They explained that last episode - and its a callback to a prior episode - its basically something special that they understand. (I do not recall the exact detail)
I think Walter seeing Peter say, “Dad, I love you.” is a powerful enough final moment for Walter. That Father-Son relationship was the core and hearing that phrase from Peter was huge. Brought a tear to my eye, as did the moment when Peter hugged him earlier.
White Tulip is the 18th episode of season 2. You can read about it here.
- He didn’t quite merge them, but he built a stable doorway between the two so they could work together to fix the universe.
a) That’s the universe they live in now, but Olivia basically forgot everything about it. She only remembers the universe where Peter lives. Walter, after being touched by Michael, remembers both.
b) Walter builds it in an alternate universe and sends it back to the past.
I loved that moment too. Also, September’s realization of paternal feelings and the final “dandelion scene” showing Peter with Etta brought the theme full-circle.
Wait–our Walter did this? Or Walternate or some unnamed third Walter?
If his name is Walter, he’s not unnamed, is he? 
I’d love to see a Walter spin-off series, John Noble playing Walter in another, unrelated alternaverse. Maybe he’s a university professor, or a doctor, aged rock star, or…? More Walter, please.
It’ll be set in 21whatever, he’s just arrived - and to prove that that the boy is from the future, he must then also prove that they can alter the brain in such a way that it would progress to the ‘Observers’…
"How do we know he didn’t invent the thing ? "
Our Walter, in the future where Peter destroys the other universe by using the machine.
Recap of major points (this will be long):
Original timeline - Both Walters are trying to find a cure for their sons. Our Walter fails, but Walternate comes up with one but is distracted by September. September tells two of his colleges that he was there because the moment was important, and they correct him by saying that the boy is important and that September has a second chance to correct his mistake and make sure that Peter lives.*
Walter sets up a machine to open a gateway between worlds on a spot on Reiden lake, which is frozen, in order to give the cure to Peter. He fights with Nina and his lab assistant because they fear that opening a gateway might damage both universes, but goes through anyway only to find that his vial of medicine is broken. He takes Peter to our universe and promptly falls through the Ice. September comes and rescues them. When Walter asks why September says it’s because the boy must live.*
Fast forward to the episodes with the machine. It appears that Peter’s purpose was to activate the machine. He steps in to (we all think) destroy one world to save the other. But instead his consciousness is pulled to his body in 2026 and he sees the result of destroying the other side. When a wormhole opens in Central park Walter says that that’s how the machine got sent back in time, and since the pieces were found then he must disassemble the machine and send the pieces through the wormhole. He has no choice. But if they can pull Peter’s consciousness from the time he steps into the machine to 2026, then when he consciousness returns to the present then he can make a different choice and they can cheat time. So Peter awakes in the machine and decides to build a bridge between worlds which will help heal them**. This, somehow, causes him to be erased from time. We learn in the episode “Making Angels” it’s because it somehow caused September to lose his blue light device, which allows people to see the past, present, and future (apparently his implant is insufficient) and without it he fails to save Peter from drowning as a boy.***
Instead of staying erased however, Peter “bleeds over” from the previous timeline to the new one. After being dosed with Cortexiphan Olivia remembers the old timeline, but forgets the new one. After searching for a way back to his timeline, September tells Peter that it’s been erased because, while Peter was important*, September’s interference corrupted the timeline, which caused Peter to have a child with the “wrong” Olivia, and so Peter was erased to fix it. But because of the Fringe team’s love for Peter, and his love for them, he came back into existence. The other observers wanted to erase Peter before he came back (it’s never explained why), but September thought that if Peter and Olivia got together and had Etta then everything would be as it should.
Fast forward to the end of season four. Bell is trying to make himself a god by destroying both universes to create a new one populated by people he mutated into new creatures. He gains knowledge of how to make a gun that shoots faster than the Observers ability to catch bullets, and a symbol that paralyze Observers when they stand on it. September says that these are “beyond” Bell, so how does he know about them? We’re never told. After Bell fails to create a new world he rings a bell and fades away. In “Letters in Transit” he was shown to be frozen with the Fringe team. What lead up to that? We’re never told.
Season five. Well, I’m sure everybody still has it fresh in their minds.
- These points show that September was talking about Peter and not Michael. The retcon gives the season a purpose (get Michael to Oslo in the future to change the evolution of the Observers) but now creates new questions, like why was Walternate curing Peter so important if the boy who had to live was Michael and not Peter?
** In the new timeline Walter is frozen in amber in 2026 and a wormhole isn’t opened in Central Park, and at the end of season five since Walter disappears in 2015 then he can’t send the machine back. But in order for everything to happen as it did up until 2015, then someone(s) must send the machine back in time. The writers never resolve that.
*** In the new timeline, did the machine seemingly go off on its own, or did someone else set it off? (I have my own theory, but the writers never gave an official answer).
nice write up nobody - the machine was not sent back thru the wormhole - Walter buried it in the past in various places - they had to use ‘the book’ to find where all the parts were - then they re-assembled it.
The amber in central park also housed a dinosaur of some sort - apparently, Walter buried it ‘way’ in the past.
In the original timeline the wormhole that opened in central park was what was used to send the machine parts back into the past. Walter explicitly says so. The details aren’t given, but Walter does use the wormhole in 2026 to send the machine back.
In the new timeline, in order for things to happen as they did, at least until 2015, then either someone else needed to send the machine back, or Walter did at a different date using a different method.
The writers not going over this isn’t too important. After all, it’s not who sends it back or when, just that it gets sent back. It would just have been interesting to see how the writers would have handled this if they had more time.
For some reason my recording stopped before the end (just when Peter looks at the envelope).
Brian
Maybe we’re saying the same thing then - he used the wormhole to bury the parts of the machine in the ‘past’ - and based on the age of the book and myth within it - the really distant past - apparently, on one of these trips - he brought a dinosaur back with him.
you said sent the machine ‘back’ - I took you to mean that he sent it back to the ‘recent’ past.
And I agree- with the ‘reset’ and the ‘current timeline’ and what we ‘know happened’ - none of it lines up correctly.
He opens it to see the ‘white tulip card’ - looks up - roll robot.
Sorry. I mean he used the wormhole to send the machine back millions of years ago.
I think they could have explained things better if the mission to rescue Michael was only 1/2 hour instead of a full hour. But then, maybe they didn’t have any explanations.
I’m going with the latter - they wanted a relatively (heh) “happy ending” with Olivia and Peter and said ‘to hell with temporal consistency, we gots time travel!’.
At the end of season three they erased Peter because the show-runners wanted the next season to be a, “It’s a Wonderful Life,” type scenario. The hell with past continuity, we’ll figure it out later.
I see some of the changes in season five being like that. Only, unless there’s a movie, there won’t be a later.
Regarding the whole “someone new had to send the machine back” paradox, you have to assume that Fringe plays by a variation of BTTF rules. That is, when you go back to change the past, you make a new timeline, and you don’t affect the time travel.
So we have
Timeline A:
Everything from season 1-3, Peter uses the machine to destroy the universe. Walter sends the machine back to the past, which creates:
Timeline B:
Everything from season 4-5. Peter sees a vision of timeline A, creates a door, disappears, reappears. In 2015 Observers invade. Michael and Walter go to the future and create:
Timeline C:
Final 5 minutes of the show. No Observers invade.
The machine still existed in timeline B. It’s just that either Peter didn’t set it off, or setting it off in his own timeline set it off in the new timeline that he didn’t exist in (originally).