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In the new Doctor Who series, we are frequently reminded that the Doctor "becomes a part of the events" as soon as he enters a time period, and is unable to (or should not) backtrack to fix his mistakes along that series of events.

Doctor Intervention

The question is (please refer to the diagram):

While he cannot travel back along a series of events of which he is a part (i.e. within the blue rectangle), what prevents him from traveling further into the past beyond those events and leaving a message for himself, or better yet, geting a proxy to help him out? Isn't this what he did in Blink? Why didn't the reapers appear then? How do we know when it's just "wibbly wobbly timey wimey" stuff and when it's time for the reapers?

Edit/Clarification: In Blink, a future Doctor stuck in the past forwards information to the present to get someone to alter his timeline. No more than one copy of the Doctor was present at any given point of the timeline, but the net effect was the same -- alteration. Why was this allowed?

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    The diagram is a result of me getting carried away while trying to explain the behavior of time to myself. If the text is unclear (SE has reduced the original image), try: i.sstatic.net/gTlVw.png
    – HNL
    Commented Feb 1, 2012 at 16:13
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    I like to call the Reapers "paradactyls."
    – Ian Pugsley
    Commented Feb 1, 2012 at 16:14
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    excellent diagram!
    – NiceOrc
    Commented Feb 1, 2012 at 21:14
  • Not stictly an answer but, he crossed his own timeline with Martha Jones (Martha's first episode, the future Doctor saw Martha on the street just prior to her seeing the present Doctor in the hospital); and he crossed his own timeline with Amy, even interacting with himself (granted the universe was imploding but, in the final episode of the 2010 series, in the museum he went back in time, spoke to himself in front of Amy and Rory and then "died").
    – Xantec
    Commented Feb 2, 2012 at 13:28
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    Sigh, everyone knows the normal course of history is black, the speaker's plans are red, and deviations are blue. Blue for normal and red for deviation is just plain wrong ;)
    – Scott Chamberlain
    Commented Oct 12, 2013 at 5:55

2 Answers 2

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From my understanding the timeline is generally pretty robust, able to change to all of the things the Doctor and others do to it. However, in Father's Day they establish that having multiple copies at one location, especially interacting with one another, is enough to "weaken" the timeline and cause serious problems (like spontaneous paradactyl attacks). If he and Rose simply observed the other copy watching instead of interfering their would have been no problem. In fact on several occasions the Doctor refers to times where he has witnessed the same event multiple times.

I don't think its the copies that cause the problem (although they raise the risk greatly) its the interaction of multiple copies (perhaps even through messages) that can cause a problem.

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  • I've added a clarification to the question...
    – HNL
    Commented Feb 2, 2012 at 9:50
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    I believe the point is interacting with another person to cause the change is far enough removed from him directly impacting himself that the wibly is wobbly enough for it.
    – Ashterothi
    Commented Feb 2, 2012 at 20:02
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The problem isn't in crossing his own timeline, it's creating a paradox by rewriting his own timeline.

For instance, the whole point of the Doctor returning to his own death in the 2011 series is that he had to preserve a sequence of events that already happened -- when he failed to do so, at a fixed point in time no less, it nearly caused time itself to implode around him. Crossing his own timeline in that case was necessary to preserve causality.

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  • This also explains why the adventures with multiple doctors are possible.
    – Christi
    Commented Apr 20, 2013 at 8:49
  • @Blazemonger The first part of your answer is right on target, not so much the second. They don't "go back", that is, don't travel back to that point in time, rather the anomalous timeline ceases to exist and is "rewound" when he and River touch. He never crosses his own timeline in this instance.
    – confusedwhovian
    Commented Jan 8, 2016 at 16:19
  • What "anomalous timeline"? There was no other timeline -- when River failed to kill the Doctor, time stopped and everything was suddenly happening at once. Time couldn't be "rewound" because it ceased moving forward entirely. Such is the nature of a fixed point.
    – Blazemonger
    Commented Jan 8, 2016 at 16:35

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