Stephen Hawking does ‘Big Bang Theory’ theme song at Comic-Con

Move over, Sheldon Cooper! There's a real theoretical physicist in the room! Stephen Hawking made a surprise appearance at Friday's "Big Bang Theory" panel at San Diego Comic-Con, kicking off the event with a pre-recorded video segment taped in his office.

The world-renowned physicist idolized by Jim Parsons' Sheldon apologized to fans for not being there in person. "Sorry I couldn't be there. I got a flat tire," he said. "That was a joke. Ha ha!"

The brilliant man, who has made a guest appearance on the sitcom, also shared a little bit about what he likes to do in his free time.

"When I'm not playing Words With Friends with Sheldon, I like to think about the universe," he revealed. And then it got really good: Hawking decided to explain the Big Bang theory in 17 words by "singing" "The Big Bang Theory's" theme song.

Hawking's appearance lent some star power to the room, as most of the cast wasn't at the event. CBS instead focused the session on the show's writers room. The panelists (including writer-producers Steve Molaro, Bill Prady, Eric Kaplan, Jim Reynolds, Steve Holland and Maria Ferrari, plus scientific consultant David Saltzberg) had barely taken their seats when Melissa Rauch, who plays microbiologist Bernadette, took the podium as moderator.

The scribes then shared some fun tidbits: a writer's own terrible wedding night was the real-life basis of Bernadette and Howard's failure to consummate their marriage, as was Amy's exaggerated illness to get some action with Sheldon.

The biggest surprise, however, came when Rauch took questions from the audience. "Reveal yourself!" she demanded of an unintelligible fan outfitted head to toe in a bounty-hunter costume from "Return of the Jedi."

Boushh the bounty hunter complied — and thousands of fans gasped to see the face of star Johnny Galecki.

"Who are you?" she asked, to which he replied with an iconic line: "Someone who loves you."

Other highlights after Galecki took a seat at the panel:

  • When season seven premieres, "we will see Leonard on the ship" conducting research with Stephen Hawking.

  • Bernadette and Amy will take their first trip sans Penny.

  • "Coitus" between Amy and Sheldon is "a possibility," with the writers acknowledging that "Amy will continue trying to get her way." But Reynolds noted that "some of the charm is their glacial pace." Even better, from his perspective, "It makes the show last longer!"

  • "It always baffles me how much Amy has gotten Sheldon to move," added Prady. "With Sheldon you have a character who comes from a place of, 'No, I'm not going to change.' It's a lot of interesting work in the writers room (proposing) 'What if Amy did this?'"

  • When an attorney took the mic to point out a legal error, the EPs countered: "You have just blown the series finale. It's a little like 'Breaking Bad': We've planted some seeds and that's one of them."

  • Galecki praised the panelists for "writing Penny as a talented actress." And pointed out that her career trajectory "is moving at the speed of the career of my friends who are actors."

"The Big Bang Theory" returns for season seven on Sept. 26.