According to M-theory, which unites the five superstring theories, there are exactly 11 alternate spatial dimensions that exist in parallel with our own physical reality. By shifting in and out of phase, it is possible to experience these alternate dimensions in varying degrees by seeing, hearing, touching, or fully interacting with individuals or objects in another phase. This level of interaction is not always reciprocal, however. An individual may see or hear those in another phase while himself appearing to be invisible to those in a different dimension. Typically, objects that are out of phase will pass through each other.
SG-1 has frequently encountered alien races and technologies that have phase-shifting capabilities. The Reetou exist exactly 180° out of phase with Earth, making them invisible in our dimension, although they have the ability, to a limited degree, to function and to manipulate matter in our phase. The transphase eradication rod relies on Reetou emissions to make the Reetou visible in our phase. Among the Goa'uld, Hathor, Nirrti, and the Ashrak had experimented with technology that mimicked the phase-shifting abilities of the Reetou in order to develop a personal cloak of invisibility. Not all cloaks of invisibility rely on phase-shift technology, however. The cloaks used by most spacecraft, for example, make the ships invisible to the eye and to sensors, but objects do not pass through them. The Tollan used trinium-based phase-shift technology to enable individuals or objects to pass through solid matter without necessarily shifting into a dimension of invisibility. The crystal skull of P7X-377 had the ability to transport an individual into the alternate dimension occupied by the aliens of Quetzalcoatl's race. An Ancient device recovered from P9X-391 emitted energy particles that created a kind of bleed-through effect giving anyone exposed the ability to see insect-like creatures from an alternate dimension. The Sodan also used Ancient technology that created a personal invisibility cloak by shifting an individual into an alternate phase. SG-1's adjustments to the Sodan cloak resulted in a bleed-through effect that allowed inter-dimensional parasites entering the cloaking field to cross into our dimension. Ancient technology similar to the Sodan cloak was used by Merlin to develop his phase-shift device which enabled him to hide his research by shifting his work out of phase. Individuals using the technologies of Merlin's phase-shift device and the Sodan cloak appear to be invisible to our dimension, but they are able to see, hear, and communicate with each other, however there is no physical interaction between the two dimensions.
M-theory is not to be confused with the multiverse theory which posits the existence of an infinite number of parallel universes, each existing and evolving concurrently with our own. Within the stargate universe, alternate dimension, alternate reality, and time travel may be considered the SG-1 trifecta.
Cross Reference: Crystal Skull, Inter-Dimensional Device, Inter-Dimensional Insects, Inter-Dimensional Parasites, Invisibility, Merlin's Phase-Shift Device, M-Theory, Reetou, Sodan Cloak, TER, Tollan Phase-Shift Technology
Episode Reference: Enigma, Show and Tell, Out of Mind, Fair Game, Pretense, Shades of Grey, Crystal Skull, Between Two Fires, Allegiance, Sight Unseen, Metamorphosis, Babylon, The Fourth Horseman, Arthur's Mantle, Uninvited, Line in the Sand, The Road Not Taken