As is the case with most science fiction though the theory of the parallel universe is strongly based on science fact. This can mean a wide range of different things though. Something loosely based on the theory of a single man which is not taken seriously can be said to be "based on science fact" as can something worked on by a group of people include post-graduate level physicists and derived from widely accepted theorems. What is not immediately clear is how likely it is that parallel universes exist and, perhaps more importantly from the viewpoint of science fiction, will we be able to visit them?

The first thing to realise about parallel universes is that their existence can neither be verified or refuted by experimental data, since, almost by their very definition, we cannot detect them. In this respect the question of their being is more one of natural philosophy than one of science. There are no right answers: only thoughts and ideas. This plays directly into the hands of the science fiction writers who argue that their use is valid since nobody can disprove their existence. Whilst this is true, it is not necessarily a good argument in favour of their usage. A common principle of natural philosophy is one known as Occam’s Razor which is commonly paraphrased to "The simplest solution is often the best one". Whilst this is perhaps an over simplification it does portray the point quite well. It would be quite easy to claim, for example, that all chairs when not being observed change into pigs and dance the Funky Chicken before returning to their exact same state upon someone once again looking at them. This is impossible to disprove since by definition it cannot be known what a chair is doing when no-one is observing it because no-one is observing it. Despite this though most people would claim this interpretation to be absurd. There is no reason to add an extra level of complexity when the explanation that a chair remains a chair at all times works well and describes what happens to them with total accuracy.

In order to believe in parallel universe it is therefore necessary not only to say that they could exist, but also to find a good reason why they should exist. The main reason argued for the existence of such alternate realities is what is called The Many Worlds Interpretation of Quantum Mechanics. To understand this fully it is necessary to first know a small amount of quantum theory. One of the main premises of this is that every event has a calculable probability of happening, but until it is observed all these possibilities happen together. The book has both fallen from the shelf and it hasn’t. The explosives detonated successfully and they didn’t. The cat is alive and it is dead. It is only when we observe the book or the cat that they "decide" whether to be on the shelf or the floor; alive or dead. According to conventional quantum mechanics, once the cat has "decided" to be alive the possibility of the cat being dead is gone. It is known to be alive. In the many worlds interpretation, however, this is not what happens. So say the advocates of this theory, when you look at the cat our universe splits in two. In one universe you see a dead cat and in one you see a living cat. Similarly, any decision you make results in the universe splitting. If this is the case then there are literally billions of universes that exist.

This is very useful for a science fiction writer. A conventional scientific theorem indicating that human decisions can produce different universes in which we can observe and absorb the differences appears to be just what the doctor ordered. As would be expected though, things are not that simple. Many scientists claim that the Many Worlds Interpretation goes against Occam’s Razor. All it does, they claim, is introduce an extra level of complexity. Once again things are looking grim for the writer who attempts to justify his story.

Some physicists go even further than the conventional many worlds interpretation. They claim that all universes which can exist, do exist. Initially this sounds even less likely but they would argue that this is not the case. They claim that instead of adding an extra level of complexity this actually simplifies things. Rather than having to know all the physical laws of our own particular universe to know the complete state of reality we merely have to accept that all possible universes exist and that we just happen to be occupying one of them.

This is both a blessing and a curse for the science fiction writers though. Although it would seem to allow parallel universe to exist, it subtracts from the amount of effect human decision have on their form.

The second question, as to whether we can travel between such universes, is possibly simplified by admitting there could be an infinite amount of universes. It is theoreticised that it may be possible to travel to different universes through tunnels known as wormholes. By removing the distinction between universe which are similar to ours but which split of somewhere and all other possible universes it makes the possibility of going through a wormhole to one of these realities seem more plausible. Unfortunately, there are currently no theories as to how the destination universe could be controlled so ending up anywhere remotely similar to our universe would be slim. For that reason parallel universe as used in science fiction currently seem almost unfeasibly unlikely, but they can make good stories, and surely it is that that is important.