In science fiction, we're often presented with "parallel universe" scenarios -- those in which the drama relies on there being more than one universe in which the action occurs. There are tons of examples, but a recent favorite is that from the dearly departed TV show Fringe (minor spoiler alert if you haven't seen past the first season!): the story involves at least two universes, in which parallel versions of events take place. Things get interesting when the universes collide and cross over, and we're often faced with fun things like the grandfather paradox, because if we're gonna have multiple universes we might as well throw in time travel too.

Whether you like science fiction or not, the question of whether parallel universes exist and, if so, how they work is an active question of science fact. In this five-minute video, Minute Physics reviews the three major models in which parallel universes might exist. If you've been wondering why your favorite sci-fi worlds operate differently, there might be some useful info in here for you. If you're a scientist looking for a super-hard problem to test, here's your ticket to fame. Enjoy: