People who played with the Axe were enthusiastic about it. But without a demonstration, it was hard to explain what the system did--and thus hard to promote it. After the Axe, Harmonix created other products, such as the music games Frequency and Amplitude; they got good reviews but never took off. The company struggled to make its products appealing to a general audience.

Success didn't come until Harmonix settled on simulating the experience of being a rock star. Before they knew anything about how Guitar Hero would work, Egozy says, he and ­Rigopulos knew that the plastic guitar needed a whammy bar. This lever, which varies string tension and produces a vibrato sound on a real electric guitar, is a key part of most people's air-guitar fantasies. Egozy and Rigopulos also knew that the game needed to make people feel as if they were onstage, being Jimi Hendrix or Stevie Ray Vaughan. It needed to give them that feeling right away, without forcing them to learn a complicated system first. By designing a game that can adjust to players' skill levels and convince them that they're in control, Harmonix found the right balance between geek credibility and accessibility.

"The thing that ends up becoming successful, you only figure out in retrospect," Egozy says. "The experiments that ended up not working too well were the ones [such as the Axe] that were a little too out there. Maybe you have to start with some things that are out there and then bring them back home. I think we did that."

But Harmonix isn't resting on its laurels. It has already announced plans for a new game based on the music of the ­Beatles; the founders are intent on creating a completely new game, not simply another version of Rock Band. And that quest to keep ­reinventing itself keeps Harmonix true to its experimental roots.

Egozy says, "The thing that MIT gives you--for a long time, I didn't know that it was there--is this sort of drive to innovate, and this notion of not wanting to copy. It's just this innovation bug that MIT really instills in you."