How can the future both be set in stone and possible to change?
 

So this really bothers me. In Tale of the Yokai, all the interactions between the Turtles and the people of the past were implied to be how history had always unfolded, even though the episode just before said Savanti tried to change the past. So far the past has not yet been altered by time travel but for obvious reasons, I think we can assume the turtles will succeed in preventing the black hole generator from destroying the earth.

So how can one episode depict time as always following a certain path while half a season is dedicated to to mission to change the future?

Just because the turtles going back in time was meant to happen doesn't mean the time line can't be changed. I'm actually pretty sure their concerns in that episode revolved around not changing things for the sake of the future.

Fixed points in time?
Hell if I know.

The turtles traveling back in time didn't change the time line, it only completed the events that had already happened.

This

The earlier events depicted in tales of the Youkai always happened - the 'Fixed point in time' analogy is a fair one. Even if the details were different (such as the turtles being present), the actual outcome could never be changed. Something would always occur that would prevent a paradox.

Ok - Overthinking hat on...


The attempt to save Earth is - at the moment - just that, 'an attempt'. They are giving themselves some breathing room here because there is no known future (beyond what Renet mentions) in the Turtles own timeline after the events of Earth's destruction. They instead go straight back.

As such, they have no set events post that point.

In theory, they can alter the outcome that would have come about by resetting the events they 'just' witnessed. If they attempt to prevent the event from occurring too early they would theoretically fail as they would be attempt to change a set event.

Technically speaking, any attempt by the Turtles to alter the events of Annihilation Earth prior to the battle around the Heart of Darkness will fail - because they know that the future is destined to reach that point. They can make the effort, but the paradox rule should kick in and prevent it from working (something will happen to reset events to still occur, albeit in a slightly different way)


The paradox here is that if they succeed in preventing the heart of darkness working, they dont need to go back in time to prevent it.

So in theory - the Battle around the Heart of Darkness will occur. Splinter will Die. The Device may even activate - but because there is no set future post that point, they could be in time to destroy it without it destroying Earth (rather like a save point in a computer game)

This largely depends on how Nick decides time travel works exactly - but yes there is a paradox to answer here. They easiest 'get out' clause would be to say that the future can only be changed very briefly after the events occur (in which case the best example is Live. Die. Repeat.)

I guess the best example I can think of is the omega 13 device from Galaxy quest (the 13 sec time jump).

Either way, to prevent a paradox they either have to find a way to send a message to themselves to go back to the past and prevent the HoD being activated, or they need to explain that there are rules of paradoxes in time travel for the TMNT universe - in a similar way to how Dr Who explains it.

I've been pondering this myself between "Annihilation Earth" and "Beyond the Known Universe" and well after BtKU aired. Obviously, the conclusion to AE is so beguiling that, like me, you just spend the time between these two episodes wondering how everything is going to be resolved. It's like imagining Bruce Wayne willingly giving up his life as Batman. It's impossible!
Now of course, Honeycutt's resolution is also beguiling what with the rules established in "Tale of the Yokai" (an episode I also believed played a large role in AE). Those same rules applied to "Gargoyles", save for the "MIA" episode where the main character (Goliath) meets someone from the past (Griff) and keeps attempting to keep him from harm in the past, while always finding it impossible to get him back to his home in that time. In the end, he came to understand (series-wise) that he can't change history--but what he can do is bring Griff back with him to 1995 where the series' present took place. I always assumed that the turtles might end up becoming responsible for the Triceratons coming to Earth in the first place or that, one way or the other, the events of AE would happen because they already did (ala "Prisoner of Azkaban").
I've been worried that, unless this show's creative team goes a route like that, they'd have to cheat by breaking their own rules or by employing something very deus ex to resolve this whole thing. Seeing them connect the events of last week's episode to "Wormquake" though gives me some hope.

I've been trying to wrap my head around the time travel paradox myself, especially in terms of that there are now technically two sets of turtles existing at the same time.

It's made a little funny considering that they have David Tennant on board. After all his years as the Doctor and continuing to love and be knee-deep in Doctor Who some way or another, he should have looked at that and said "Um yeah. There's a paradox here, here, here, and here."

Two sets of turtles makes sense to me, I just hope they remember this fact when "Earth's Last Stand" comes around.

Maybe the past turtles will heroically sacrifice themselves, but they'll have to do something with them.

One thing Fugitoid mentioned in BtKU is that the black hole has an effect on localized time travel. Perhaps this may also resolve a paradox.

Well, time will tell :)

But if I'm right then the simplest way for Nick to proceed is that Splinter will get killed, and the generator will go off - despite the turtles best efforts.

Further, the space turtles will be present at the final battle, but will not be able to see or team up with their counterparts. Its likely that one or more of them will try to save Splinter and be unable to.

Once the generator goes off, the space turtles will probably see their counterparts being picked up by past Fugitoid. The space turtles will then probably meet up with Bishop (explaining why he just wandered off) and between them get into the generator and destroy it from inside.

They will somehow survive this and then stay on Earth, continuing the series. The 'past turtles' that were picked up by Fugitoid, go back 6 months and repeat the process, creating a kind of time loop, where each time the earlier group of turtles will time travel back, while the ones who just went through the space arc will continue onward living on the freshly saved Earth.

That way, the turtles still go back in time, but the 'present' day turtles remember everything that happened, save the earth, and dont cause a massive time paradox

This is gonna sound really farfetched, but what if season 4 April has the ability to plant the memories of the original timeline into the past versions of Casey and the TMNT. She's been working on her powers a lot and I'm willing to bet by the time they get back home she'll be on a level similar to Martian Manhunter!

Renet came from the far future and knew of the Turtles as legendary heroes that saved the world. From her perspective, all of this has already happened. From her perspective, the outcome is fixed. We're just watching how they get there.
The Turtles travelling back in time is just a part of the process that will create the future that Renet has already established for us; just as their going back in time in Tales of the Yokai was part of the process that created the timeline established in the series already.

It's not that confusing really, when you consider that we've already had a glimpse of a future coming to pass where the triceratons fail.

The issue with this theory is that the past Turtles witness the Earth's total destruction before they travel in time. They literally watch it break apart and crumble into nothing.

Yeah - that bit I cannot explain away :)

All I can think of is either it will be destroyed regardless and then 'reconstructed'

or

They will somehow stop that future and then send the 'past' turtles back into space to restart the loop

or

They'll quietly gloss over that problem ;)

They are basing this moment on hitch hikers guide. I the story Arther Dents world is destroyed. Then at the ending the series he is able to return to earth in his time and despite knowing what was going to occur he dies with the world anyway.

So the probability is while the turtles of the past see the distraction of earth while the present day turtles realize they have to enter the vortex to destroy it