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Time Travel in the Doctor Who Universe

By Stephen Pritchard

In writing this article I have tried to keep the language as simple as possible, but when dealing with a subject as complicated as time travel it becomes very difficult. If you give up halfway through then jump to the second part of this article, which will deal with time continuality in the Doctor Who universe.


Every week on TV for nearly 23 years we saw the Doctor travelling back and forwards in time in the TARDIS. Battling monsters and evil races intent on conquering the universe, but in reality is this sort of time travel possible? The conventional view of scientists has always been that the laws of physics logically cannot permit this to happen because of possible paradoxes it could cause.

Suppose a man with a time machine goes back into the past and kills his grandfather before he has any children. If he died childless, how could he have had grandchildren let alone be murdered by one of them? Thus a paradox is formed.

Some philosophers have questioned the validity of this argument, saying that it only proves that a person cannot go into the past and kill their grandfather.

But what would prevent a person, who had travelled into the past with the intention of killing his grandfather, from pulling the trigger of the gun? Logically something must. Probably the gun would jam or as (I think it was) Poul Anderson wrote in The Coal Sack the universe would simply stop them from getting into that situation by preventing them from building the time machine in the first place. In the case of The Coal Sack the sun simply went nova, killing the people before they could build it.

This kind of time travel is set in a single-universe model and depends on whether so-called Closed Time-like Curves, or CTCs, exist. This is where an object may be able to travel into its own past without changing its future. However, this mean s the loops have to be planned out beforehand, so how do they form in the first place?

For decades theorists have known that certain solutions of Einstein's equations of general relativity, which describe the structure of space-time, permit such CTCs to exist.

When quantum mechanics is applied to the analysis of paradoxes, the result is not a single universe as above but a multiple universe where all possible histories happen simultaneously - this is the so-called 'parallel universe' interpretation of quantum theory.

The quantum version of space-time can be thought of as resembling an infinitely long shelf of books, each book being a single universe which is similar to its neighbours except that in each of them a slightly different history occurs.

If CTCs are present in these parallel universes, something qualitatively different happens. When a space-time region loops back into the past, it does not necessarily join up with the same copy of the space-time it left. For instance if the time traveller went into the past and changed something, when he returns to the future he will return to the future with that change in it, leaving the future he left from originally still intact on the shelf. He just swapped books as it were. So in this way it becomes possible to travel from one universe to the next.

Exactly how the universes link up depends on what interactions take place in them. In particular, it depends on what the inhabitants decide to do.

This helps avoid the grandfather paradox, as history consists of two concurrent branches. Initially, they are identical, both containing the grandfather. In one of them, that person (not actually a grandfather in that history) is murdered by someone who emerges from the time machine. In the other history, no one emerges, and the grandfather goes on to have children and grandchildren, one of whom enters the time machine, travels to an earlier time in the other universe and becomes a murderer.

In this way quantum mechanics provides as many linked universes as there would be conflicting possibilities.

However hard a person tries to create paradoxes by travelling in time, the linked, multi-sheeted structure of the quantum space-time sets itself up in such away as to allow the person to do what they set out to do, yet without inconsistency.

In this way the free will on the part of the time traveller is preserved. If this was not the case, then everything the time traveller did would have to have been mapped out beforehand to prevent the paradoxes occurring as in the single-universe model. This means that the time traveller would be merely acting out what has already happened. This type of thinking is called 'determinism', and contradicts the idea of free will. The most obvious example of this is in the Doctor Who story Earthshock.

In this story the Cybermen tried to use a space freighter to destroy a meeting of the planets on Earth in the future. But thanks to the Doctor and Adric the freighter crashed into prehistoric earth, killing the dinosaurs and Adric. However, since Tegan who was with the Doctor at the time was a product of a reality in which the dinosaurs were destroyed by the freighter collision, then it follows that in a single-universe model the freighter was going to hit no matter what the Doctor or Adric did. This in turn means that they were only acting out a predefined event.


There have been a large number of script writers for Doctor Who over the years, all of whom have had a slightly different idea of how and when events occurred in the time stream of the Doctor's universe. Because of this there have been clashes and contradictions over the years in various stories, and different stories can be used to support either a single-universe or multiple-universe model. Trying to find a definitive answer in the stories could keep an argument going for years, so I will only look at a few of the ideas raised by them. (Besides which, I can't remember a lot of them.)

A good example of the problems in time travel is raised by the story Genesis of the Daleks. In this story the council of Gallifrey order the Doctor to go back in time to Skaro and prevent the Daleks from existing. He was to do this by destroying the labs in which they where first made and by killing their creator Davros. The problem this story creates is simply that if the Daleks never existed, what happens to all of the races they have killed or subdued over the years? Thus a major paradox is formed in a single-universe model. There is also the complication of the Dalek wars with the Cybermen and the Movellans. If the Daleks had not gone to war with them, then they would have caused almost the same level of destruction as the Daleks - or maybe even more.

At this point we can see a way for a single-universe model to survive by making CTC in which the people killed by the Daleks simply get killed by either the Cybermen or the Movellans. This means reorganising the whole time stream so in any event it makes a continuous time stream model impossible.

In City of Death the main villain was Scaroth. Scaroth was a Jagaroth pilot whose ship exploded on lift off from Earth in 400,000,000 BC. The explosion of his ship caused the spark which created the very first single-celled organisms to form from the prehistoric goo.

Scaroth was caught in the warp-field and was split into twelve segments. These were scattered across time to reappear during various periods of Earth's history.

Not being one to give up easily, Scaroth influenced the way science developed on the planet in order for him to be able to rejoin and prevent his ship from exploding. Naturally the Doctor was able to prevent this from happening, but it raises so me serious questions.

If Scaroth had managed to lift off from the earth the second time, how would he have got there in the first place, as the technology he used was that of a reality in which his ship exploded splitting him into 12 pieces. Unlike Genesis of the Daleks, there is no obvious way to form a CTC in order to save a single-universe model, if he does take off.

This story is also an example of determinism, as no matter what Scaroth does he will ultimately fail in a single-universe model. On the other hand in a multi universe model it is quite feasible that this could and has happen in a infinite number of different realities. One of the reasons I believe in the multiple-universe model in the Doctor Who universe is that I would like to think that the Doctor actually has some form of free will unlike that shown above.

There are many more examples of the problems of time travel. I have tried to illustrate three examples - one in which a CTC exists, a second in which there are problems but a CTC could be made to fit, and lastly a story in which it is impossible to fit a CTC.

This I hope shows that when people talk of mapping out a single timeline for the Doctor Who universe there are many variables that need to betaken into account, and that it is not really possible to justify such a map as the time line is continually changing; i.e. one minute there might be a Dalek war at the year 3000, the next minute due to a change in realities there might be a Cyberman war in the year 3000.

I could go on for ages about other stories, but instead at this point I will end this article and let you get on with rest of TSV. If anyone disagrees with anything in this article then please write into TSV and say so, as I would be interested in hearing people's ideas on this subject.

This item appeared in TSV 30 (September 1992).