Although pure parallel dimension theory rarely appears in time travel stories, its offspring, divergent dimension theory, is more common. It is used in fiction to avoid paradox.
The salvation of this theory is also its problem: each trip creates a new universe diverging from the previous one. Although this faces most of the objections to parallel dimension theory, it avoids one: travelers never go to the same universe, but to one that did not exist prior to their arrival.
Universe creation is a problem for physicists: this must create a universe as large as our own, but conservation of matter and energy declares this impossible, as it would require the energy in all matter and energy in this universe. However, in most stories the mechanics of time travel are secondary to those of the effects, and this theory manages effects well.
The theory's strength derives from the facts that the traveler is not impacting his own history but that of another world exactly like his own. This prevents paradox while giving the illusion of time travel and allowing free action. However, the traveler never changes his own history, but that of someone who would have become him had he not interfered. Returning to the future, he finds there is another version of him there who has a better claim to his identity.
Some fiction, such as the role playing game Multiverser, suggests that the universe is constantly dividing at every point where anyone makes a choice or anything could happen or not, dividing into one universe for each event that could have happened. If you could go somewhere or stay home, there must be one universe in which you go and another in which you stay. The number of universes so created is staggering. There are at this moment hundreds of things you could do--go to bed, eat, read this page, read another page, rob a bank, assault the person in the next room. You are doing one. Multiplying the number of universes by the things you could do, even that meager list makes six universes. A second person with six choices makes the number of universes thirty-six; a third makes it two hundred sixteen--increasing exponentially with each additional person. The population of Earth is billions, with innumerable choices for each, recurring each second (or less) (because you could change what you are doing now, or now, or now). Thus the number of universes expands at an inconceivable rate.
More challenging, though, is there are at least as many wicked terrible harmful selfish things anyone could do as good wonderful helpful selfless things, but the theory multiplies by one universe for each thing. Nearly everyone alive could have assaulted someone several times today, but most did not. This theory means there must be many universes in which most people did assault someone today, and tomorrow ours might be such a universe. All possible universes existing, ours is extremely unlikely, and almost certain to turn toward the barbaric tomorrow; yet it never does. This seems fatal to such a multiverse theory, which makes for fun fiction but does not fit reality.
Sideways time sometimes applies to divergent dimension theory much as it does to parallel dimension theory, but it is more common in divergent dimension stories to assert that such travel is impossible. Divergent dimension theory then resembles replacement theory, in that the universe that was is inaccessible to those who left it. It is sometimes unclear in stories whether the original history has been replaced or the travelers have been isolated from it.
Travel to the future under this theory is more confused. In most versions, such travel takes the traveler to the future of the world he created; however, in some versions any travel through time creates a new dimension, and in others a traveler will return to his own universe if he travels forward the same temporal distance that he traveled back. The mechanism for this is unclear, failing to answer what happens if the traveler heads for a time in the future of his place in the past but the past of his originating point in the future.
Divergent dimension theory is found in Back to the Future Part II, and something like it may have been behind Donnie Darko and Primer.
Next time we will consider replacement theory.
Comments
Conservation of Energy is not as absolute as we were thought. You can violate it on very small scales, but the lost energy is always taken from somewhere else. It seems the Conservation of Energy is tied more closely with locality than it is with an absolute tenant of reality.
The matter and energy found in parallel universes does not exist in this universe and thus does not count to the matter/energy sum total. And when things move between the two parallels, the balance will be auto-corrected. This makes sci-fi a bit problematic since that adjustment would probably come from matter spontaneously disappearing or exploding.
The number of universes ARE staggering when you consider all the possibilities over all time. But also consider that the physical space we CAN observe is also staggering. Just because something is inconceivable doesn't make it not-realistic. It just means we don't have a way to describe it to others or even ourselves.
I just don't believe that number is inf
Before you start running around saying "because there is an infinite number, it must be like this" consider that although the number is really really really big, it is not infinite. Yes it expands exponentially, but the source material is still limited to the raw number of possible states over time. Our version of the universe (the one we experience) is one of many probable.
So yes, there would be locations out there with no experience of WWII because Hitler was born female and couldn't get into politics in that era. But, although you can bend probabilities to the point a time traveler with knowledge of possible future events appears from nothing, the probability is so low that it ranks up there with cartoon characters jumping off the page and saying hello.
When looking at how probable the universe you're sitting in is, consider all the consistency of history. THAT's the amazing thing. Cause and effect is just as un-absolute as conservation of energy.
Regrettably, Waggs, space is limited in these comment boxes (as you discovered). I will be adding your comments to future answers to questions in a future article. For the moment I will say only 1) I agree (and have not disagreed) on several points but 2) I think you are confusing issues between the parallel and divergent dimension theories, which although similar have some very important differences.
Thanks for raising these issues.
--M. J. Young
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