I have a problem with the Doctor Who timeline in several episodes. For example, in the 10th Doctor episode, "Blink", the Doctor and Martha Jones were trapped in 1969 and are able to leave a message for Sally in 2007 to assist them getting back the TARDIS, which was stuck in 2007. After the problem was solved,

Sally encountered the Doctor in 2008, but at that time the Doctor didn't even know Sally, and Sally passed the Doctor the clues to escape from 1969 - in particular, the DVD Easter Egg script that Larry wrote in 2007.

In my point of view, Sally wouldn't be able to solve the problem if the Doctor didn't know about Sally before the Weeping Angel sent them back to 1969, and the Doctor can't leave a message to Sally in 1969, because the Doctor couldn't know the exact events that would happen in 2007.

Sally needed to know about all this before the Doctor was trapped, and her source of information is the Doctor, and at that time the Doctor don't know about the trouble he was going to encounter.

This complex timeline of Doctor Who is a "chicken and egg" paradox, isn't it? There are other examples of this in Doctor Who, such as:

  • The 11th Doctor episode "The Big Bang" - how did the Doctor get out of the Pandorica?
  • The 10th and 11th Doctor "River Song" story arc.
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    Time is like a big ball of wibbly wobbly timey wimey... stuff. Commented Aug 9, 2011 at 8:33
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    Can we have a wiki explaining "Time"? Commented Aug 9, 2011 at 15:31
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    I think this is also called the Bootstrap Paradox. Information/item created from nothing. An example being a man going back in time to give himself a time machine which he then uses himself to give to himself. Where did the time machine come from in the first place?
    – riv_rec
    Commented Aug 10, 2011 at 14:09
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    "wibbly wobbly timey wimey" is a quote from the Doctor. c.f. the TV Tropes page about this: tvtropes.org/pmwiki/pmwiki.php/Main/TimeyWimeyBall
    – Tony Meyer
    Commented Aug 12, 2011 at 10:58
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    Modern understanding of physics suggests that, given time travel, that particular variety of paradox is entirely possible, because it's self-consistent (unlike the famous grandfather paradox, which renders itself impossible). There is nothing that says the information actually has to have an origin, rather than its history being a cyclically infinite regression. Or, as the Doctor puts it, "People assume that time is a strict progression of cause to effect, but actually — from a non-linear, non-subjective viewpoint — it's more like a big ball of wibbly-wobbly... timey-wimey... stuff."
    – Darael
    Commented Feb 1, 2013 at 18:20

9 Answers 9


That's more or less what's explained in the episode.

Remember the video of The Doctor explaining what time is?

Time is like a big ball of wibbly wobbly timey wimey... stuff.

There's a "Children in Need" mini-episode on that theme, too. The Doctor (10th) find himself stuck with The Doctor (5th) on the TARDIS. The only way to save himself is to press a series of buttons on the TARDIS console.

Which he knows because he (the 5th) saw himself do it (the 10th). And the 10th knows, because 5th knows and they have the same memory.

It's a paradox, but it's not, because it's Doctor Who.

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    I think that "wibbly wobbly timey wimey" pretty much explains any paradox in Doctor Who.
    – Tango
    Commented Aug 9, 2011 at 21:24
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    @TangoOversway: Not quite; it's the signal that the writer is declining to explain. (Quite rightly; sometimes it's better writing to give no explanation than invent a bad one.)
    – Tynam
    Commented Dec 18, 2011 at 18:01
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    There's also the Time/Space pair of mini-eps that play with "we got out because we told ourselves how to get out". Commented Apr 24, 2012 at 19:09
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    "THE WIBBLY LEVER!" Commented Apr 25, 2012 at 7:33
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    "You remembered being me, watching you doing that. You only knew what to do because...I saw you do it" Commented Oct 8, 2015 at 11:20

Believe it or not, these sort of "closed timelike loops" aren't a paradox at all. They've been debated by better brains than yours or mine, and the mathematics holds out.

Quoting from Wikipedia's article on the "Novikov self-consistency principle":

Joseph Polchinski ... argued that one could avoid questions of free will by considering a potentially paradoxical situation involving a billiard ball sent through a wormhole which sends it back in time. In this scenario, the ball is fired into a wormhole at an angle such that, if it continues along that path, it will exit the wormhole in the past at just the right angle to collide with its earlier self, thereby knocking it off course and preventing it from entering the wormhole in the first place. Thorne deemed this problem "Polchinski's paradox".

After considering the problem, two students ... were able to find a solution beginning with the original billiard ball trajectory proposed by Polchinski which managed to avoid any inconsistencies. In this situation, the billiard ball emerges from the future at a different angle than the one used to generate the paradox, and delivers its younger self a glancing blow instead of knocking it completely away from the wormhole, a blow which changes its trajectory in just the right way so that it will travel back in time with the angle required to deliver its younger self this glancing blow.

This is the model that current "Doctor Who" writers follow whenever "wibbly wobbly timey wimey" is invoked.

In summary: time travel (whether using closed timelike loops or TARDISes) cannot be used to create a contradiction, which is a paradox -- but it can be used to create self-consistent loops.

  • would that not be perpetual-motion-like though?
    – fbstj
    Commented Sep 3, 2012 at 5:54
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    @FallingBullets: Yes, but if you have a time machine, perpetual motion is no problem. Certainly not compared to the Bootstrap paradox. (Having said that... the Doctor generates information paradoxes all the time, but he's careful to avoid physical object paradoxes. The note in "The Big Bang" is a perfect example of both - after he receives the note he knows information which is generated out of nowhere by paradox, but he's careful to throw it away and write the same note so as not to have a paradoxical object around.)
    – Tynam
    Commented Sep 3, 2012 at 8:27
  • Setting aside quantum limitations, it would be possible for each iteration through the time loop to end up with the billiard ball's path being closer to an equilibrium path where it would knock itself into the exact same path repeatedly. Even if there would have been no reason for the ball to get onto the equilibrium path perfectly, its path could get so close as to be practically indistinguishable. Add in quantum limitations and the path could be indistinguishable.
    – supercat
    Commented Oct 16, 2014 at 22:43
  • @supercat -- no, that's not it. The ball never "gets closer" to the equilibrium path because it's always on the equilibrium path. Commented Oct 17, 2014 at 13:58
  • @Blazemonger: Setting aside quantum effects, a ball could never "enter" the equilibrium path; the only way it could ever be on the equilibrium path would be if it had always been. On the other hand, if the equilibrium path served as an attractor, it would be possible for a ball to get arbitrarily close to the equilibrium path even if it started out on some other path.
    – supercat
    Commented Oct 17, 2014 at 20:32

Basically, the point of that episode and others like it (Such as Big Bang 2 when the only reason the Dr got out of the box is that after he got out he went back and told Rory to get him out.) is that time isn't "linear" in respect to itself - a person always moves forward along his own timeline, but the universe's timeline may have "loops". In the Doctor Who universe, time may be circular (and as noted by others, Paradoxical).

Look at it this way in Blink - once the Doctor was able to help Sally save the Tardis from the Angels, it was inevitable that Sally would, in her future, share the story with the doctor. Once the doctor received the information from Sally, it was inevitable that he would help her, in his future. It's a perfect, unbreakable circle - once entered at any point it cannot be left.

  • But how can the circle be started? Commented Aug 9, 2011 at 15:30
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    it depends on who you are looking at. From the Dr.'s perspective, the circle starts when he receives the information from Sally. In his timeline, that is the start and the end is when he saves the Tardis. From Sally's perspective, the circle starts when she first receives contact with the Doctor and ends when she gives him the information. The struggle to understand is that you are looking at it as if there is a single, linear, timestream. Really, there is a timestream for each person, and those streams can intersect - you just can't loop back on yourself. (Think Madame De Pompedour.)
    – user158017
    Commented Aug 9, 2011 at 15:58
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    @gunbuster363 Circles don't have a start, or an end either. That's the whole point. Commented Feb 1, 2012 at 18:50
  • They don't have points either!! ;~) | The circle starts relative to who you are looking at, @user158017 states above
    – user001
    Commented Oct 8, 2015 at 11:05

i believe that the doctor may have sent out a different message "earlier" (in a previous continuity of time) to sally sparrow after meeting billy in 1969, either through a video or through billy (a simple message such as "send the TARDIS back" may have enough) later in 2008 she meets the doctor outside the dvd store and gives him some sort of account of what happened, so that when the doctor "later" (for the doctor this happens later on) gets stuck in 1969, he checks that information, and this may happen many times until it stabilizes into a healthy loop. the truth is, that once she tells the doctor even a little, "before" (from the doctors perspective) he travels to 1969, a time loop is created that will loop over until the same exact events happen in 1969 twice (think about it) so without a time loop there would be a problem.


Yes, it has a paradox. No, the billiard ball comment is not relevant.

There are different types of paradoxes. The type all these comments seem to discuss is a paradox where one changes his/her own past through time travel, causing a circular loop that cancels itself out. For example, if you travel back in time and kill your past self, you create a paradox. Since past-you died, future-you never existed, thus future-you never killed past-you, meaning future-you does exist, so future-you kills past-you, and so on.

The paradox in this episode is essentially the same, with one major difference. The problem here is not that future-you can't kill past-you, but rather that future you can't save past-you. The difference between the paradoxes is that the "kill" paradox can theoretically occur, while the "save" one can't.

Let me explain. Theoretically, and particularly in Doctor Who, time is a dimension, yet each person has a personal timeline, or progress. In DW we follow the Doctor's timeline.

-If you are at a certain point in your timeline, then jump to another point in the "past" and kill the "past" version of yourself, you are stuck in a continuous loop, but you had some way of getting there. The paradox started at a certain point. It is logical.

-If a future version of yourself saved you, there is no connection or progress. You could never reach that "future" point in time because you died. Meaning, you could only survive because you saved yourself, and you only saved yourself because you survived. The events are not a part of any timeline, and are dependent on each other.

The "kill" paradox could occur in nature, although it may result in a black hole, or some "Back to the Future" type of situation. The "save" paradox can NEVER happen.

Sally survived because the Doctor warned her and guided her, but the Doctor was only able to help her because she survived. Every single aspect of the episode was impossible, from the transcript to Sally's eventual meeting with the Doctor. Steven Moffat tried to wave if off as Wibbly Wobbly, but DW is a science fiction show, and he can't just disregard science (that hasn't stopped him, though).

To be honest, this episode might be impossible, but it's still fantastic and pretty clever. One of my favorites.

  • It's a science fantasy show with science fiction elements
    – user001
    Commented Oct 8, 2015 at 11:09
  • More importantly... It's a kids show and story takes precedence
    – user001
    Commented Oct 8, 2015 at 11:10
  • I agree, the story was great. But the question wasn't if the story was good or if it's easy for kids to understand, the question was whether or not in contained a paradox, which it does. Commented Oct 9, 2015 at 1:58
  • In real world yes but in the show's internal logic no. I was referring to the fact it's not sci fi, it's sci fantasy with scifi elements, so paradoxes don't exist unless they need one
    – user001
    Commented Oct 9, 2015 at 7:14
  • But that's not what a paradox is. A paradox is just an inner contradiction, and DW can't decide paradoxes don't exist for the sake of lazy writing. A paradox is equivalent to a plot hole, especially when it's not acknowledged in the episode. Commented Oct 9, 2015 at 7:31

How about a multi-universes answer to this question:

All of the universes where the 'send the information to the future' method did not work - don't exist because the story ends there.

We are just watching one of the examples of the 'information successfully arrived in the future' outcomes and that makes the story work!

This means that while there are possible cases where Sally does not solve the problem (or even get the message) are automatically outside of the world case we are observing.

This is the same for the case where the Doctor remembers pushing the right buttons in the Tartis -> In all of the cases where they push the wrong sequence of buttons the 10th doctor does not exist.

In another way of saying it: Time travel could be naturally iterative (and wibbly wobby) thus every event that takes place is one iteration (or the last iteration) case where everything that didn't work.... has now worked itself out.

How happy ending....


Well remember. He didn't know her because he had never seen her face. He only read the transcript. But also remember just because he was in 2008 doesn't mean he had been to 1969 yet.


I think time uses all possibilities. The possibility Doctor does all to free past doctor, to be still alive, if not there again, and again stays created new possibility Doctor to use new possibility. Time Ping-Pong Paradox.

  • Is this canon?? Commented Nov 14, 2013 at 17:57
  • Canon? In Dr. Who? Always a slippery notion. Didn't really exist until several decades into the franchise at least, and was basically optional for the writers until the reboot. Commented Feb 6, 2014 at 16:16

Blink was a paradox/bad science or badly explained, but I am banking on the former since RTD is not as good as Steven Moffat. Yes, time-streams do not run linearly, but what happened in Blink could not conceivably have happened unless Sally, figured out on her own the first time, which I doubt that she did.

Regarding the Pandorica, easy explanation is that it was not an impenetrable prison after all, and that the Doctor escaped somehow and acquired a vortex manipulator since he no longer had access to the TARDIS.

The River Song arc is not paradoxical at all.

Let's say that Doctor A (which is the very first instance of the Doctor) reached the Field of Trenzalore and River A (First instance of River was not stolen from The Doctor's timeline). The Silence then go back to steal River B (Younger instance of River) in order to prevent The Doctor from reaching that point. River B grows up to attempt to kill Doctor B and uses up all her regeneration energy to save him, and the Doctor does not find out about his death since it has not happened yet. Eventually, River B ages some more, and is caught by Kovarian and sent to kill Doctor B (An instance of the Doctor after Doctor A), who is the first Doctor and ONLY instance of the Doctor dying at Lake Silencio. River B goes back in time to the Donna era with Doctor C and dies (First time that the Doctor is called to the Library since River is the one who beckons for him to come). The second instance of The Silence intervening results in a River C that uses up all of her regeneration energy to save Doctor C (older instance of the doctor), who finds out that he dies at Lake Silencio. The Teselecta dies in Doctor C's place for all of future instances of this scenario when River C kills it after finding out that the Doctor has no intention of dying, and River D and Doctor D (delayed invite for The Doctor) are invited to this event. River C does not suggest to Amy that she already knew that it would happen, just that she knew that the Doctor did not die. Eventually River C calls Doctor E to the library and is saved this time because Doctor D gave her his screwdriver in anticipation of her death. After it happens once again with River E and Doctor E being invited to the event, River D tells Amy that she already knew that the Doctor D wouldn't die as she had been invited to the event when she had been younger. I believe that the Pandorica occurred as a result of The Silence eventually finding out about Doctor D (and future instances) surviving to reach Trenzalore, resulting in the second Big Bang first occurring in Doctor E's timeline. Yes, I do believe that many extra points in time were created for future instances of The Doctor as they could not have feasibly occurred in previous scenarios where the timeline was being fixed so that The Doctor could reach Trenzalore.

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    -1 for "RTD is not as good as Steven Moffat"
    – deworde
    Commented Oct 13, 2011 at 14:56
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    -1 for an offhanded "Blink sucks because the writer isn't my favorite" followed by a mass text unrelated to the original question. Commented Apr 24, 2012 at 19:09
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    Super bonus -1 for "RTD is not as good as Steven Moffat" since Steven Moffat wrote that episode..
    – KutuluMike
    Commented May 24, 2012 at 20:24

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