One of the more interesting innovations in time travel was popularized by the television series Quantum Leap, in which Dr. Samuel Beckett traveled to moments in the past by having his mind take over the life of someone else while they temporarily lived in his body in a small room in the future. Something like that is happening in Source Code: Captain Colter Stevens does not travel bodily to the past, but his mind or soul or spirit or some other intangible part of himself takes over the body of Sean Fentress.
What the film never asks is, what happens to the mind of Sean Fentress?
On one level we don’t care. Sean Fentress died when that train exploded earlier this morning, and since we are specifically told that those people are all memories having no reality we, like Stevens, take the view that they’re not real, they have no minds or spirits or souls, and the fact that Fentress is not controlled by Fentress is not significant–Christina presumably is not controlled by Christina, either, but by Stevens’ mind tapping the memories of Fentress. Yet in the end we discover otherwise: somehow the spirits (and bodies) of everyone in the world have been duplicated in this divergent universe. The one exception is Sean Fentress, whose body is present but whose mind is missing. Why was it not copied? Or if it was, where did it go?
We are apt to miss the complication here, because to our minds the divergent universe began at the moment Stevens arrived, and so the fact that Fentress was not copied means only that we did not make a copy of his mind. However, from the perspective of the divergent universe, they have always existed, and at some point Stevens arrived and possessed Fentress, disposing of his original self and controlling his body thenceforth. People in that world have every right to say, as Christina once in jest says, What have you done with Sean Fentress?
In most iterations of the universe it is a question that will never really be asked. Fentress dies eight minutes or so after Stevens possesses him; he would have died in any universe in which that did not happen. Yet because in the final universe Stevens survives as Fentress, the question of what becomes of Fentress in that and every other universe must be asked; and it is not something the film ever answers.
One guess is, Fentress was sacrificed in the process of creating a new universe. We know that matter and energy are neither created nor destroyed, but divergent dimension theory breaks this law of thermodynamics entirely, creating enough matter and energy to construct a duplicate universe. We do not know how much matter or energy exists in a human spirit. Some would suggest that as one of the few things in the universe that are eternal, they individually are infinitely greater than the entirety of the temporal universe. If that were so, sacrificing one such spirit would provide sufficient energy to build a new universe–but not to create all the spirits of those who live within it, so in solving one problem it creates another: how do we get the duplicate spirits for all those duplicate bodies? If the answer to that is that spirits are merely byproducts of material functions, then the loss of one gives us nothing from which to build a universe–and forces us back to the question of how the mind of one person can possess the body of another.
We won’t find the answer to this dilemma in the movie, because for most of the movie we are told that this is not a divergent dimension but an imaginary construct. We have already noted some of the problems and inconsistencies in that, and we will uncover more as we continue. What we know is that when Stevens possesses Fentress’ body, he creates a new universe in which at that moment an alien consciousness (Stevens) possessed a body whose original consciousness (Fentress) at that moment ceased to exist, and in which everything else was as it had always been. Perhaps Fentress is in Stevens’ comatose body for the moment, and then dies either when he returns to his own body or when Stevens’ body dies, but no one ever asks the question so no answer is ever offered.