Once we have decided that it is possible for a time traveler to alter the past of the world from which he came, and that such changes are neither self-sustaining nor self-correcting, we have to determine what happens when history is changed. In this, the causal chain must be preserved–that is, no effect occurs if it is not preceded by its own cause. Pots of water do not boil if heat is not applied, the baseball does not fly out of the park if the bat fails to hit it, and events which are (always) the result (effects) of other events (causes) do not happen if the events which cause them do not happen. Thus Bob’s arrival in the past is the effect of his departure from the future, and if he does not depart from the future he does not arrive in the past.
However, before he can depart from the future, there must be a future from which he can depart, a chain of causes and effects which create the world at the moment of his departure; and since the future cannot have occurred until those events create it, Bob cannot have departed from the future until there is such a history of the world, a history in which he did not arrive in the past. This we call an original history, original in the sense that it is what happened when Bob did not arrive within it.
It should be noted that the use of the word “original” here is with reference to this trip alone. Readers sometimes observe that it might be possible that other time travelers had already interfered with history. What matters is that the first time this time traveler makes this trip, he does so from a history in which he has not altered history by arriving in the past.
Fixed time theorists object that his arrival in the past is always part of history, because he will depart from the future. Notice, though, that the one who departs from the future has knowledge of the events which occurred in his past, now the future, because he experienced them as if in time. Thus the causal chain must have reached that point sequentially prior to his arrival in the past. To illustrate, Traveler awakens Monday morning, has a hamburger for lunch, dines with his girlfriend, goes home to bed, awakens Tuesday morning and sees on the news that his girlfriend was murdered Monday night. He leaps into his time machine and travels back to Monday afternoon, hoping to prevent whatever events led to her death, having such knowledge as was in the news concerning her murder. He cannot arrive on Monday afternoon until he departs from Tuesday morning; he cannot depart from Tuesday morning until he has lived through Monday. Whether or not he can otherwise alter events on Monday night, tracing the causal chain backwards demands that he has experienced the events of a history in which he did not interfere. The fixed time alternative not only means he cannot change history, it means that the future is already determined as well, and he is not really making any decisions but merely playing a pre-written script.
Thus if it is possible to change history by means of time travel, there must be an original history which is unchanged, and the arrival of the time traveler, in and of itself, alters that history. The issue of temporal anomalies arises, because as it happens there are exactly three ways in which a time traveler can alter events: he can create a new history that is self-sustaining, that is, which his younger self will create in the same way (an N-jump); he can restore the previous (or a previous) version of events and so create a cycling causal chain between two (or more) iterations of history each of which causes the other (an infinity loop); or he can create a new version of events that is different from all previous versions and will not repeat any of them (a sawtooth snap). These will be considered in more detail in the articles ahead.