"The Variable," Lost's landmark 100th episode and the 14th this season, focused on an aspect of time travel that we've already discussed in this column a few times before: free will. Frazzled physicist Daniel Faraday has decided to tell everyone what he knows—that he and the other "Lostaways" are from the future, that Miles is Chang's grown son, that something horribly catastrophic is about to happen on the isand—because he is now convinced that he can alter the past to change the future. But if Oceanic 815 never crashed on the island in the first place, then the survivors wouldn't find themselves on the island in 2007 or 1977, so... then what? To straighten things out, we turned once again to Richard Muller, author of Physics for Future Presidents, Lost fan and time-travel expert.

Until now, Faraday's take on time travel has been constant: You cannot change the past. But upon returning to the island after doing some "research" at Dharma HQ in Ann Arbor, Mich., "you can't change the past" Faraday has changed his tune (trying to save your true love from death by time jumping an island will do that to you). "I've been spending so much time focused on the constants that I forgot about the variables, and do you know what the variables are? Us. People. We seek the reason, we have choices, we have free will," he tells Jack. "If I can, then that Hatch will never be built and that plane will land just like it's supposed to in LA."

It seems that Faraday now believes in free will, which, as Muller puts it, is the aspect of reality that is not part of his physical equations. "In quantum physics, there are similar things," he says. "For example, physics is fundamentally unable to predict the moment when a radioactive atom will explode. Faraday is saying that there are things in his equations that are not predictable, and that's human behavior."

Even though the survivors currently exist in 1977, it's their modern selves that are back in time, according to Faraday—he uses the fresh wound on his cheek as an example—and therefore, they can still die. How can one exist in two places at the same time? "All this can happen as long as free will is constrained," Muller explains. "Nothing in physics prevents Miles from seeing himself as a baby," Muller says, unless you put in place spiritual constraints. "Some people believe that there is a 'spiritual' identity that is beyond the realm of physics, the inner person, the soul—something that may or may not survive the death of the body," Muller says. "Of course, this is not physics—but neither is it ruled out by physics. The issues that Faraday is wrestling with are issues that are not part of current day physics, and (I think) never will be." —Erin Scottberg