In The Constant, it doesn’t look like the barrier that they flew through that caused Desmond’s trip is very far off shore, and its safe to assume that Desmond didn’t follow the correct bearing when he left in the Elizibeth so perhaps his lengthy absence was due to time dialation similar to Daniel’s rocket. But why didn’t he experience any mental time traveling when he was in the Elizibeth?
Well, are we sure he didn’t? I think we can be reasonably sure he didn’t. He certainly didn’t appear to be expecting anything to happen when he was in the chopper. He was certainly confused when he first traveled in Flashes Before Your Eyes. So for the sake of this post, I will assume he did not time travel in the Elizibeth.
So why didn’t he time travel? I think it is the same reason Sayid and Frank didn’t, he hadn’t been exposed to a heavy dose of radiation yet. At first I assumed the radiation that Desmond had been exposed to that caused him to travel in The Constant was the three years of exposure in the Swan Station. But it seems clear that it was the failsafe event that was the source of the exposure. When he set sail in the Elizibeth, he hadn’t turned the fail safe key yet. After he turned the failsafe key, he started having Flashes and then later when crossing the barrier, he traveled.
This is interesting, because all of these elements seem to tie in together. You have the Swan Station that is like a cork in the bottleneck of some sort of energy the Island produces. The failsafe key blew that cork, which triggered the Purple Sky. The Purple Sky was a diffuse form of the energy the Swan was holding in all that time, the same energy that doused Desmond, being at ground zero as he was. The side effect of being drenched in this energy and leaving the Island is a form of time travel. The side effect of physically leaving the Island at the wrong bearing is a literal form of time travel. And then we have the Orchid which is possibly associated with either teleportation or time travel, or an amalgum of both.
In summary, it seems that various aspects of the Island are related to each other and are facets of one single phenomenon. The bearings, the magnatisim, the Swan, its all the same thing. Maybe that’s what Candle was talking about when he mentioned the Casimir effect.