Carter had been attempting to rewrite the safety protocols on Merlin's phase-shift device, and she had used a force shield to seal her lab and take it out of phase. At that exact moment, in an alternate reality, Carter and Dr. Bennett were conducting a very different kind of experiment designed to extract energy from parallel universes. Ordinarily this type of energy retrieval is extremely dangerous, leading to the creation of exotic particles in one or both universes, but Carter had found a way around that and had designed the machine so that the bridge between universes would only open for a microsecond. The idea was to steal small amounts of energy from thousands of different universes, thereby minimizing the fallout in any single one. The device was never designed to bring ordinary matter through the inter-universal bridge, however, because she tapped into our reality at the precise moment that Carter's lab was taken out of phase and protected by a two-way force shield, the unexpected effects on the inter-universal bridge led to Carter and her lab being pulled through the bridge and drawn into the alternate universe where a power spike caused the capacitors to overload and alternate Carter and Bennett were killed in the massive explosion.
In the alternate reality, Carter had been a major rather than a colonel, once married and divorced from dot-com millionaire smart-ass Rodney McKay. The SGC was under the command of Major General George Hammond. Since there had never been a mission to Atlantis, Major Lorne had not been stationed in the Pegasus Galaxy, and instead he was the commanding officer of SG-1. Mitchell had been an F-302 pilot who was shot down over Antarctica and hailed as a hero, then abandoned by his government. He had left the military, and, now in a wheelchair, had become an embittered alcoholic who considered his situation to be the price for standing up for his principles. Teal'c had returned to the Jaffa several years ago, Daniel had been captured by the Ori a few weeks before, and Vala was occupying a cell at Area 51. Hank Landry was the president of the United States, but three years ago during Anubis's attack on Earth, the US was forced to reveal the existence of the Stargate Program to the world. People had panicked and rioted, and the President had declared martial law. He moved his offices to level 27 of the SGC, and the Prometheus had become Air Force One. Earth had begun devoting its full resources to planetary defense and had reduced exploration, abandoned the Alpha Site, and cut ties with the Jaffa and most alien allies. Although many feared the destruction of civil liberties, Congress was forced to cancel mid-term elections the year before, due to widespread unrest, and the temporary ruling led to the indefinite suspension of elections and a simple yes/no vote to keep the President in office. Three weeks earlier, a single Ori warship had attacked Earth, but the Ancient weapons platform in Antarctica had successfully defended the planet, leaving the ZPM nearly depleted. Major Carter's experiments had been intended to bolster power reserves with energy captured from other universes to replace the ZPM, and intelligence indicated that the Ori were assembling a fleet for a full-scale attack on Earth within five days.
Unable to complete Major Carter's research in so little time, Colonel Carter proposed an alternative. She and Dr. Lee brought Merlin's phase-shift device from storage and prepared to take the entire planet out of phase, a feat that required 700 gigawatts of power, roughly 80% of the power generating capacity of the continental US. By tapping into the US power grid, Carter shifted all of Earth out of phase just as five Ori warships arrived and launched an attack, their weapons fire passing harmlessly through the planet. Carter was hailed as a hero, presented for interviews and public receptions, appointed as special advisor to the President, and used by Landry as political capital for reelection. However, Carter sought McKay's help to find a way to return to her own universe, and she used her celebrity as a platform to speak out for improved foreign relations and the restoration of civil liberties, including a live appearance on "Inside Access" with Julia Donovan that was abruptly shut down by the President's Chief of Staff. Finally, Carter was deemed more trouble than she was worth, and Landry agreed to let her return home. Instead, McKay was pressured into filling her advisory position, and with McKay's help, the inter-universal bridge was recreated. As Carter prepared to return to her own reality, two weeks after arriving, she urged McKay to also be the persistent voice of reason to the President.
Cross Reference: Alternate Reality, Alternate Reality - P3R-233, Alternate Reality - Quantum Mirror, Alternate Reality - Ripple Effect, Dr. Bennett, Samantha Carter, Charlie, Julia Donovan, George Hammond, Inside Access, Hank Landry, Bill Lee, Major Lorne, Rodney McKay, Merlin's Phase-Shift Device
Episode Reference: The Road Not Taken