In our description of time travel scenarios we will make several assumptions. Let us look at a sequence of events as an illustration. Point A will be a moment in the past and Point B will be a moment in the future. In our time travel scenario a traveler leaves Point B and returns to the past arriving at Point A.
Our first assumption is that the sequence of events from Point A to Point B must first be described without any visitor returning from the future. Our reasoning is that when Point A arrives the traveler from the future has not yet been sent back, that event will not occur until we get to Point B.
Therefore, any attempt to describe the events leading up to Point B must first be described with no visitor from the future at Point A. There must be an original chronology unaltered by visits from the future.
Our second assumption is that at the moment the traveler leaves Point B the past just described no longer exists; it has been changed by the arrival of the traveler at Point A. When we first encounter the decision of the traveler to return to Point A they are leaving from a future in which no traveler had appeared in the past. Once the traveler leaves Point B that is no longer true. There is no past in which a traveler did not appear at Point A. Because of this we cannot continue to describe events that occur after Point B as that particular Point B no longer exists. Any meaningful Point B must now include the arrival of the visitor at Point A.
Before we can continue we must understand this new past, created by the arrival of the traveler at Point A.
Therefore, at the moment the traveler leaves Point B, we must return to the past, to Point A, and describe the sequence of changed events, changed by the arrival of the traveler at Point A.
Our third assumption is that as we describe the events leading from the traveler's arrival at Point A up to Point B the same decision to return must be made. If the traveler at Point B does not return to Point A we have a paradox because that event has already occurred. Although paradoxes are an interesting concept when discussing time travel, we want to avoid them here. The timing of the decision to return, that is the location along the timeline of Point B, might change and the reason for returning to Point A might change, but the return to Point A must occur.
Therefore, once the traveler arrives at Point A, we must be able to describe events from there to Point B wherein the traveler again makes the decision to travel back to Point A.
The last assumption is that there is only one timeline. We are never describing multiple parallel timelines (more on this below). There is not one past in which the traveler did not arrive and another in which the traveler did arrive. There is only one past. This past can be changed, which therefore changes the future, but it is still one continuous timeline from the past into the future.
Therefore, any meaningful description must be a single continuous chronology from Point A to Point B and must include all visits from the future and all future decisions to return to the past.
As one of the goals in this process is to describe the sequence of events through the first three stories as one continuous sequence with a past and a future we want to avoid circular causality. This is where an event occurs in the future only because that future sent something back to the past, which allowed that future to develop.
Many have argued that Terminator depicts a case of circular causality and, if only that movie is taken into consideration especially the deleted scenes, a very strong argument for this can be made. The only reason Cyberdyne is able to develop Skynet is because they have the chip from the future. The only way the future could have that chip is if Cyberdyne developed it in the past. Cyberdyne is able to develop the chip because after they developed the chip it was sent back in time allowing them to develop the chip. It's a never-ending circle. In this type of time travel situation, there is no original unaltered timeline. The future can only develop if there is a visit to the past from that future.
Although circular causality is an interesting concept, we want to avoid having it occur here. Our goal is to describe the events as one continuous timeline where events can occur without intervention from the future.
Another concept is what we call instant causality. Duration along the time line is an aspect of consciousness and not of the time line itself. Because a decision to travel back in time from Point B will be made then it has already occurred when we first reach Point A and there is no scenario in which events that eventually will occur do not occur. In this way, there are never altered chronologies, there is only one and it contains all visits from the future.
Interesting as these ideas are, we prefer the idea that we have a chronology that gets changed by a future decision to return to the past and this is the way we will describe events. One of the reasons for preferring this explanation is that it makes sense of the fact the Kyle tells Sarah that Judgment Day will happen in 1997 when it actually happens in 2004. The reason it happens in 2004 is because the chronology in which it happened in 1997 has been changed by a visit from the future.
We have frequently been asked why we do not include the concept of Multiple Parallel Timelines. There are several reasons we rejected this concept.
Time Travel Doesn't Work - Ever
Let us consider a simple situation. A traveller leaves a point in the future and returns to a point in the past. At the instant the traveller arrives, a new timeline is created.
Above we have the original T0 timeline. The traveller leaves returning to the past. However, the instant they arrive a new timeline T1 is created. The traveller does not show up in the past of T0 they arrive in the T1 timeline. To the inhabitants of T0 the traveller never made it. To those who sent the traveller back, it is as if the time displacement device did not work. No matter how many times they send someone back, it never seems to work because no one ever arrives in their T0 past. To them, Time Travel Does Not Work - Ever.
Infinite Loops Loop Infinitely
The next issue we have with travel back through time creating new timelines is the creation of an infinite loop. Let's look again at a single traveller leaving the future and returning to the past.
The traveller A0 leaves on T0 creating the new T1 timeline. On T1 the traveller who leaves from the future is A1 who travels to the past creating a new T2 timeline. Remember, A1 cannot suddenly return to the past of T1 any more than A0 could return to T0 because, by the rules of Multiple Parallel Timelines, a return to the past creates a new timeline. This means that each time we reach that point in the future where the traveller returns, we create a new timeline. This will go on forever.
Another possibility, and admittedly an intriguing one, can be seen below.
Here the traveller leaves Point B, creating the new T1 timeline. In this timeline, the traveller is never sent back into the past. This solves the infinite loop problem. For example, let's say the T‑800 travels back, successfully kills Sarah, prevents John's birth and the resistance never succeeds against Skynet, which never needs to send a T‑800 to the past to kill Sarah. In the single timeline theory that we support, this would be a paradox. The T‑800 must be sent back because the T‑800 has already arrived in the past.
Multiple Parallel Timelines neatly gets around the paradox issue as well as the infinite loops issue by presuming that what happens in the future of one timeline does not control what happens in the future of another. Intriguing as this concept is, it does not serve.
Everyone to whom we have talked who are believers in Multiple Parallel Timelines always seem to describe each future as having basically the same events as the original timeline. The T‑800 will always be sent back to 1984 to kill Sarah regardless of the changes wrought. A traveller returns, creating a new timeline, but in that timeline the traveller is again sent back. Because of this assumption that the future of each timeline created is basically the same as the previous we seem stuck in the realm of infinite loops.
I've Crossed This Rubicon Before
We have often heard supporters of Multiple Parallel Timelines state the following: we move along the timeline, the Terminator and Kyle get sent back creating a new timeline, the timeline of Terminator.
However, this cannot be. The T‑800 returns to the past, creating the new T1 timeline. Kyle is still on T0 and when he returns he creates his own new T2 timeline, he does not magically arrive on the timeline the T‑800 created. We would now have three timelines: T0 wherein no one arrived from the future, T1 wherein the T‑800 arrived from the future and T2 wherein Kyle arrived.
The only way it can happen as many have suggested is if Kyle somehow crosses to the timeline created by the T‑800 rather than creating his own and this makes no sense to us. Kyle's return creates a new timeline and in that timeline no T‑800 arrived from the future, that only happens on the T1 timeline.
We can make this less confusing by assuming that the Kyle that returns to the past does not come from T0 but comes from the T1 timeline.
As we can see, this seems to make a little more sense and gets around the Rubicon problem. As Kyle arrives after the T-800 did and his creation of a new timeline only occurs at his arrival it does not effect the earlier arrival of the T-800. However, this explanation raises another issue.
Kyle is sent back in response to the T-800 departure. On T1 the T-800 is not sent back, that only occurs on T0 and so begs the question, why is Kyle sent back? If we assume that on T1 a T-800 is sent back as well, then we have the infinite loops problem discussed before. The T-800 leaving from T1 would create a new timeline which, because of the Rubicon problem, would be a different new timeline than the one on which Kyle is going to land.
Another issue is the abandoned timelines. If the T-800 leaves and creates a new timeline, what happens on the timeline from where it came? No one who supports Mutliple Parallel Timelines ever seems concerned with these abandoned timelines. The only concern seems to be for what changes will be wrought on the new timeline. In our Single Timeline theory, we are intimately aware of what occurs after visitors leave in the future as there is only one future, what changes are caused by the travelling are crucially important because it is that single and only future we are concerned with.
So, for the infinite loops problem and the Rubicon problem, we do not see that Multiple Parallel Timelines serves any useful purpose or actually addresses the issues surrounding time travel. We maintain there is but one single timeline and it must be described in such a way as to include all visits from the future to the past.