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time travel - Is the Doctor's inability to backtrack along his own timeline consistent?


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?



Answer



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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