I'm not sure if this has been expounded elsewhere on the site, but traveling back in time in principle should not make the Island disappear to onlookers in the present except under the following conditions (among other less likely possibilities that I will not go into, e.g. optical illusion, etc.):
1) The Island somehow reverts to a previous time period, but it is destroyed in the events that ensue once the people from the present reach the time destination in the past. Therefore once they time-travel, the Island is effectively destroyed within the present because all of the past events will immediately changed the present (à la Back to the Future). This enters us nicely in the Free Will area of the philosophy of the show, in the sense that until Ben DECIDES to turn the Wheel, the consequent events that ensue in the past from the time-travelers has not become reality and the Island exists. The Island could re-appear in the present if new people (Oceanic 6?) also travel back in time (another free will choice) and alter the course of events again (superimposition of past events alteration). There may be a paradox that follows from this however, which I leave unresolved:
A. If the Island no longer exists in the present, what becomes of the people who crashed on the Island? Did they not ever arrive on the Island since it had been destroyed in the new present timeline? I do not think this to be the case; rather, since the past destruction of the Island is contingent on events that must take place in the present (Ben turning the Wheel), time travels in a curly-Q fashion: first, linear up until Ben turns the Wheel (what we have seen so far) --> people are transported back --> events ensue resulting in the destruction of the Island --> since the Island is clearly in a separate time plane from the rest of the World, its time plane is destroyed with it and only "Outside the Island" time plane exists, but the effects of the destruction of the alternate time plane (i.e. the Island) can be seen from the "Outside the Island" time plane.
2) Another possibility is that the Island can move back in time OR forward in time. Since most indicators suggest that Ben's turning of the Wheel in "There's No Place Like Home" sends the Island back to a previous time period, it is also possible that from the point of view of the past, the Wheel is turned again to send the Island FORWARD in time to a point in time years after Ben's Turning of the Wheel. This means the Island will re-appear at some point in the future, i.e., at some point after the events of "There's No Place Like Home". Let's say that from the past it travels forward in time to 2010. This means that the Island will effectively be invisible/not exist between December (?) 2004 and 2010, at which time it will re-appear. Jumping forward in time fits nicely with events we have already seen; the most obvious one that comes to mind is Ben's appearance in Tunisia in 2005 (months or one year after his turning of the Wheel, although he experiences it as immedia! tely following his turning of the Wheel).
These are two possibilities that I believe must play into the shape of things to come. :)
Otherwise, it is difficult to reconcile the fact that the Island disappears when it moves, since if no cataclysmic event occurs after the time-travelers go to the past, all that should happen is momentaneous changes in the present state of things on the Island (suddenly the Dharma Initiative re-appears because the Hostiles/Others Purge was avoided). This not being the case, and having seen the Island disappear from the present, something more along the lines of the two possibilities I have fleshed out above seems more likely.
Theory by Francisco
1) The Island somehow reverts to a previous time period, but it is destroyed in the events that ensue once the people from the present reach the time destination in the past. Therefore once they time-travel, the Island is effectively destroyed within the present because all of the past events will immediately changed the present (à la Back to the Future). This enters us nicely in the Free Will area of the philosophy of the show, in the sense that until Ben DECIDES to turn the Wheel, the consequent events that ensue in the past from the time-travelers has not become reality and the Island exists. The Island could re-appear in the present if new people (Oceanic 6?) also travel back in time (another free will choice) and alter the course of events again (superimposition of past events alteration). There may be a paradox that follows from this however, which I leave unresolved:
A. If the Island no longer exists in the present, what becomes of the people who crashed on the Island? Did they not ever arrive on the Island since it had been destroyed in the new present timeline? I do not think this to be the case; rather, since the past destruction of the Island is contingent on events that must take place in the present (Ben turning the Wheel), time travels in a curly-Q fashion: first, linear up until Ben turns the Wheel (what we have seen so far) --> people are transported back --> events ensue resulting in the destruction of the Island --> since the Island is clearly in a separate time plane from the rest of the World, its time plane is destroyed with it and only "Outside the Island" time plane exists, but the effects of the destruction of the alternate time plane (i.e. the Island) can be seen from the "Outside the Island" time plane.
2) Another possibility is that the Island can move back in time OR forward in time. Since most indicators suggest that Ben's turning of the Wheel in "There's No Place Like Home" sends the Island back to a previous time period, it is also possible that from the point of view of the past, the Wheel is turned again to send the Island FORWARD in time to a point in time years after Ben's Turning of the Wheel. This means the Island will re-appear at some point in the future, i.e., at some point after the events of "There's No Place Like Home". Let's say that from the past it travels forward in time to 2010. This means that the Island will effectively be invisible/not exist between December (?) 2004 and 2010, at which time it will re-appear. Jumping forward in time fits nicely with events we have already seen; the most obvious one that comes to mind is Ben's appearance in Tunisia in 2005 (months or one year after his turning of the Wheel, although he experiences it as immedia! tely following his turning of the Wheel).
These are two possibilities that I believe must play into the shape of things to come. :)
Otherwise, it is difficult to reconcile the fact that the Island disappears when it moves, since if no cataclysmic event occurs after the time-travelers go to the past, all that should happen is momentaneous changes in the present state of things on the Island (suddenly the Dharma Initiative re-appears because the Hostiles/Others Purge was avoided). This not being the case, and having seen the Island disappear from the present, something more along the lines of the two possibilities I have fleshed out above seems more likely.
Theory by Francisco
