One of this year's new time travel movies previously mentioned was the new addition to Marvel's popular X-Men superhero series, X-Men:  Days of Future Past.  Having seen it once in the theatre, and having an open slot for an article, we are providing a quick snapshot of the problems involved.  Once it is available on video for closer inspection, we hope to provide a more detailed analysis.  We correctly predicted this to be a temporal disaster, and indeed it has met that expectation; but it has also met the expectation of being a highly enjoyable film.

The first uncertainty presented in the film is whether we are viewing the future, a sort of Terminator-type glimpse of an apocalyptic disaster, or the present, an alternate history in which all that has happened thus far in the X-Men stories has culminated in this very different world.  Professor Xavier's opening narration speaks of it as the future, but most of the mutant characters already known to us in that era--Xavier and Magneto, Storm, Rogue, Iceman--are played by the same actors (as also are cameos by Jean Grey and Beast) not much older than they were in the 2006 X-Men:  The Last Stand.  It thus gives us the impression that this is happening now, in some alternate version of "now" triggered by events in their parallel universe.

Kitty Pride demonstrates a new talent, the ability to send someone's present consciousness into their past body, something like what happens in the Butterfly Effect franchise or the recent film About Time, but that she can only do this to someone else, and it is very dangerous for them.  In the present, mutants have been hunted nearly to extinction by robotic "sentinels" that can detect the mutant gene--who have extended their hunt beyond mutants to normal humans who carry the gene and might have mutant offspring.  Kitty's power enables the mutants to stay one step ahead by sending a companion back when the attack comes to alert their past selves that they have been discovered and have to move elsewhere.  We see her do it once, but are told that it is their strategy, creating scores of infinity loops based on the transmission of information to the past that undoes the information.

Professor Xavier and Magneto determine that the turning point was during the Nixon administration at the end of the Viet Nam war, when Mystique a.k.a. Raven believed that sentinel inventor Bolivar Trask was responsible for the deaths of many of her mutant friends, and killed him, resulting in the United States funding the sentinel program because of the perceived mutant threat.  The irony is that she is captured, and her mutation modeled to enable the sentinels to adapt to the weaknesses of their specific targets.  They decide that if they can send Xavier's present consciousness back to the Nixon years, they can get Xavier and Magneto to work together to stop the assassination and derail the program.  Problem:  Kitty says that the damage of such a long trip would be fatal to anyone.  Solution:  Wolverine can regenerate any damage of any type, so he will make the trip.

That he succeeds means a massive infinity loop; but the problems go beyond this.  Kitty explains that while she is maintaining his connection to his past self, time moves normally, minute by minute, at both ends of the trip, but when the connection fails history will be changed and he will return to the altered future but with the memories of the original history.  We thus have the problems of whether (or when or how) he is changing history, and why he remembers and does not remember various histories.  We have seen these problems in other films, of course, but there are a few minor twists here worth considering.

We see what we make to be two trips to the past, with presumed returns, in this film, but the opening backstory tells us that the first one we see is the last of many like it.  Thus as we unravel it, we also unravel a dozen others which we have not seen.

The Sentinels, an army of adaptive androids programmed to identify the "mutant gene" and kill anyone carrying it, are seeking our heroes wherever they hide.  The team has its best spotter, Warpath, as lookout, and the instant the enemy is detected everyone moves into action.  Kitty Pride and Bishop retreat to the most defensible point of their present shelter, and she sends his consciousness back a few days into his previous body so that he can alert their younger selves.  The younger selves having been alerted break camp and relocate elsewhere.  As a result, the Sentinels do not find them, as they have already left.  Significantly, they have left long enough before the arrival of the Sentinels that the Sentinels never arrive; the trail is cold, there is no reason for the Sentinels to be somewhere that the mutants left days before.  The film appears to get that part right.

The immediate problem, though, is the problem always faced when we intentionally change the past in a way that erases our reason for doing so; first, though, we must agree on a theory of time.

It is evident that this is not a fixed time theory story; every trip made successfully alters the future.

Stories of this sort are sometimes resolved by some version of multiple dimension theory, in that the time traveler changes the history of another world.  Our first example trip, though, tells us this is not happening:  as the Sentinels reach Kitty and Bishop in their secluded section she rather rudely tells them they are too late, and then everyone vanishes.  It is quite clear that what she does changes the world from which the time traveler leaves.  We thus have some form of replacement theory, in which the time traveler arrives in his own past and changes his own history, the history of the world from which he departed.

There is an obvious problem at this point, though.  Our mutants are hiding somewhere, let's suggest a castle.  They see the Sentinels approaching and launch their escape plan.  That plan involves sending, via Bishop's consciousness, the knowledge of the time of the arrival of the Sentinels.  Thus a few days earlier they receive that knowledge and leave.  That means a few days later the mutants are not in the castle, and the Sentinels do not arrive, and Kitty does not send Bishop back with the information.  Yet if she does not send the information, they will never receive it, and we have an infinity loop, the outcome of a grandfather paradox in which the actions of the time traveler make themselves impossible.

Stories which allow a time traveler to alter the past and return to an altered future in which the past is now different usually rely on the somewhat dubious interpretation of Niven's Law, to the effect that once the past has been changed it remains in that changed state without intervention from the future.  This solution is fraught with problems, but in the present film few of them are evident.  We can thus give the story the benefit of the doubt and allow that their escape plan works, that receiving a warning from the future they are able to relocate and so escape the danger, and never have to send the warning that enables them to escape.

However, we have several other problems.

We noted that under ordinary rules of replacement theory every time Kitty sends Bishop to the past she ought to create an infinity loop, but that if we allow the application of Niven's Law we can resolve that.  That matters, of course, because absent that their trick will only work once, and they have done it many times.  Kitty explains what it is they do in some detail, and when we see them do it they work like a well-oiled machine.  Everyone knows his part and does it smoothly.  They know what they are doing because they have reportedly done it several times before.

Only they have not.  They have in fact never done it; not once.  Even Bishop has never actually done it--he has probably several times been possessed by his future consciousness and delivered the message, then been returned to his right mind, but he has never been on the sending end.  Kitty has never sent anyone to the past, she only believes she can.  Sure, they have received the message many times--a dozen, a score, a hundred perhaps--but they have never sent the message.  Every time they send the message, they erase the future from which it was sent.  Every time they defend themselves against the Sentinels, they undo the battle such that the Sentinels never found them.  In one sense they have executed this plan many times; in reality, it is a plan that has never been tried.

We can forgive the preparations the mutants have taken, and the effective way they respond to the threat.  After all, we already know that the Sentinels have killed many mutants, and these are the few that escaped to tell the tale.  The X-Men, individually, are practiced fighters, and they have done some work on teamwork.  That they can set a perimeter defense and respond to an incoming attack is not surprising; they do not need to have done this before to do that part effectively.

However, when Kitty sits down to send Bishop to the past, it is something she has never before attempted; it is entirely theoretical.  She believes she can do it solely because at some point Bishop told her, and everyone else, that he was Bishop from the future and that Kitty had sent his consciousness back into his earlier body to bring them this warning, and they believed him.  Note that they believed him, repeatedly, despite the fact that having fled their various refuges they never had any evidence that the Sentinels ever would have reached them there.  Yet they are desperate enough that they dare not ignore a warning that might be real from a trusted teammate.

Further, Kitty seems to know far too much about a skill she has never attempted when Professor Xavier and Magneto ask her about it.  She says that she might be able to send someone back a month, but that no one could survive a trip back decades, but how does she know this?  She knows that Bishop claims to have been sent back from a few days in the future; she has never sent him, nor anyone else.

That means she does not really know how to do it, or what effects it would have on her or on her subject, or what happens while she is doing it, or for how long she might be able to maintain it or how far she can send someone.  She knows nothing.  She has had no practice with this, no experience.  Every time she does it she changes history such that she will have never done it and will not remember ever having done it.  Further, Bishop cannot tell her how it works--all he knows is she put him to sleep and he awoke in his old body.

So that was not well considered.

Of course, someone has to know, because it is plot exposition and has to go somewhere.  Still, it would be much more plausible if Kitty did not know how it worked, and possibly if she did not know whether it was working.  After all, she knows that she can do it because she has been told in the past that in the future she will do it, but every time she does it is the first time she has ever actually attempted it.

There is something of a science problem that has nothing to do with time travel but something to do with time.  It seems that by 1973 Doctor Bolivar Trask has devised a means of detecting the mutant gene in a subject within perhaps a few meters.

First, the notion that there is "a mutant gene" stretches credulity.  The X-Men concept was brilliant in itself.  Before this, every superhero needed a backstory to explain his powers--Superman is an alien, Green Lantern was recruited by the alien Green Lantern League, The Flash was victim of an explosion in a chemistry lab, The Atom invented his mass and density controls, and on the Marvel side Captain America ingested a supersoldier serum, Thor is a God from another dimension, Spiderman was bitten by an unusual spider (originally radioactive, updated to genetically modified), The Incredible Hulk was in an accident in a radiation lab.  But now the world is full of radiation and mutagenic chemicals, so it is natural for us to expect that some incredibly small percentage of the population would develop these mutations that give them special powers, and given the billions of people in the world a few per million is still thousands.  (We conveniently overlook that the vast majority of genetic mutations result in defects and unsurvivable specimens; it makes better story to have the bulk of them be useful.)  But this requires that there are at least hundreds and possibly thousands of separate distinct mutations.  There is no sense to the notion that the same gene that produces the powers of a Jean Gray also produces those of Mystique, and Iceman, and Quicksilver.  In fact, the plot relies on the notion that Mystique is genetically unique, that her abilities come from mutations not shared by others and thus useful for Trask in a way that no other mutant's genes would be.  The notion of a single mutant gene shared by all mutants does not fit.

Nor does it make any sense that any such gene would be found exclusively in mutants.  Of course, the film catches that, because the Sentinels ultimately began to identify and destroy unpowered humans who carried the gene.  Yet this point is so obvious in the world of genetics that Trask would certainly have been aware of it.  We cannot suppose that he did not understand basic principles of genetics, and therefore it must be that he intended the destruction not only of all mutants but of all who carried the genetic material that would produce the mutants.

There is also the problem of how Trask identied this supposed gene.  It is natural for us to speak of the functions of individual genes, but the Human Genome Project did not produce a complete map of our genetics until 2003, and there was little in terms of useful results prior to the mid to late 1990s.  For anyone to have identified a specific gene in 1973 is extremely unlikely--and Trask is not a geneticist but a weapons designer.

Allowing that this is not impossible, that it was plausible to identify specific genes associated with specific conditions, the next problem is how such genes could be detected by the sentinels.  Detecting a specific gene remotely is not something we can do now; we detect such genes by taking a tissue sample and extracting the DNA for careful examination by computerized electronic microscopes.  In 1973 we could not even do that.

Of course, it might have been possible to detect mutations by some means other than remote DNA identification.  We know that certain conditions release faint odors and other cues--trained dogs can detect some cancers, and can identify when a diabetic's blood sugar is out of range.  So it might be that there is a way to detect people with certain mutations, even in 1973.  The complaint here really is that the pseudo-rational explanation used in the film does not pass muster, and they would have done better by omitting the explanation than trying to make it scientifically plausible.  Tell me that you have a means of detecting mutants and I can accept that; tell me how you do it and you are likely to make a mistep if you do not know both the science and the history of science at least as well as I do.

What has that to do with time travel?  Not much--only that if you are going to attempt to create the world of the past, try to do it in a way that people who were alive then will believe.

It is a popular trope in movies, and in some cases it almost makes sense.  It is the idea that a minute elapses in the future for every minute you are in the past.  We see it with any story that assumes time travel by wormhole--Deja Vu, Timeline--and sometimes in other films where it is done without good explanation.  Here the implied explanation seems to be that because Wolverine's body, and thus his brain, is in the future, his mind has to stretch across time in such a way that his brain in the future is controlling his body in the past, and the two must move forward moment by moment in synch with each other.

That actually does not sound quite as ridiculous as it seemed.  However, we have the problem we encountered in Frequency:  if Wolverine is sending his thoughts to the past and receiving sensory information transmitted from the past (or more precisely, if Kitty Pride is transmitting his thoughts and sensory information between the past and the future), then every time he sends a thought to his past self all of history must change to incorporate that change before he can receive the sensory information from the event and send the next thought.  The past would in this case be changing kaleidescopically, each instant creating a new history that played through to the point in the future where the next thought could be sent.  Dangerously, any thought he sent to the past would impact his own memories of the intervening history and might undo or alter the sending of that specific thought.

The film has an answer to this:  the changes do not take effect in the future until the moment Wolverine disconnects from controlling his past self and returns to his present self.  Yet this raises another issue:  is he in fact changing history, or not?  Logically, once he reaches the past, everything he does in the past ought to become part of the future, in some sense "immediately", because from the perspective of the future his entire time in the past is, plainly, in the past.  It is thus at the moment of his departure from the future that all history changes; his moment of return to the future is irrelevant.

To illustrate, imagine that Kitty sends Bishop back twenty minutes to warn them that the Sentinels are coming.  Bishop arrives and starts spreading the word; everyone starts packing.  Kitty, in this version of history, does not send Bishop to the past.  Yet if it takes him more than twenty minutes to get everything organized, the future to which he intends to return will be in the past.  Where did his consciousness go?  Does it return to his past self?  If he can only stay in the past while Kitty maintains the link, at what point did the link fail because Kitty was not maintaining it?  Did it fail at the moment of his departure, bouncing him back to himself twenty minutes later?  Does it not fail because until he voluntarily returns to his other self they all exist in the other world?

We might suppose that the time traveler is in some sort of dream state, that in his own mind he creates all the events that will happen in the past, and at the moment he awakens all those events become part of the past.  However, his past self is interacting with others.  Are we to suppose that he imagines their reactions, and then provides them?  Even were we to allow some such breach of free will, there is still a moment in which Xavier in the future seems to have a momentary contact through Wolverine with Xavier in the past--something that must happen in real time for both.

Of course, they do this for plot reasons.  Wolverine must complete his mission in the past before Kitty is interrupted in the future.  The problem is that that tension is never actuated.  There are tense moments in the future when they are not certain it will be completed soon enough, but despite the action and the occasional mention of a time limitation at the past end, our heroes almost seem to meander over the course of several days, driving to Xavier's home, finding Quicksilver, freeing Magneto from the Pentagon, flying to Paris, returning to Xavier's, flying to Washington.  Theirs is an exciting effort, but we never have the sense that they are racing against the time Wolverine will be with them.

So the notion of two times running concurrently makes no sense, and while it gives us two stages for action it fails to create the tension in the past that we ought to expect.

We are given an explanation of the time travel experience that says the time traveler's body sleeps as his mind takes over a past version of his body--something like astral projection but that it is temporal and the spirit must be in a body.  That introduces our next problem.

It is perhaps the most hectic moment in the film, as Xavier's team bursts into Trask's private meeting just as Mystique has been discovered by the Sentinel tracking system.  They prevent her from killing Trask, but then Magneto attempts to kill Mystique to prevent Trask from ever being able to obtain her DNA (messy solution that, since her body would still be full of her DNA and it would be difficult to remove it from the premises without losing some along the way), and she escapes but is injured.  Wolverine is in pursuit when he sees a familiar face, someone who is going to cause him a great deal of pain in the future, pain which he remembers vividly now and which disrupts his connection to his past body.

At the future end, Kitty is worried that she is losing the connection, and that Wolverine is awakening.  This is odd.  There is no indication that Kitty has any knowledge of what is happening in Wolverine's history, and we were told that the changes would not be effected until he awoke; he has been in the past for probably over a day by now (and Kitty must be exhausted) since he has already run the Pentagon breakout and flown to Paris.  There is no reason for Kitty not to allow him to awaken; of course, there is no indication that she has anything to do with that, but she does seem to be struggling to hold the connection at that point.

Meanwhile, at the past end, the past consciousness of Wolverine emerges.  He does not remember Xavier or Beast, or understand how he got where he is.  This disoriented state continues for what seems several minutes, long enough for Xavier to invent an explanation and move Wolverine to safety, and for Mystique to escape.

The question for us, though, is what happens to the time travel?  Our understanding is that Logan's mind is in the past because it is connected to Logan's past body, but it seems at this moment to have become disconnected; is it returning to the future?  Does it then return to the past?  Is this still the same trip, or another?

If it is another trip, then by the logic put forward in the film once Logan leaves the past all of history should play through to the future.  It is already a different future--Mystique did not kill Trask in Paris, although she was still seeking to kill him; Beast and Xavier were already working to prevent that; Magneto was also aware of the situation and acting in his own interest in this.  We cannot reconstruct the future with any certainty, but the suggestion is that the Sentinel program is going to receive even more funding now that Mystique attempted to kill Trask, and still has Trask to make it work because she failed.  Their situation in the future might be worse; they might not have had this chance at all.  On the other hand, it might be that Wolverine's actions have already been sufficient, that Xavier and Beast working together, possibly with others such as Quicksilver, might succeed in creating a better future; in that case, Wolverine would not return, because once he leaves the past Kitty will not be waiting to send him back again.

We thus must conclude that it is still the same trip, that despite the disruption Wolverine's future mind is still in the past--it just is not controlling his past body.  It might be bouncing forward a bit, picking up his body a minute or two later because he skipped to that point; it might still be in his past head, as it were, but temporarily overcome by his past mind.  When he reconnects, it is still the same trip; the only disruption is that for a few minutes he was not in control of himself.

Wolverine gets credit for recognizing his own limitations and the value of the team.  He realizes that he is entirely the wrong person to help Professor Xavier learn to control his power, but that in a sense within his own mind are the memories of the older Professor Xavier, the person who can help.  Since the younger Xavier is able to see into Wolverine's mind, he is able to find the older Xavier in the memories.

There are two problems with this; both relate to that question about why it is bad to contact your past self.

Wolverine remembers a version of Xavier who does not yet exist.  That Xavier has come to terms with how to use his power, and has taught many others how to use their own.  The younger Xavier finds wisdom in that encounter with his future self.  He even quotes himself a few minutes later--when he says to Beast, "Just because someone stumbles and loses their way doesn't mean they're lost forever" he is repeating what his older self said in Wolverine's memory.  We have a small predestination paradox, but it is easily resolved:  Xavier could have picked up that thought anywhere, and that in this version of history he learned it from himself does not become a problem since once he knows it its origin is almost immaterial.  He perhaps loses the thought process which brought him to that conclusion, but in time he will fill the gaps.

The other problem, though, takes us back to the question of how this works.  Immediately after this encounter, future Xavier comments that he had a glimpse of the past, strongly suggesting that Xavier's mind reached Xavier's mind through Wolverine's mind.  We had gotten comfortable with the notion that Wolverine's mind had in a sense traveled to the past, and thus we can accept the idea that the young Xavier finds Wolverine's image of the older Xavier within Wolverine's mind.  However, if the young Xavier has reached through Wolverine to the mind of the older Xavier, if the two connected across time, we have a separate time travel event with a second time traveler--one of those two Xaviers carried information from the future to 1973, and changed 1973 in the process.

It complicates our situation significantly, but reinforces what we already recognized:  the idea of the film, that the brain in the future is on a moment by moment basis controlling the body in the past, has serious temporal implications (akin to those in Frequency but considerably more complex) not easily resolved by any theory of time.  If we accept the possibility of this future mind/past body connection, it is not more difficult for Wolverine's decades-spanning mind to become the conduit for Xavier's thoughts.  It just makes it much more difficult to make sense of a time travel event which already does not work in any way that makes sense.

This is a problem we often see in time travel films, particularly when the time traveler takes control of his own self in the past--The Butterfly Effect franchise, and more recently About Time.  Yet sometimes we see some of it in otherwise solid time travel stories like Back to the Future (part 1).  Frequency attempted to address it, but the solution was not entirely credible.  The problem is, what does the time traveler remember?  The particular complication is that the time traveler has duplicated himself, and so we have to ask what each of him remembers.

The easier one to answer is the harder one to explain:  When Wolverine returns to the future, he returns to the history he has created but he remembers the one from which he departed.  In 1973 he and the others prevented the launching of the Sentinel program--the program which has, within the history he knows, resulted in the deaths of thousands of mutants and many more humans with the mutant gene.  His return to the future puts him in a world in which no one knows anything about that program, save perhaps Professor Xavier, Magneto, Beast, Mystique, and himself.  There is prima facie sense to this; after all, he lived through the other history, and now has leapt over the history that has replaced it.  Yet it is an outcome which does not withstand scrutiny.

Wolverine, pierced by metal bars, sank to the bottom of the Potomac.  The last thing he remembers is drowning.  He was salvaged, and from that moment forward he experienced much that had happened to him in the other history--the military covert operations, the secret experiments which replaced his skeleton with adamantium, the connection to Xavier's school.  However, it was all within a different context, one in which there was not a growing Sentinel program trying to extinguish mutants, one in which Magneto and Mystique faced a lesson in building bridges instead of weapons.  We know it was different in other ways as well, because Jean Gray is alive (she died in a previous movie).  Through all this altered history, Wolverine--the other Wolverine, the one fished out of the river--has been part of it all.  He apparently taught class yesterday, and is late for a scheduled lecture today.  When told that the time traveler should not be able to remember the erased original history, people ask what happens to those memories, but the real question is, what happened to the memories of the Wolverine who lived through what is now the only history that ever was?

The harder one is also answered for us:  when future Wolverine temporarily loses contact with his past self, the past self is disoriented and confused.  He does not know what has happened to him or how he arrived where he is.  We assume that much the same thing must have happened with Bishop, that the future Bishop took over the body of the past Bishop and delivered his message, and then the future Bishop returned to the future and someone in the past to whom he delivered the message has to deliver it to him.  We thus have blackouts as we did in the original Butterfly Effect, but these are indeed caused by the visits of the time traveler from the future.  It makes the entire system rather unstable:  the person who sends the traveler has never done it before every time, the person sent has never been sent before, and the younger version of the person sent has repeatedly had blackouts after which someone tells him that he delivered a message from the future.  The message is always that they have a few days to move before the Sentinels find them, so they do; but since they move, the Sentinels never find them, and the message is never confirmed.  The only good reason for them to move when Bishop tells them is that if he is right and they don't, they might not survive.

It was an exciting movie; Marvel lives up to its reputation for action adventures.  It was not terribly credible as a time travel story, though.  But then, movies in which time travelers intend to alter the past are rarely very good at the time travel elements.