Periodically readers criticize this series based on the assertion that since no one has ever traveled through time (out of sequence) we cannot really know what would happen, and our assertions that this must have happened for the story to work or that this could not have happened as presented in the story are not defensible. There is certainly a level on which this is true. But it is not rational to say that anything is possible, nor to say that we know nothing simply because we cannot demonstrate it experimentally. For example, we have a pretty good idea what would happen if gravity were reversed, even though we have no means of reversing gravity. Thus it might be worthwhile to put a few articles into the question of explaining what we think we know and why we think we know it, and to look at the assumptions behind these conclusions.
If we are going to look at each of the three major theories, it makes sense to begin with fixed time theory, in part because it is highly popular among physicists (which in some minds makes it the true theory scientifically) and in part because quite a few time travel stories attempt to explore worlds in which fixed time is assumed. It also becomes the base against which other theories are contrasted, with parallel dimension theory also having a following among physicists but its distinguishable cousin divergent dimension theory being preferred in fiction, and various incarnations of replacement theory, some defensible and some not so much, being the most commonly presented understanding of time in most stories. Before we examine these individually, let’s be clear on the distinctions between them, in simple terms.
Fixed time theory views all of history as already completed; time is not exactly an illusion, but the way we experience it is. It might best be understood by speaking of a road that runs from your house to the store, and your travel along it. The road exists; the store awaits ahead. Your movement does not cause the store to come into existence, but only brings you to it. In the same way, your death (for something that is ahead for at least most of my readers) stands at a specific moment in the future, and you are approaching it. It has in some sense “already” happened; you just have not yet reached the moment where it exists. You feel as if you are making choices that bring you forward in life, but in some sense you are only acting out what you are destined to do. Thus if you are going to travel to the past, your departure from the future has already occurred and so has your arrival in the past, and everything which from your perspective you are going to do you have from another perspective already done. Thus you cannot change the past, because the past is already written; you cannot change the future, either, because that, too, is already written.
This flies in the face of our belief in our own ability to change the world, that we make choices which become causes which have effects. There is a degree to which it seems irrational to claim that were we to travel to the past, we could not make choices that would change the past. This thus gives rise to multiple dimension theory, in its two major forms. The one more embraced by scientists asserts that parallel dimensions already exist, and that were you to attempt to travel to the past you would find yourself in the past of a seemingly identical universe, which you could change without incident because it is not really your past, but the past of a world you cannot distinguish from your own until you alter it. The one more embraced by writers suggests that when you arrive in the past, that instantly changes the past, but since the past cannot change it creates a separate universe, a divergent dimension which diverges from that point of arrival and continues with its own history into the future, without changing the events of your universe of origin, from which you vanished.
There are quite a few variations of replacement theory, but they share in common the idea that if you travel to the past you can change your own history. Whether there are other dimensions is irrelevant; there is only one history of the universe, but it is mutable. Whether the future already exists is also irrelevant; we can change it as easily as we can change the past.
With that groundwork, we will look a bit more closely at the assumptions and logic of each theory.