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human
[Fringe] Major spoiler question inside. Don't read unless you've watched the whole thing.

At the end of the show, Walter and the anomaly go into the future to prevent the creation of the Observers. We then see Peter and Olivia with Eta in the park, etc.

However, going way back.....

Right after "our side" Peter died, Walter was watching Walternate in the lab. September arrived on the other side and distracted Walternate at the exact wrong moment so that Walternate didn't notice the result of the cure he had made; he didn't know it worked, but Walter (watching through the window) did.

As a result of this, Walter travels to the other universe with the intent of curing the other Peter. Things go wrong, and Walter brings Peter to "our side" and never brings him back. This starts the degradation of the other side. Once Walternate finds this out (via Olivia), he attacks "our side" and the Pattern ensues.

But this all started with an observer distracting someone. If the observers never existed, Olivia and Peter never would have met; they never could have met, as they lived in different universes. So how are they chilling with Eta at the end of the show?

The main plot arc of the series hinges on that event (September distracting Walternate)....so how did it still happen after the observers are no more?

The same could be said for any post-Machine episodes. Peter was erased from both sides; no one remembered him. IF he had never been taken, the degradation and war would never have happened.
gpt
So there are 3 Universes at play here.


Blue, Red and Orange for Prime, Alternate and Altered. 


First, it should be noted that the Observers (other than September) are not seen in the Red verse; this is probably due to it not having a future at that point, so other Observers not being interested.  While Blue did have a Future even if it was black holed, and it’s all so very complicated.

In Orangeverse though we do see they are able to travel to the alternate at will; but notably not the original Red universe, the altered red, which we should probably have another name for, Magenta or something.  

However a key assumption here is that the observers from blue and from orange are the same future people. 

Take this, what if the reason the Blue observers are so interested in Septembers mistake and are watching the events at Fringe divisions so greatly is because it is the destruction of their future.  They are not an advanced team from the future of an upcoming invasion, they are literally watching their future fall apart.  Them standing on the lawn the moment Peter disappeared was the final act of watching their peaceful future wink out of existence with a rough idea of what was to come.  Everything they ever did, every action they ever took, was winked out of existence.

At the moment the Orangeverse was created, the Orange observer future was created.  We see that events played out differently in the Orange, so why not everything we know about observers.  Yes several threads that seemed set up be Blue observers were picked up by Orange observers; but that is only because the Orange also had actions back that far.   Orange September, is not technically the same as Blue September; however their actions mirror each other almost perfectly. 

So Orange September has a kid, and Walter takes them to the future to stop Orange observers ever happening and show there is another way.  This other way created a peaceful future, where people travel to the past only to observe it.  Essentially it created Blue Observers universe again.   This is why they did nothing about Blue universe ending.  This is why they didn’t stop September.  Their existence is a contained time loop that continues forever.

Blue September knows to go to the lab and distract Walternate because he knows Orange September did.  And Orange September goes to the lab, because Blue September did.  But how did they know of each others actions you ask?  They didn’t.  However they can see timelines and probabilities of things happening.  They both saw a moment in time where whole universes were created and are destroyed, where whole timelines and different future people are created and then disappear.  They see the spark of Walters threatening to tear the universes asunder. They can see in that moment the intersection of the Blue and Orange universes.   It fascinates them because both are so different and yet similar; and it’s such a small act.