Entropic cascade failure is a side effect that can occur when traveling to an alternate reality. It is caused by the increased entropy created by multiple versions of the same person in one reality. The effects, which only affect the non-native versions, continue to get worse over time, eventually resulting in death. The only way to prevent this is for them to go back to their own reality. Entropic cascade failure does not occur if a person's counterpart in another reality is deceased. It is marked by violent and painful convulsions as the foreign matter is, in effect, rejected by the host reality.
In 2006, when multiple SG-1 teams emerged from the Stargate, Dr. Bill Lee speculated that entropic cascade failure did not occur because of the proximity of all the realities to each other. (SG1: "Ripple Effect")
- Entropic Cascade Failure has been defined by Stargate SG-1 writer Joseph Mallozzi in this way: "The Multiverse theory of quantum physics posits the existence of endless alternate realities co-existing with our own. Some hold that the proximity of these realities is dependent upon the similarities between the various universes - i.e. the more similar the realities, the closer together/easier to access via an inter-universal bridge, while the more dissimilar, the further apart/more difficult to access. Some theorize that the temporal effects of entropic cascade failure are directly proportional to the proximity of the "universe of origin" from which a given element is introduced, ranging from dangerously high impact in distant, off-setting cases where the elements are more likely to be "out of sync" to negligible in closer, more attuned elements."
- It would appear that entropic cascade failure does not affect multiple versions of the same person living in a timeline where history has been altered by outside influence; in Stargate: Continuum, Dr. Daniel Jackson spent a year in a timeline where Ba'al changed history so that the Stargate was lost in the Arctic in 1939 without any adverse effects despite the fact that his other self in that reality was still alive and living in Egypt- Colonel Cameron Mitchell had never existed in this reality and Samantha Carter's other self had died four years earlier; most likely, since this was technically still his timeline, entropic cascade failure wouldn't affect him as he was still in 'his' universe even if history had been altered. This idea is supported by the later Stargate Universe episode "Twin Destinies", when future versions of Dr. Nicholas Rush and Colonel David Telford travel back in time twelve hours, with Rush making no reference to the possibility of entropic cascade failure being an issue despite the fact that he was almost certainly aware of the possibility.
- The idea of entropic cascade failure not being a factor when time travel is involved in the circumstances that lead to versions of a person being present is also supported by the events of "1969", as all four members of SG-1 traveled back to a time period when their younger selves were present and experienced no trace of discomfort.
- Entropic Cascade Failure is not mentioned when Rod appears in the episode "McKay and Mrs. Miller", although Atlantis' limited contact with alternate realities and the limited amount of time Rod in this reality may account for that.