Although last time we noted that fixed time theory seemed to be preferred by scientists, not all believe that time travel is governed by it.  Quite a few hold that there are parallel dimensions, our universe duplicated outside our perception, and that a time traveler would land in a parallel universe.

There are two distinct notions about these parallel universes, one, that they are all identical unless a time travel event interferes, and the other, that they are all different due to random events altering history, compounding over time.  This latter version is the basis of Sliders, the television series in which a student and his friends cross the Einstein-Rosen Bridge into parallel universes and cannot return home.  In this version of parallel universes, the world in which you arrive probably would be not merely different but very different; we would not call such an event time travel.  This remains an objection to all versions of Parallel Dimension theory:  no matter how similar two universes are, they are not the same universe, so this is not really time travel.  Travel to a parallel universe, whether identical or distinct, is often referred to as sideways time, a term that has different meanings under other theories.

Even if all universes initially are identical, time travel changes them, causing them to diverge from each other.  It might be thought that given an infinite number of universes this would not matter, because if a time traveler changed only one of those universes so as to prevent his departure from it, the next time traveler probably would not land in the same one.  However, since those infinitely duplicated universes are all identical unless altered by the acts of a time traveler, then "half an infinite number" (also an infinite number) of time travelers have departed from "half an infinite number" of universes and prevented their own departures from the other "half an infinite number" of universes, and thereafter any time traveler has only a fifty-fifty chance of landing in an unaltered one--a chance that decreases with each additional trip made by anyone.  People think that making the number of universes "infinite" solves these problems, but it only makes them more difficult to discuss.  It is also implausible, as although our universe is vast we have found no part of it that is infinite.

The existence of the future in such systems varies with the version.  In one, there are an infinite number of such universes in a continuum such that moving sideways across them is like traveling through time:  a distance of one minute sideways places the traveler one minute to the seeming future or past.  In such a version, the current existence of a linear past or future is irrelevant.  History might be being created instant by instant and falling into non-existence as it passes, or it might exist in full form from eternity to eternity in each timeline.  Also in such a world, if you traveled to the past and changed history, then came forward to the present, you would have moved to another universe and back, so nothing you changed would be different in your own universe.  This prevents paradoxes, as you are not affecting your own past:  you can enter the past of another universe, but no one from that universe could ever enter the past of yours, which is positioned to its future.

Without this or similar limitation, parallel dimension theory does not avoid paradox:  if a traveler from world one alters the past of world two such that a traveler from world two alters the past of world one, the fact that they are distinct worlds is no longer relevant, as the causal chain now connects them:  the change in each world is dependent upon the change in the other.

Other versions of parallel dimension theory differ in whether the future exists in the present universe or not.  This often reveals itself by whether the traveler to the future arrives in the same universe or another, since to travel to the future the future must already exist.

However, parallel universe theory remains popular among theoreticians, partly because there is scientific evidence suggesting the existence of parallel dimensions.  It is not conclusive that these exist, nor whether they are like ours, but if so they would enable something like paradox-free time travel allowing the traveler to change history.  Although varied parallel dimensions have been considered in the context of other time travel venues such as Dr. Who and Star Trek (which will be discussed in connection with sideways time), it is rare for identical dimensions to substitute for time travel in stories.

Next time Divergent Dimension Theory will be considered.