“Don’t try to understand it,” a scientist tells the protagonist of Tenet, as she briefly explains the physics of Christopher Nolan’s $205-million, time-traveling spy thriller. Sure, the physics is often unrealistic and confusing, but it’s fascinating. And with its many Easter eggs, Tenet...

If you are familiar with the laws of thermodynamics, you may recognize the second law as dealing with entropy. In the realm of physics, entropy represents the degree of disorder in a system. Because systems tend to degrade over time, thermodynamic energy becomes less available to do mechanical...