I am now into the fifth season of my rewatch, and I was really trying to figure out why Jack, Kate, Hugo, and Sayid were the only ones who traveled to the past. It seems like there would be some kind of complicated time traveley type of explanation, but I think It is actually pretty simple. There are two phrases in LOST that explain it. One is "whatever happened, happened", and the other is "you weren't supposed to leave Jack". You see, if Jack and the group had stayed, they would have been time traveling just like the rest of they're friends and they would have ended up in the seventies too. That is where they were supposed to be, so when the plane went down they were course corrected and transported to where they should have been in the first place. I also think this has something to do with Jacob touching the Losties. I don't think he touched Sun, and she didn't time travel. So I wonder what would have happened if Jack didn't leave with the rest of them. Would everyone have experienced the flashes except Sun and Lupidus? I apologize if this is common Knowledge to all of you, but it just hit me like a brick.