Stephen Hawking, Lucasian Chair Emeritus of Cambridge University, and self-described "physicist, cosmologist, and something of a dreamer" has recently published instructions on how to build a time machine.
However, Professor Hawking only provides ideas for time machines that travel into the future. A time machine that travels into the past is theoretically impossible, according to Hawking, and he uses a story to demonstrate why (italics are mine):
I don't like the way scientists in movies are often described as mad, but in this case, it's true. This chap is determined to create a paradox, even if it costs him his life. Imagine, somehow, he's built a wormhole, a time tunnel that stretches just one minute into the past. Through the wormhole, the scientist can see himself as he was one minute ago. But what if our scientist uses the wormhole to shoot his earlier self? He's now dead. So who fired the shot? It's a paradox. It just doesn't make sense. It's the sort of situation that gives cosmologists nightmares.
This kind of time machine would violate a fundamental rule that governs the entire universe - that causes happen before effects, and never the other way around. I believe things can't make themselves impossible. If they could then there'd be nothing to stop the whole universe from descending into chaos. So I think something will always happen that prevents the paradox. Somehow there must be a reason why our scientist will never find himself in a situation where he could shoot himself. And in this case, I'm sorry to say, the wormhole itself is the problem.
So, Professor Hawking worries about effects that are their own causes. And rightly, so...no one wants the universe to descend into chaos. However, we have seen one paradoxical object already on the Island: Richard's compass. I predict that we'll see another such item by the end of this season: Daniel's Journal.
Perhaps this is part of the unique essence of the Island...the laws of cause and effect may be suspended. The entire flight of Oceanic 815 is in essence, a paradox. Only due to the Island's existence was an effect (the 815ers on the Island) able to negate its very cause (the crash of Oceanic 815).
Perhaps Jacob has spent his entire existence hoping that the men and women he brings to the Island will be able to end (that is, '...end once') the bastion of paradox that is the Island.