The Loki TV series, released this past summer, revealed that anytime a Marvel character makes a decision, a new “branch” of reality is created for each possible outcome. To understand the science behind Marvel’s branching timelines, you need to think about the multiverse on a much smaller scale. This field of physics is called quantum theory. It describes the behavior of extremely small systems, made up of subatomic particles smaller than atoms. (Think about the movie Ant Man, where characters shrink down until they’re so small they enter the “Quantum Realm.”)
In our universe, all large-scale physical interactions are governed by forces that affect an object’s motion in predictable ways (like the pull of gravity). Consider throwing a baseball, says Muir. If we know how hard the ball was thrown, we can use that information to predict where the ball is going, how fast it’s moving, and when it will get there. But describing the behavior of electrons, subatomic particles with a negative charge, or photons, the smallest particles of light, is different—and much stranger.