Ready for some mind-bending musical physics to take you into the weekend? A Capella Science is back with a new parody video, “Entropic Time,” set to Billy Joel’s classic pop hit, “The Longest Time.” The twist: if you look closely, the video footage is running in reverse—a visual play on the subject of the song.

Tim Blais, the mastermind behind A Capella Science, said on Twitter that he based the lyrics on this lecture about the arrow of time and the origins of the universe by Caltech physicist Sean M. Carroll. Since I’m married to Carroll, I’ve heard a lot over the years about the subject.


Why does time run forward and not backward? Blame the second law of thermodynamics. The universe has a general tendency toward disorder and decay, a phenomenon known as entropy. Entropy dictates an arrow of time that runs only in one direction: forward. Heat only flows in one direction, so a cooler body can’t pass heat to a hotter one, any more than a melted ice cube can refreeze of its own accord. Time, therefore, is what physicists call an “irreversible process.”

But mathematically, things are more complicated. Every major physics theory exhibits something called “time-reversal symmetry” in its equations. In other words, the value for time (T) can be either positive or negative. The equations allow for the possibility that time can flow forward or backward, even if real-world physics precludes it because of entropy. The math says there is no uniform direction in which time must always flow—in direct opposition to our daily experience.


That’s the arrow of time, or entropic time. Why is there such an arrow? It’s because the entropy of the universe is higher today than it was yesterday. And the entropy yesterday was higher than the entropy the day before, and so on, all the way back to the birth of our universe. As for why our universe started out in such a low-entropy state in the first place—well, that’s a question that physicists like Carroll continue to grapple with.

Blais also has a behind-the-scenes video showing just how they achieved the time-reversal effects in “Entropic Time.” Enjoy!