The Dimension Theory stated that for every decision made, the alternative was played out in another dimension. (RD: Dimension Jump) This could be seen as a fifth dimension or co-existing reality, where objects shared the same space but were unaware of each other's presence. Arnold Rimmer suggested that it sounded like his parents in bed. (RD: Parallel Universe) Dimensions were synomonous with parallel universes or alternative realities, although it is unknown whether the seven main universes at the Omnizone counted as such.
Several methods have been seen for crossing dimensions.
- The Holly Hop Drive was a transport device that malfunctioned, throwing Red Dwarf into a parallel universe where the crew met their female opposites, or in the Cat's case, a Dog. (RD: Parallel Universe)
- Ace Rimmer's ship, Wildfire, was designed to break the speed of reality. This ship crossed many realities and Ace met many versions of himself (RD: Dimension Jump)
- A dimensional bridge that passed through non-space was created by a spatial anomaly that hit Starbug. (RD: Ouroboros)
- Flying into a black hole could lead a ship to the Omnizone, a place where dimensions converged. (Better Than Life)