if [a story] doesn't break a fundamental law of physics, then it doesn't matter how far you stretch it."



Ultimately, advisers understand they're not creating award-winning research. "You have to accept that the goal is to tell a story first," says Kevin Hand, a planetary scientist at NASA's Jet Propulsion Laboratory. Science & Entertainment Exchange director Marty Perreault agrees: "We're not the science police." They also realize that these films could get young viewers interested in science. "I can write a book where I explain real physics and reach several thousand people," Carroll says, "or I can help create Natalie Portman's character in a movie that will reach 10 million people. And some will be young girls who see that Natalie Portman's playing a scientist."



Now that Carroll's done with Thor, he's moved on to Doctor Strange, about a surgeon who becomes earth's Sorcerer Supreme. Carroll's job is to apply limits to Strange's powers. "You need constraints to provide tension," he says. A world where anything can happen makes for a very boring movie. It's when science imposes boundaries on what a superhero can do that the real drama begins.