I think the Hawking book was a definate clue, so was the guy's name who was reading the book.. Aldo.. short for.... who can guess.

Anyway, I think it has to be a wormhole type thing, because also included in the answers show was Damon's quote "they are not dead, they are somewhere along the space/time continuim" I could see a force field, but i can't go with that, because I think if that were true, a lot more castaways would be on the island. A wormhole would only allow the 'lucky' few to hit that exact point in space/time.