# How is Harry able to cast the Patronus to save himself & Sirius?

When we read about the Dementor attack down by the lake in Prisoner of Azkaban, we experience the situation twice. The use of the time turner affects both as both Harry's are present in both situations, we just see it from two different perspectives, first being before the use of the time turner when he believes he sees his father but is actually himself after using of the time turner.

What I don't understand is - the first time the Dementor attack ever happened there could not have been the version of Harry after the use of the time turner to cast the patronus in the first place. The problem lies it the fact that in order to get to the situation where Harry is able to use the time turner he requires there to be a version of himself which has already used the time turner - so if the Harry that used the time turner hadn't saved them, he wouldn't have been able to get into that position.

Presumably the Dementors would have succeeded in kissing him the first time so he wouldn't have been in a state to be able to use the time turner. Even though that is a presumption of what would have happened, the original part still doesn't make sense.

• time travel my friend, time travel. – Himarm Aug 3 '16 at 12:59
• Buckle up, kid. – DavidS Aug 3 '16 at 13:12
• Wibbly wobbly, timey wimey stuff... – tilley31 Aug 3 '16 at 15:04
• Keep in mind, also, that physicists are quick to point out that just because we experience time in a linear fashion, past to present, as we do, does not mean that the rest of the universe does. Trying to apply "this must have happened first" to a concept like this, fictional or not, is not going to fit. – PoloHoleSet Aug 3 '16 at 20:01
• – Oriol Aug 3 '16 at 20:48

What you are describing is often called the bootstrap paradox, or a causal loop. Essentially, Event A causes Event B, then Event B leads to time travel to before Event A, and CAUSES Event A. From outside the loop, there appears to be no outside cause for the chain of events.

It is a fairly common trope in fiction involving time-travel. Best not to think about it too hard.

• This kind of thing is exactly why I don't like "time travel paradox" story lines. Both physics and logic argue that you cannot go back, only forward. I know fantasy is supposed to be about doing the impossible, but time travel is the dividing-by-zero of plot devices. – T.E.D. Aug 3 '16 at 20:46
• @T.E.D.: multiple-timeline time travel (like e.g. Back to the Future) does indeed require pretty much complete suspension of logic. But the single-timeline approach involved in bootstrap-paradox situations is pretty internally coherent (when done well, and Rowling does it pretty well in Prisoner of Azkaban). At any given moment, ordinary physical causality holds, except for the single moment where the time traveller “arrives” from the past/future. “Globally” there may be loops in causality, but “locally”, everything is causal. – PLL Aug 3 '16 at 21:05
• @PLL - I'll agree those are indeed better. However, you still have to somehow posit limits on what your traveler can do, or that they can "change" what has in fact already happened. Of the two I vastly prefer the former (which is almost never done, unless you count Dickens), but the obvious solution is that the physicists are right and it just can't be done. – T.E.D. Aug 3 '16 at 22:09
• @PLL You have it backward. Single timeline time travel with causal loops is what requires the suspension of logic. Multiple timelines are basically attempts to get away from the suspension of logic needed for single timeline causal loops – Kevin Aug 4 '16 at 1:01
• "Best not to think about it too hard." i love this sentence. – Hardik Vaghani Aug 5 '16 at 9:20

As @Himarm and @Irishpanda mentioned, it's time travel.

Presumably, in the Harry Potter universe, where there's the existence of time travel, the effects of the time travel are within the same timeline.

In pre-time turner, Harry and Hermione have already gone back to before Buckbeak was executed. The same 'sort' of thing happened there as well, Hermione tossed the rock that alerted the group to the incoming executioners.

Side Note: This kind of gives credence to the whole, "Why didn't anyone go back using a time turner and take care of Voldemort?" Well because if they had, it would have already happened.

• Re: your side-note: I'm not sure. Although time-travel as depicted in HP:PoA works that way, Hermione describes cases where it worked very differently. It's possible that she was mistaken, but I think we're to assume that it doesn't always work the same way (due presumably to factors we don't know about). – ruakh Aug 3 '16 at 23:50
• True. Just wanted to add it as food for thought.... Time Travel is weird :P – John Grabanski Aug 4 '16 at 12:13
• @ruakh Didn't she also describe that it had grave consequences? That time doesn't like it when somebody messes around? Of course, it is quite a bit of antropomorphication of time, but in the HP universe, it's not too weird. – Luaan Aug 5 '16 at 8:22
• Your side note reminds me of the Orson Scott Card book "Pathfinder" which explores this sort of paradox in amusing detail. (Definitely not his best book, though.) – Wildcard Sep 9 '16 at 6:39

The answer to this lies in how the book treats the timeline. Time travel as used in Prisoner of Azkaban seems to follow a one-pass method.

There are a few different ways of handling time travel in fiction, but for the purposes of this answer only one distinction is particularly relevant; that being, whether it is a one- or two-pass timeline.

A two-pass timeline is like that of Back to the Future. In a two-pass timeline, we first see the original, unaltered events, and then later see them in the second timeline with the changes enacted by the time travel.

A one-pass timeline is subtly different in that we are never shown the timeline that existed without the effects of the time travel, if it existed at all. One-pass timelines don't occur very often in fiction, as they can be quite confusing to the reader.

The trick here is that Rowling writes our first look at the scenes in question with our viewpoint from a distance, carefully obscuring the events. During the execution, the trio hear the swing and strike of the axe and Hagrid sobbing and draw the sensible conclusion, as do we as the readers. Then, when we learn about the Time Turner, we think that there is now the chance to go back and change the events.

What Rowling does masterfully here is trick us into thinking that we're looking at a two-pass timeline when it really is a one-pass timeline. The events of our first look at the timeline are actually perfectly consistent with the changed version; the axe still strikes, Hagrid still sobs, the Patronus is still cast, but the context we were missing the first time through leads us to believe that these were the 'wrong' events, that they have to be changed. It's only the second time we see the events that we realize that the actual events worked out just fine, and it was just our limited knowledge that made us think they needed to be changed.

So if you're trying to wrap your head around how Harry cast the Patronus the first time, instead try to make sense of this: there was never a version of that scene that didn't have both Harrys in it.

• Very well explained. I've never heard the "one pass" versus "two pass" method of explaining it. This is, IMO, the best answer. – Wildcard Sep 9 '16 at 6:40

What I don't understand is - the first time the Dementor attack ever happened there could not have been the version of Harry after the use of the time turner to cast the patronus in the first place.

But that's not how it went down: the Dementor attack only ever happens once.

There has 'always' been a second Harry that sprung into existence shortly before the originals travel down to Hagrid's while carrying memories of events that have not yet happened. The existence of the second Harry is later explained when the original makes use of the time turner.

By using the time turner, Harry and Hermione did not change anything — it simply explains how events could have happened as they did.

Because JKR use a deterministic approach to time travel ( at least in this instance ). You have to imagine that the time flows in a straight line which can't be changed, so everything that happens is already written and meant to happen. The Harry who calls his Patronus always existed to call his Patronus for the other Harry.

On the other hand there is the non-deterministic approach, you see this in Back to the Future. Here the time flow may branch and each action is not pre-written. You see Marty go back and forth in time and each time he interacts with something (leave back the sport book). The future changes reflecting different branches of the flowing of time.

• In other words, post-timeturner Harry didn't go back in time and change the past, he only went back in time and played his part in what had already happened. – Kevin Aug 4 '16 at 9:26
• @Kevin pre-timeturner Harry didn't cower before the dementor, he only played the part of cowering that had already happened. With deterministic time travel, everything has already happened in a sense, and everything is "playing a part". Or nothing is. – Yakk Aug 4 '16 at 15:13
• @Yakk Good point, these kinds of loops make 0 sense if the future weren't pre-determined. The entire timeline is already set in stone, the current time just follows the path. – Kevin Aug 4 '16 at 16:36

A lot of the answers here go along the "it doesn't make sense" and "best not to think about it too much" lines. If it hurts to think about it, then yes, just enjoy it! But there is a way of thinking about it that makes (a sort of) sense.

The many worlds interpretation of quantum mechanics states that every decision we make results in a new universe being created. If we reach a fork in the road and decide to turn left, then we create a universe which represents that decision - but another universe is created in which we turn right, so both outcomes actually occur.

How does this help? Well, when Harry makes his decision to use the time-turner, he is creating two universes, one in which he used it, and one in which he didn't. In the one where he didn't, he never went back and stopped the dementors. But in the one where he did, Harry is saved. The point is that that "possibility" wasn't made real until Harry used the Time Turner. The Harry that was saved, was saved because that was the decision that created that universe (and therefore timeline).

Having said all this, this explanation probably doesn't help that much. Quantum Theory comes with its own set of bewilderments and counter-intuitions. But it is real, testable and pretty much right, in terms of its ability to explain what happens in nature. Further reading:

http://www.preposterousuniverse.com/blog/2014/06/30/why-the-many-worlds-formulation-of-quantum-mechanics-is-probably-correct/

or just google Many worlds interpretation.

• This explanation makes no sense. Harry couldn't have forked the universe, because he would already be dead. Forking makes sense in some time travel stories, but not this one. HP uses predestination, not forks. – Mooing Duck Aug 5 '16 at 0:16
• @MooingDuck: Actually, HP uses an inconsistent mashup of various time travel systems. If everything happened according to predestination, it would be impossible to change the timeline, but we know from Hermione's comments that it can be changed. – sumelic Aug 5 '16 at 11:55
• @sumelic: Yes. Characters say and imply that under specific circumstances (seeing themselves), they can disrupt the timeline, but we never see this occur. You're right that the laws mean it's probably possible though. – Mooing Duck Aug 5 '16 at 16:47
• @MooingDuck Saying that Harry would "already be dead" means you are still thinking in terms of a single universe with a single timeline. The many worlds view says that there are in fact uncountable numbers of parallel timelines. In some of these, Harry is dead. In others, alive. What I am suggesting is that the Time Turner allows the user to travel, not just to a different timeline, but to a different point in that timeline. Thus it is perfectly possible for Harry to be alive in one timeline, then travel to another in which he saves himself. – bornfromanegg Aug 8 '16 at 15:18
• The only potential forking timeline explanation if is there was a timeline that's never implied in the books where Harry didn't get caught by the dementors, who traveled to the book timeline where everything was different, saw in this timeline he was to be killed by the dementors (wildly different than his own recollection), and saved himself, thus initiating the loop. But that theory revolves around the forked timelines happening well before the dementor attack ,as well as a new timeline where he never needed saving. – Mooing Duck Aug 8 '16 at 18:20