In considering the logic of time travel, the first issue to address is the nature of time itself. Two principle forms are logically plausible: either all of time exists in some static form from beginning to end (or eternity to eternity), or only the present moment exists, the past lost and destroyed, the future not yet created. Either view is defensible. However, from a time travel perspective, if only the present moment exists time travel is impossible nonsense: you cannot go to a place that does not exist. Thus we can have the Rip Van Winkle-type story exemplified by Buck Rogers in which the character sleeps or is in suspended animation and emerges in the future, and stories like Minority Report and Next in which the character is able to predict future events, but not one in which a character travels to the past. Time travel of that sort seems to require that something like a “timeline” exists, that the past is still the present “somewhen”, and the future is “somewhen” to which we can travel.
Some avoid the “timeline” with a rather more complicated “timeplane”, that is, movement along many parallel timelines which are staggered along the same events. By moving laterally across time, you travel to what appears to be the past or future, but what is actually a separate universe lagging or leading our own. Thus the future and past of our universe do not exist, but are duplicated in other universes also without futures or pasts. We cannot travel to our own past, but we can travel to something indistinguishable from it, and change it with impunity because it is not our universe. This will be considered further in discussing parallel dimension theory.
The very concept of a “time line” suggests that time is a dimension–not in the sense of another universe, but in the sense that length, width, and height are dimensions. Some insist that this is not true, that time is different in kind from these; yet despite the differences, time has the same impact on reality as spatial dimensions, other than that it is what we can call the dimension of change: absent time, everything would remain the same, because change implies time, a “before” and an “after”. We can see that time is like a spatial dimension with a simple thought experiment.
The corner of the desk is a point in space; if you place a book there, it occupies that point in space, and you cannot place another book in the same place. Suppose that the book is actually an object with no dimensions, which occupies a point to the exclusion of all other objects. In space with zero dimensions, that is all that can exist. If, though, we take the edge of the desk to be the first dimension, we see that we can place other books along the edge, but only one at that point that we call the corner. However, if we then take the perpendicular edge as defining a second dimension, the surface of the desk becomes a two-dimensional space, and we can place a second book at the same point along the first edge by placing it at a different point along the second edge. It is thus in the same place in one dimension by being in a different place in the other. We can repeat this by adding a third dimension, stacking the second book on top of the first, at which point both books are in the same place, on the corner of the desk, in the two dimensions that define the desktop, by being in a different place in the third.
We could imagine a fourth spatial dimension, such that two books can be in the same place on the corner of the desk touching the desk by being in different places in this fourth spatial dimension; but we can also arrange for the two books to be in exactly the same place in those three dimensions by removing the first and replacing it with the second. We then have an object in the same place in three dimensions because it is in a different “place” in the fourth. Time is thus a dimension similar to the spatial ones, and arguably could exist very like a line or fourth axis on a graph.
In order for time travel to be possible, time must be a dimension very like this. The questions are whether we can move to other points along it (which is assumed by the concept of time travel), and whether it is mutable or fixed. That is the issue to be addressed next…time.