The SGC has encountered several races that are capable of invisibility. The Nox use the power of the mind to disappear, and to conceal others for protection. The crystal skull of P7X-377 teleports one to the alternate dimension or phase of Quetzalcoatl's race in which one passes through solid objects and appears invisible to those outside the dimension. The Reetou exist in an alternate phase in which their matter waves are exactly 180° out of phase with Earth, so their light and sound waves are undetectable to us.
The Goa'uld have experimented with invisibility, and many Goa'uld ships use cloaking shields. Hathor and Nirrti had mastered phase-shift technology based on the Reetou in order to create personal invisibility devices. A transphase eradication rod (TER) can recognize emissions and counteract the invisibility of the Reetou and phase-shift technology. The Ashrak who infiltrated the Alpha Site also used a personal phase-shifting device which was counteracted by generating a high frequency electromagnetic field within a range of 400 to 700 nanometers in order to excite the phase particles enough to bring them into the visible light spectrum.
Asgard technology has also produced cloaking and personal invisibility devices. The NID off-world operation had attempted to steal Asgard personal cloaking devices from PX3-595, an Asgard protected planet.
The technology of the Ancients also experimented with invisibility by phase-shifting into alternate dimensions. Objects that are out of phase typically pass through each other. The inter-dimensional device from P9X-391 enabled one to see insect-like creatures which live in an alternate dimension normally invisible to us. The cloaking device of the Sodan is Ancient technology that shifts the individual wearing it into an alternate phase, but those who are wearing the activated devices are visible to each other. The SGC acquired several of the Sodan cloaking devices, however adjustments made to the technology resulted in a bleed-through effect that allowed inter-dimensional parasites entering the cloaking field to cross into our dimension. Merlin used technology similar to the Sodan cloak to develop his phase-shift device that allowed him to hide his research by shifting his work into an alternate dimension. Although an individual using Merlin's phase-shift device and an individual using the Sodan cloak can not physically interact with each other, they are able to see, hear, and communicate with each other while out of phase. Merlin's phase-shift device was eventually adapted to enlarge the displacement field enough to take an entire village out of phase, and in a parallel universe, Carter's modifications succeeded in cloaking the entire Earth.
The cloaking shields used to hide spacecraft typically do not rely on the same sort of phase-shifting technology used by the Ancients. While a cloaked ship will remain invisible to the eye or to sensor scans, it does not shift out of phase, objects do not pass through it, and it may continue to give off a faint energy signal. Cloaking devices for spacecraft are considered safe without side effects to those within the ship, although SG-1 has reported an incident in which Colonel O'Neill was standing in the engine room of a mothership, right next to a cloaking generator when it was activated, and he was bombarded by the particle field being emanated by the device. He became invisible, and he liked it.
Cross Reference: Alternate Dimension, Ancients, Asgard, Ashrak, Nicholas Ballard, Crystal Skull, Hathor, Inter-Dimensional Device, Inter-Dimensional Insects, Inter-Dimensional Parasites, Merlin's Phase-Shift Device, Nirrti, Nox, PX3-595, Quetzalcoatl, Reetou, Sodan Cloak, TER, Tollan Phase-Shift Technology
Episode Reference: The Nox, Enigma, Show and Tell, Out of Mind, Fair Game, Pretense, Shades of Grey, Crystal Skull, Rite of Passage, Allegiance, Sight Unseen, Metamorphosis, Avatar, Babylon, The Fourth Horseman, Arthur's Mantle, Uninvited, 200, Line in the Sand, The Road Not Taken