Parallel universes (Stargate)

Parallel Universes are differing dimensions in which details are changed, minorly or majorly, from the original universe. They play an important role in the Stargate universe and often act as a catalyst for major points in the shows, Stargate SG-1 and Stargate Atlantis. Daniel Jackson first crosses into one using a quantum mirror, a device which transports an individual to a set parallel universe.

The concept of parallel universes was first established when Daniel Jackson is separated from the rest of SG-1 when he accidentally triggers the quantum mirror in the episode "There but for the Grace of God". Subtle differences are noted, such as changes of rank of common characters, and names, such as the change of Stargate Command to Stargate Administration. Daniel is later transported home when he figures out how to operate the controller for the mirror and returns with information of the impending Goa'uld attack.

In the episode "Point of View" an alternate Samantha Carter and Charles Kawalsky come through the quantum mirror in hopes of finding help against Apophis' attack on the alternate earth.

Multiple SG-1 teams start showing up through the stargate in episode "Ripple Effect" it doesn't take long for the SGC to figure out they are from Parallel Universes.