During our second Ars Live event earlier this month, screenwriter/producer Ed Solomon (Bill & Ted franchise) joined physicists Sean Carroll (Johns Hopkins University) and Jim Kakalios (University of Minnesota) and Ars Senior Reporter Jennifer Ouellette for a rousing discussion on the science and logic of time-travel movies. The discussion was inspired by last fall's Ars Guide to Time Travel in the Movies, written with the objective of helping us all make better, more informed decisions when it comes to choosing our time-travel movie fare—and having a bit of fun while doing so. You'll find the entire discussion in the video above, complete with a transcript.

Not all time-travel movies are created equal. Some make for fantastic entertainment, but the time travel makes no scientific or logical sense, while others might err in the opposite direction, sacrificing good storytelling in the interest of technical accuracy. The best strike a good balance between those two extremes.

We started off by letting Carroll recap his fundamental rules for time travel in the movies: (1) You can't go back earlier than whenever the time machine you're using was built; (2) it's easy to travel to the future, and special and general relativity give us ways to get to the future faster; (3) it may or may not be possible to travel to the past BUT.... (4) if you do, you can't change the past. Whatever happened, happened.

Kakalios had slightly less stringent criteria. "I don't sweat too much of the physics as long as it doesn't do anything so egregious that it takes me out of the story," he said. "I don't go to the theater with a pen and paper going, 'Ooh, my physics sense is tingling.'" Also, the shorter the trip back in time, "the easier it is to make the movie on a really small budget."

As for Solomon, he drew a distinction between physics in the real world and the physics of the film itself. "All works of fiction in my mind have their own rules and their own internal physics," he said, and a good screenwriter won't break those internal rules—at least not beyond a couple of "buys," which is Hollywood lingo for the elements you just ask the audience to accept for the sake of the story. And he offered a corollary: If you're making a comedy, the jokes have to be consistent with the rules and overall tenor of the film as well. "It's like a key signature," said Solomon. "You might not be able to listen to a symphony and say that it's in E minor, but you'll know if a note is wrong."

We didn't cover time-loop movies in the Ars Guide because it's a subgenre all its own, but per Carroll, the fact that there's always at least one person who remembers the prior loops makes such films intrinsically illogical. Still, Groundhog Day and its many successors—Palm Springs was a recent favorite among the panelists—succeed because they treat that illogical element as a given (the "buy") and otherwise follow their own internal rules. The consequences, at least, make sense.

There's plenty more to pique your interest. Give it a listen to hear why Carroll credits Bill & Ted's Excellent Adventure for the best use of time travel in a movie; why Tommy Lee Jones called Solomon an "a-hole" during the writing of Men in Black; Carroll and Ouellette's failed pitch for a time-traveling detective TV show; Carroll's advice for science consultants in Hollywood; why Primer is overrated both as a movie and for its time-travel physics; and Kakalios' spirited defense of how Superman was able to go back in time by reversing the Earth's rotation in 1978's Superman. (It involves being able to control inertia, space/time curvature, and essentially creating a "Tippler cylinder.")

Listing image by Aurich Lawson/Getty Images