Time Turner paradox

Question

Its clear from the HP books that more than one time turner existed. What would have happened (before the Order of the Phoenix) if two persons at the same time use two time turners to change the past?

For instance, lets suppose, had Snape used a time turner to kill Sirius and Harry had used another one to save him. What would become of Sirius?

Here is a great essay on time travel in Potterverse at the Sugarquill website which you might like. It's thorough and deals with pretty complex ideas regarding time travel. I think, though, that your question is very subjective -- it is contingent upon which theory of time travel a person subscribes to, and people are going to have multiple ideas about how time travel works. Unfortunately I'm going to VTC based on this, but others might feel differently. I like the question; I just think it's too theoretical. — Slytherincess, May 10 '14 at 10:24
I've no idea why someone's voted to close. This seems very answerable with canon quotes. — Valorum, May 10 '14 at 10:38
@Richard - 1. People not knowing canon 2. people not knowing the SE definition of "opinion based" (hint: google "Good Subjective") — DVK-on-Ahch-To, May 10 '14 at 13:23
@DVK - From the FAQ about types of questions you shouldn't ask - "you are asking an open-ended, hypothetical question: “What if ______ happened?” Just because it may be answerable, doesn't make it a a "good subjective" question. It's still a "what if this happened" question. I dv and flagged for that reason. — JohnP, May 10 '14 at 13:42
I think this question, removing the Harry Potter references, resembles many common time travel paradoxes e.g. the Grandfather paradox, just slightly altered. I've VTC'd because this isn't really appropriate for this site - moreso for the philosophy SE — Often Right, May 11 '14 at 01:35

Valorum · Answer 1 · 2014-05-10T10:52:26.857

JKR herself seems to subscribe to the principles seen in the Terminator films, e.g. that time travel results in a variety of differing timelines upstream, but that these don't generally effect the timeline (butterfly effect style) downstream unless major changes are made. The shorter the jump, and the less interaction you have with other actors, the less impact you're likely to have.

Although the first Terminator film appeared, superficially to obey the rules of the predestination paradox, we later learn that the ability to make informed choices about the future allows time travellers to affect the outcome of events.

Bringing us back to time-turners, while in Prisoner of Azkaban we only see a causal loop (e.g. Hermione and Harry play their parts in an event that has already taken place) we learn from Pottermore that unmoderated use of time travel can result in un-births (e.g. unstable temporal paradoxes)

Any and all time travel within the Potterverse has the potential to result in the user's disappearance therefore a second time loop existing within your own original loop (e.g. caused by the use of a second time-turner by another person) would simply amplify the risk of this happening...

Ellie: Why didn't Harry use the time-turner to save his parents?

JKR: Oh, that's a very good question, that. But it would take us into "Terminator" territory, if you've ever seen the "Terminator" films... but never mind. Well, the time-turner was a very difficult invention for me, because it created as many problems as it solved. And anyone who's read Order of the Phoenix may have noticed that during the climactic scene in which they chase through the Ministry of Magic, they shatter all the time-turners, thereby preventing them using those in the future.

and

J.K. Rowling's thoughts

I went far too light-heartedly into the subject of time travel in Harry Potter and the Prisoner of Azkaban. While I do not regret it (Prisoner of Azkaban is one of my favourite books in the series), it opened up a vast number of problems for me, because after all, if wizards could go back and undo problems, where were my future plots?

I solved the problem to my own satisfaction in stages. Firstly, I had Dumbledore and Hermione emphasise how dangerous it would be to be seen in the past, to remind the reader that there might be unforeseen and dangerous consequences as well as solutions in time travel. Secondly, I had Hermione give back the only Time-Turner ever to enter Hogwarts. Thirdly, I smashed all remaining Time-Turners during the battle in the Department of Mysteries, removing the possibility of reliving even short periods in the future.

...a second time loop existing within your own original loop..., What is meant by original loop? — Tom Lynd, May 10 '14 at 11:12
@TomLynd - The original loop is turning the time turner, travelling back to an earlier time, then waiting until you catch up to yourself again/ — Valorum, May 10 '14 at 11:13
@TomLynd - A second time loop could either exist within your loop, or overlap it — Valorum, May 10 '14 at 11:13
I mean the other way.. I mean to say that two persons at the same time makes use of two time turners. In this way the loops are independent — Tom Lynd, May 10 '14 at 11:15
@TomLynd - That's what I'm referring to. It doesn't seem possible to use the time turner to create a secondary loop — Valorum, May 10 '14 at 11:19
You mean to imply that only one person can use the time turner at a time? — Tom Lynd, May 10 '14 at 11:21
@TomLynd - No, I mean that you can only create one loop per timeturner. Multiple people can use the timeturner if they're bounded by the timeturner's chain. — Valorum, May 10 '14 at 11:23

score 1 · Answer 2 · answered Jul 30 '14 at 14:43

Easy answer - it wouldn't be possible for two persons to use the time-turner at exactly the same moment. One would be some fraction of a second earlier than the other one - he would go to the past and the second time-turning action wouldn't happen until he has completed his loop. Once he is back to the same situation he will fist see his self disappear and a fraction of a second later the second one enters his own time-loop.

score 0 · Answer 3 · answered Nov 23 '18 at 03:23

Based on what we know from Prisoner of Azkaban, I'd say your example couldn't happen.

It would not be possible for Snape to go back in time and kill Sirius, because if Snape would truly go back in time and kill Sirius then Sirius would have already been dead. If Sirius survived the first time around then nothing can change that. Harry was only able to go back in time and save Sirius because Sirius had been saved all along. Had Sirius been killed all along, Harry would not have been able to go back in time and save him.

The reason this is so is that the way time travel is depicted in the book is that the events of the present already take into account the effects of future time travelers. This is why Harry was saved from the dementors — his future time-traveling-self's patronus already existed in the present and was able to save him.

So essentially, whatever happens in the present is what will remain. So if Sirius is saved in the present then Harry would be able to go back in time and save him, and Snape would not be able to go back in time and kill him. If, however, Sirius is killed in the present then Snape would be able to go back in time and kill him, and Harry would not be able to go back in time and save him.

This results in time travel being pretty counter-intuitive because you can only go back in time to do something that has already been done, which makes it kind of pointless to go back in time (other than for pure observational purposes) unless you already know that your future self was the cause of something, in which case you would have to go back in time to complete the events.

Time Turner paradox

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