Recently, while discussing Marvel Two-In-One #50, I talked about Reed Richard's theory that any attempt to change the past via time travel inevitably leads to the creation of an alternate timeline. This rule came to be known (in Marvel comics, anyway) as the Conservation of Causality, or Kang's Third Law of Time. Here, this is easier:
Fantastic Four #553 revisits the theory, and Reed has to make some drastic revisions. Dwayne McDuffie’s all-too-brief writing stint on FF ended when a time-traveling Doctor Doom came back from 75 years in the future to stop Reed from implementing Idea #101. The elder Doom insists that in the future, Reed has become a power mad tyrant and therefore must be stopped from continuing to implement his list of ideas. The FF of 75 years hence arrive, still spry and more powerful than ever thanks to Reed's longevity advances. The centenarian Doctor Doom then demands asylum from “our” FF. Long story short, the two teams fight, reconcile, and have an appropriate parting with Doctor Doom. It’s a good story, as was the rest of Dwayne McDuffie and Paul Pelletier’s FF run. Ultimately, Reed has this to say about the Conservation of Causality:
This new development is quite a relief. I was worried that I was creating alternate timelines every time I had to choose between having a turkey or a roast beef sandwich. As stated, the timeline will remain the same regardless of my lunch choice, so that’s a load off my shoulders. Using Reed’s logic here, deaths can be undone, marriages can be never-were-ed, and origins can be re-written with impunity, provided they don’t mangle history so much that it creates a new timeline. So Harry Osborn can come back via magic devil trickery and the rest of the world “adjusts’ its collective memory and history to allow for his never having died. Of course, Harry may be doomed to die again, too. Mephisto also went back in time to alter the past so the Parkers were never married, and the timeline was robust enough to allow for that. In other words, had Mephisto gone back and killed Spider-Man on his wedding day, that would have been enough to launch a branching timeline, but this more subtle tweak kept the timeline intact, with minor alterations.
Of course, devil-powered time travel is magic, which has even fewer rules than science-based time travel, so that may be a poor example. My point is that from now on, time travel becomes a much more elastic concept. And of course, severe, transformative events are going to stick, even under the new rule. Presumably, the bigger the problem, the harder it is to change via time travel, as in Reed’s Lincoln example. The heroes may, for example, be able to go back and prevent Stamford from blowing up, and thus prevent the Superhuman Registration Act, but something else would happen to cause Civil War anyway.