The H-Bomb that Juliet set off at end of last season was the cause. This is what Jack wanted. But (of course) it didn't work out quite the way Jack (who got this notion from Faraday) expected. Instead of just eliminating the energy pocket which later would cause the first plane (815) to crash (which started Lost, it seems all those many years ago), this H-Bomb blast caused a split in reality, one in which 815 indeed did not crash, and the other in which 815 did indeed crash, just as it did in the Lost series pilot.
Now the idea that an H-bomb can cause reality to split in two is a chestnut in science fiction - I first came across it years ago in a Superman comic (the Man of Steel tries to contain an H-bomb explosion, but, although the bomb is contained, the suppressed power of the explosion created two realities, with two diverging story lines). Presumably something similar happened on the island.
So, one of the two realities has 815 not crashing, and landing in LAX. But bear in mind that that reality came into being years earlier, when the H-bomb went off, which was back with Dharma still on the island, Sawyer as LaFleur, etc. And from that instant on, that new reality had an increasing number of small and bigger differences from our original reality, in which 815 crashed.
Among the differences we see on the plane: Boone left Shannon in Australia. Desmond is sitting next to Jack on the plane (I was glad to see this, because it suggests we'll perhaps find out more about the coincidence of Desmond and Jack meeting at the stadium, and then on the island in the hatch in our original Lost reality - see my Keys to What Is Really Going On for more on the significance of these inexplicable coincidences). But Desmond is nowhere to be seen as the passengers take their seats for the landing. Meanwhile, Charlie doesn't care if he chokes to death on a heroin bag on the plane (he doesn't - Jack saves his life). And Locke later claims he did go for the walkabout in Australia.
All of these and who knows how many more differences between the original Lost reality and this new one, with the plane not crashing, were caused by the H-bomb going off, and not allowing Dharma and the island to proceed as they originally did. In principle, not only Boone and Charlie, but almost any character deceased in the Lost world we know could be alive in this new alternate Lost reality - Charlotte, Eko, Michael, Libby, Ana Lucia, Alex, Keamy - take your pick. (Possibly not Faraday, because Eloise was presumably killed in the H-bomb blast, but Jack et al survived it in this reality, so who knows.) And they can appear at any time off the island - Alex, for example, could have left the island in this new reality with Ben - a field day for the writers!
And there also are some great new story lines in this premiere, entirely new with our characters in this new reality, as Kate escapes from her marshal captor, and has some great small interactions with Sawyer, Hurley, and Claire. And Jack and Locke, in this new reality, also have a wonderful exchange, in which Jack tells Locke to call him, if Locke ever wants a second, more optimistic opinion about his legs. (Jin and Sun also have an interesting if less surprising new story line, as Jin gets held at customs for carrying a lot of cash, and Sun keeps up her pretense of speaking no English.)
And that's just the half of it.
Because, back on the island, in the other half of this reality split, our characters have indeed crashed on the island - twice, now - and discover to their near-despair that the H-bomb explosion did not get them off the island after all. Juliet, almost rescued from the bottom of the rubble in the hatch, dies in Sawyer's arms. (She wants to tell Sawyer something before she dies. Miles, who can read the dead, tells Sawyer that Juliet realized that "it worked" - this could be very significant, evidence that characters in this reality can have knowledge of the other reality.) Sayid is near death, Jacob tells Hurley to bring Sayid to the Temple, where Sayid can be saved, but it seems that Jacob's people only drown Sayid. I had a feeling he wasn't really dead, and, sure enough, he comes back to life at the end. But will he still be Sayid? People on Twitter are saying Sayid could now be Jacob - just as Jacob's nemesis is now Locke - and that does make a lot of sense.
So the stage is set for a really powerful finale season of Lost - and I'm looking forward to savoring every minute.
10-min podcast review of Lost Season 6 Premiere
See also: Three Questions Arising from the Lost Season Six Premiere: Linkage Between Two Realities, Dead Bodies Inhabited, Who/What Survived H-Blast?
More Lost - see : The Richard-Locke Compass Time Travel Loop ...
and Lost Returns in 5 Dimensions and 5.3: The Loops, The Bomb ... 5.4: A Saving Skip Back in Time ... 5.5 Two Time Loops and Mind Benders ... 5.6 A Lot of Questions ... 5.7 Bentham and Ben ... 5.8 True Love Ways ... 5.9 Two Times and a Baby ... 5.10 The Impossible Cannot Happen ... 5.11 Clockwork Perfect Time Travel ... 5.12: Ben v. Charles, and Locke' Slave ... 5.13: Lost Meets Star Wars and the Sixth Sense ... The Problem with Baby Aaron and the Return of the Oceanic Six ... 5.14: Eloise, Daniel, and Obsession Trumping Paradox ... 5.15: Moral Compasses in Motion ... Lost Season 5 Finale: Jacob and Locke
The Plot to Save Socrates
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