Fans of the hit TV series Lost are awaiting the big event next week: the premiere of Season Six on Tuesday night. The show is famous for its mysteries and plot twists, so this year has a special status: it's the final season, where everything that's going to be revealed will be revealed. That might not be absolutely everything, but it should be a lot. Lost has always played with time and narrative -- characters' backstories were told through elaborate flashbacks, lending a richness of nuance to their behavior in the main story. But time travel as a plot device was established as a central theme during Season Five. One happy consequence was the invention of Lost University, through which fans could learn a little about physics and other real-world subjects underlying events in the show. Naturally, scientifically-minded folks want to know: how respectable is the treatment of time travel, anyway? We are, as always, here to help. My short take: Lost is a TV fantasy, not a documentary, and it doesn't try all that hard to conform to general relativity or the other known laws of physics. But happily, the most important of the Rules for Time Travelers is very much obeyed: there are no paradoxes. And more interestingly, the spirit of the rules is obeyed, and indeed put to good narrative effect. The potential for time-travel paradoxes helps illuminate issues of free will vs. predestination, a central theme of the show. And what more can you ask for in a time-travel story than that? Details below the fold, full of spoilers. (Not for the upcoming season, of course.) See also discussions from io9, Popular Mechanics, and Sheril. The way that time travel works in Lost can be analyzed on three separate levels: physics, logic, and metaphor. (Or by ignoring all these high-falutin' ideas and just enjoying the show, but where's the fun in that?) Physics Make no mistake: the point of Lost is not to present a realistic depiction of time travel according to the laws of physics as we know them (or ever expect to know them). As explained in Chapter Six of From Eternity to Here, a remarkable feature of Einstein's general relativity is that it provides a context in which we can sensibly talk about the idea of traveling in time. Space and time are curved together, and the amount of time elapsed between two events is affected by motion and gravity. Traveling near the speed of light, or lingering in a powerful gravitational field, you will "move into the future faster" than someone floating freely in empty space. It's easy to imagine -- likely impossible to construct, but easy to imagine -- curvature so intense that you can hop in a space ship and come back before you left. One particularly evocative mechanism for dramatic spacetime curvature is a wormhole, a shortcut through spacetime through which one could easily reach tremendous distances or wildly separated times via a relatively short journey. But it would still be a journey, involving relatively conventional means of transport; no flashing lights, no dematerializing and popping into existence elsewhere or elsewhen. The tremendous amounts of energy and spacetime curvature necessary to maintain a realistic wormhole don't fit easily into the island milieu of Lost. So the show simply doesn't bother with such details. Characters, not to mention the island itself, do indeed pop randomly from one time to another. Even more divorced from realism, Desmond and other characters have their consciousness travel through time ("temporal displacements"), appearing in their physical bodies with all the memories and feelings of their future selves. Neither Einstein nor anyone else suggests any way that could happen in the real world. Which is fine; it's a TV show, not a science documentary. It's an invented world, not the real one. But the writers do nevertheless hint at a scientific basis for time travel within this invented world, one that borrows from real physics. I probably was not the only viewer to laugh during Season Three when one of the hated Others was shown deeply engrossed in A Brief History of Time. More directly, in one of the Dharma Initiative orientation videos "Edgar Halliwax" (Dr. Chang) explains that the island contains a pocket of exotic matter, perhaps sustained by the Casimir effect, which lets them conduct unique experiments in space and time. That's all on the right track. Even though general relativity lets us talk about wormholes, under ordinary circumstances we wouldn't expect them to be useful for purposes of time travel -- even if a wormhole were created, it would collapse to a singularity before anyone could cross it. A hypothetical way out is to invoke exotic matter, which would have a negative energy density and prevent the wormhole from collapsing. And how can we get negative energies? Perhaps from the Casimir effect, which arises when materials alter the energy contained in quantum vacuum fluctuations. Again, it's not a full-blown respectable and realistic theory of time travel; but I'm happy that the show nods in the direction of real ideas, which will hopefully inspire the occasional viewer to dig more deeply into them. Logic It's much more important that time travel in Lost makes logical sense -- it's consistent and obeys rules, even if the rules are not those of the real world. Most fundamentally, you can't go into the past and alter the future; there are no alternate histories or any such cheap ploys. Daniel Faraday says at one point, "What happened, happened"; Sawyer just says "What's done is done." Dr. Chang, confronted in the video above with a worker who jokes about going back and killing Hitler, reacts in anger: "Don't be absurd. There are rules!" And the main rule is that things happen in a unique way at every place in space and time. If we have good reason, based on memories or some other form of records, to think that events played out in a certain way, then that's what they did. There's no changing things, and more than we can imagine changing the past under ordinary circumstances; the past already happened. As far as I can tell, the events we've been shown conform very well to this principle. Of course, there are certainly mysteries, and we'll have to see how those are resolved in the season to come. There is one seeming exception to this rule: Desmond's visions of future events. He can see something happen in the future, and then take some action to prevent it (at least for a while). But as long as we're being sticklers, we have to admit that a vision of the future isn't the same as having that future actually happen. There is no paradox; only one thing ever happens in the real world, it's just not necessarily the thing Desmond sees in his vision. When Desmond shuttles information back and forth between the past and present, it doesn't conform to our ordinary notions of causality, but there's nothing inconsistent about the complete history through time. I'm inclined to grant this bit of poetic license in the cause of interesting storytelling, as it still respects the no-paradox rule. Despite the importance of this rule, fictional invocations of time travel tend to violate it all the time. Most such stories are all about changing the past, acting as if there is some narrative "meta-time" with respect to which events unfold, independently of the good old time we measure with physical clocks. (Think of Back to the Future, where Michael J. Fox does something in the 50's and conditions "immediately" change back in the present day -- erggh.) Personally I find the restrictions of logic to ultimately provide a more satisfying story structure. By the end of Season Five, Faraday has become convinced that you can alter time, and hatches a plan to donate a nuclear bomb in 1977 to ultimately prevent everything we've later seen happen on the island. Faraday is killed by his mother, Eloise Hawking, but Jack and the other survivors try to carry out the plan. The finale of Season Five ends with a bright flash of light. We don't know exactly what this means -- that's what cliffhangers are all about -- but presumably this is the "Incident" referred to in later Dharma Initiative videos. I hope so, anyway; after all this wonderfully consistent if complicated narrative, it would be a shame to throw out a universe and start all over again. Metaphor Why does time travel fascinate us, anyway? Why do we find it so interesting? Part of it is the interest in changing the past -- all of us have things we'd like to do over. But part of it is the fear of predestination. We like to think that, while the past is set in stone, we can make choices about our future -- we have free will. But if we are able to travel into the past, then our future is part of the time that already happened -- so in fact we don't have complete freedom of action. Whatever it is we do when we get to the past, it must ultimately be consistent with how we know that past ultimately evolved into the present. That seems a bit irksome, even if it does respect the laws of physics. This is where I think Lost really shines. One of the major themes of the show is destiny vs. free will, as embodied in the characters of Locke and Jack. Are there places where we are "meant" to be, or can we choose our paths for ourselves? Well, there's a balance. I can choose to turn right or left at a fork in the road, but I can't choose to simply float into the air -- there are the laws of physics to be obeyed. Lost uses the device of time travel to play with this tension -- we think certain things are destined to happen, but we don't know how. The logical restrictions of time travel are used as metaphors for the competition between predestination and choice. A great example is the idea of "course corrections," explained to Desmond by Eloise Hawking. Even if you see the future and try to prevent it, ultimately the designated fate is going to come to pass, perhaps in a different way (as with Charlie's death). As a physicist this originally annoyed me, as that's not how the laws of nature work -- things happen or they don't, but they're not teleological, working through multiple channels to fulfill some crudely-specified goal. But taking off my physicist's cap and thinking more as a storyteller, I came to really appreciate this conceit as an interesting metaphor for how we try to think about fate. Determinism and the laws of physics are not the point; it's simply that certain kinds of conditions pretty much inevitably result in certain kinds of outcomes. (Ever had two friends get together, and you knew from the start that it wasn't going to last?) In our human lives, the rigid inevitability of the underlying physical laws isn't very relevant to figuring out what's going to happen next, but there is still some degree of predictability. The battle of destiny vs. free will isn't one that has a winner and a loser; we are both constrained by circumstances, and free to make choices within that framework. That's what makes life interesting. Ultimately the idea of free will is tied to the arrow of time. Given perfect information about the present, in principle we could predict both the past and the future, without any wriggle room. But we don't have perfect information. Because of the low entropy boundary condition in the past, we can nevertheless reconstruct what already happened with a certain amount of reliability; that's why we think the past is unchangeable. But the future has no such boundary condition, and many possibilities are open. Otherwise I would tell you what's going to happen over the next eighteen episodes of Lost.