Almost everything temporal in the film is possible, and most of it is at least plausible.  However, what we see cannot all fit in the same history of the world--it is possible only if we assume that history has changed and we are viewing some pieces from different versions of it.  There are several complications here, though.

The first complication is that we have no way of determining how many trips have been made to the past, who made them, or what impact they might have had.  In the first moments of the story we meet Vincent Taub, dressed in fourteenth century peasant garb and dying on a highway in the desert of New Mexico.  It is mentioned that he has traveled to the past ten times.  On the last trip he was trying to retrieve William Decker, who also had made many trips the last one with security chief Frank Gordon and not with Taub.  Security officers Bill Baretto and Jimmy Gomez are also experienced time travelers when we meet them.  Of these, only Frank Gordon was with Professor Johnston when he made his trip.  That means we have at minimum a dozen trips to the past prior to the one we observe, all of them focused on the same place.

We also can only guess at how long this has been happening.  Our best guess is based on the fact that Decker, who probably had a standard business haircut similar to those of the other ITC security personnel, has long hair--long enough to estimate that at least half a year had passed since he was left there.  Also, he rose from being a severely wounded fallen man to being the Aide-de-Camp of the commander of the English forces in that time, a position of significance.  Yet he was an overly experienced time traveler who did not think he would survive another trip, so his many previous trips must have been before that time.  Further, there is a strong suggestion that since he was presumed dead in the past Gordon has only made one more trip (with Johnston), and Taub made one trip seeking Decker.  If in not less than six months they had made only the two trips, the second demanded by the professor, we are forced to question the frequency of these trips.  We would be justified in concluding that the time machine has been functioning for at least two years, possibly as long as five, and that people have been changing history in minor ways all that time.

The second complication is that we are introduced to the history of the target location by an archaeological team.  These are experts in the subject, and know as well as can be known what transpired when at this site they are excavating.  We learn the history of the battle from them--but it is not the history which occurs during the course of the film.  This has to be reconciled.

The third complication is that our time travelers not only change history, they leave their mark on the very archaeological site they are excavating, and they find their own contributions to that site as they do so.  If this were fixed time, it would be perfectly reasonable for them to discover what they will do in the past; however, the past they create does not match the past they report, and the changes they make become part of the history they uncover.

One footnote:  this analysis will frequently refer to the "time machine".  The author is aware that the device in question is actually a matter tranmitter (a.k.a. teleporter or transmat) and that the time travel is an accident incidental to the discovery of the wormhole.  The term "time machine" is used for convenience.

All of this will take some explaining and examining, but hopefully we will uncover just what does and does not work in Timeline.

We have the significant complication that there have been many trips over several years, each of which has had some unknown impact on the critical events of the history in question.  Apart from Decker (who stayed in the past), everyone who traveled to the past prior to the trip made by the professor stayed for a matter of hours and then returned to the future; but they still may have impacted the past.

The complications created by a technology that utilizes a wormhole have been discussed in connection with the analysis of Deja Vu.  Those complications are both increased and mitigated in the present story.  On the one hand, once someone travels to the past, he is changing history moment by moment, and every one of those changes has to propagate to the future; on the other hand, it might be concluded that when the machine is not active nothing is traveling through the wormhole, and thus there are not infinite anomalies but only specific individual ones when trips occur.

Despite the number of trips made and the time over which they occur, we can conclude that certain aspects of the past remain unchanged, for various reasons.  First, it is clear that the English occupied the area of France in which Castlegard, its monastery, and the overlooking Castle La Roque are found; this is certain because the Hundred Years War lasted longer than our time machine has been operational, and the Lady Claire commented that the war has been fought in her territory since before she was born, and thus the time machine could not have reached a moment prior to the English occupation of the area.

It is also fairly certain that the interference of the time travelers did not alter the outcome of the war.  Had the English not ultimately been driven out by the French, the political situation in the world would have been different enough in the centuries which followed that the formation of the United States might be in doubt, and with it the creation of the time machine.  The time travelers are doing their best to be unobtrusive observers, and it would require a major incident to change even the battle at La Roque.

If we assume that the history of the battle as reported in the beginning of the film is correct, our time travelers probably did not change that, either.  That is, Lady Claire was hung from the battlements, enraging the French such that they overwhelmed La Roque and slaughtered the occupying English forces.  Nothing done by time travelers prior to those we see caused or prevented that.

Any of these details might have been changed.  However, even a small change at that distance in the past could have sufficient impact on the future as to make itself impossible, undoing the lives of those involved in making the change or the creation of the time machine itself.  Since those trips reportedly were made and the time machine still exists, we can reasonably conclude that no major changes were made in these critical historic details.

There are still problems, however.

Of all the trips made to the past prior to that of the professor, the one made by Frank Gordon and William Decker is the most problematic.  These two are pursued by soldiers.  Gordon flees, activating his recall device (called a "marker") and returning to the future.  Decker takes three arrows and is down, but survives.  The United States Marine Corps may want to disown Frank Gordon, because he left a man behind, and that man survived.

This means that everything Decker did in the past is already part of history when Gordon reaches the future.

This confuses people, and the nature of the time machine is in part to blame.  It is indeed the case that for the wormhole, for every minute that passes in the present at the future end, a moment passes in the present at the past end.  Thus when it is said that Professor Johnston left the future seventy-two hours before, that logically means that when they exit the wormhole in the past he will have been there for seventy-two hours.  Similarly, however long it has been since Decker left from the future, he will have been in the past that long upon the arrival of any travelers who make the trip subsequently.

The confusion arises, though, because it is then incorrectly believed that time travelers can stop Decker from changing the future before he changes it.  That is, Decker traveled to sometime perhaps in October or November of 1356, and he intends to help the English defeat the French in April of 1357.  This would so alter history that our archaeologists might never be born.  Thus we imagine that if they arrive in April they can prevent Decker from changing the past and so protect the future as they know it.

The problem is, whatever changes Decker made are already part of history.  Frank Gordon left William Decker on the battlefield in France, and returned to the future.  He skipped over six and a half centuries, but the history of those centuries must have happened in the time that Gordon skips.  Complicating it, in the history we know Decker is not mentioned, and in the history we see he is killed by Marek, another traveler from the future who has not yet made the trip to the past.  That means Decker dies in the past at a time and place unknown to us.  We can only guess what changes he will make to the future.

Decker is also problematic because it is clear that he is responsible for the deaths of numerous French soldiers and peasants.  This is of particular concern relative to Francois, whose ancestors come from France.  It is not even necessary that Decker prematurely terminates one of Francois' ancestors:  he could kill someone who would have married someone who otherwise would have married someone who otherwise would have become one of Francois' ancestors.  That is, even one death can significantly alter the children born in the next generation by a ripple effect in the selection of mates.

Two things play in favor of a preserved history, however.  The first is that so many French died in the Castlegard area anyway that it is likely those Decker killed would have died on the blade of some other English soldier.  The second is that the slaughter at Castle La Roque was so complete that it is unlikely Decker survived it.  So although his presence in the past is extremely dangerous, it need not be fatal to history as we know it.

We are introduced to the story by the appearance of Vincent Taub, a time traveler who leaps from the woods near Castlegard to the desert of New Mexico while fleeing an English knight.  This introduces us to the time travel concept and some of its risks.  It is of interest to us primarily because it is the last trip to the past prior to that made by the Professor, and whatever history he left behind is the history of the world before the professor becomes involved in it.

That is probably the history we hear recited in the lecture given by the professor and Marek, in which Lady Claire is hung from the battlements of Castle La Roque, enraging the French such that they overrun the defenses and, at great cost in lives, destroy the English and recapture the fort.  It is at least implied that the Lady Claire dies, and stated at one point that her death brought about the overthrow of the English.

It must be considered as possible that the archaeologists have the story wrong.  They do not have everything right, after all.  The young archaeology student Kate Ericson comments that she was completely wrong about an entire wing of the manor house in Castlegard.  It does not appear that the English Lord Oliver intends to hang Claire by the neck until dead, but rather that he intends to strap her to the walls in such a way that an ill-aimed trebuchet load or stray French arrow would be likely to kill her.  This is unclear, because Oliver's words indicate that intention, but the rope with which they are preparing to secure her is clearly a noose around her neck, and so the film sends conflicting messages in this regard.  It is possible that Oliver did not kill Claire, but dangled her in front of the wall to demoralize her brother, French commander Lord Arnaut, and that the French troops not only overran the castle but also rescued the lady.  Somewhere in translation, then, the statement that she was hung from the wall was misinterpreted, and historians believed she died at the hands of Lord Oliver.

Absent this "mistaken history" theory, it is clear that Marek changes history; that is, if Lady Claire died in the history they knew before they traveled to the past and not in the events as they transpired once the time travelers interfered, then fixed time is not a possible explanation for the film.  We could resolve this to a fixed time story only if we assume that any inconsistencies between the reported history and the actual events are mistakes in the historic interpretation.

That, though, feels like a cheat.  From a story perspective, the reason we are told that Lady Claire was executed by Lord Oliver is because it happened, and we are going to be faced with the complication that Marek changes history by rescuing the lady.  That is several problems to which we will return; for now, what matters is that the history of the world after Taub's trip includes the execution of Lady Claire from the battlements of La Roque, and the successful storming of the castle by the French, with the associated devastation of the English forces.

What was said of Decker remaining in the past is an even greater problem in connection with Professor Johnston:  anything he is going to do he has already done long before Gordon arrives in the future without him.  He has made whatever changes to history he is going to make, and removing him from the past only changes history yet again.

This fact is underscored in the film by the discovery of the documents and the eyeglass lens in the lower level of the monastery.  Professor Johnston placed them there in 1357 after being separated from Gordon and left behind, and in one sense they were "immediately" present in that location six and a half centuries in the future.  They were, of course, there for all that intervening time.  That means that before the time travel group makes its trip to rescue the professor, all that time has elapsed.  The professor lived and died in the past, and any changes he would have made had he not been rescued he did make, because he was not rescued in this history.  No one can leave to rescue him until that history is established as the history of the world.

However, unlike Decker, Professor Johnston could make a major difference in the history of the world.  He has been captured by the English, and in an effort to keep himself alive has identified himself as a magister, that is, a scientist, and promised to produce Greek fire, something that acts much as the flaming oil the Byzantines used in their naval battles during the Crusades whose formula is lost to history (probably a petroleum-based naphtha).  He has a formula that will create a fire that is spread by water, and since he cannot escape without help he undoubtedly creates enough of this to arm his captors.

The fear is that this weapon will reverse the outcome of the battle.  If Oliver has and uses this flaming oil against the attacking French, he could win this battle.  If he wins the battle, he changes history in a major way.  He will undoubtedly keep his magister and the formula for producing this weapon, and use it against the French in future battles.  That could change the outcome of the war, weakening France and strengthening England.  Henry V might never fight at Agincourt.  The Hundred Years War might not have reached the century mark.  It is difficult to predict the long-term consequences of such an imbalancing of the power in such a struggle.

Further, the weapon would have been ready sooner.  In the events we see, the Professor escapes and is recaptured some distance from La Roque, and when he is brought to the castle the necessary materials are already there.  Had he remained captive in the manor house, he might have reached La Roque sooner and been there when the materials arrived.  Oliver could have had his demonstration and his supply sooner, and turned the tide that much quicker.

It is possible, though, that the materials would not have been available significantly sooner; and it appears that the demonstration Oliver desired could not be performed until there was a target on the field.  Thus he would not have had his weapon earlier than he did, and without the assistance of Marek perhaps he would not have had it quite so early.  If we allow that Claire was still hung from the walls and the enraged French managed to win despite the new weapon, we can be fairly certain that the professor would have died in the slaughter of the English.  His impact could be contained that way, and history preserved, although undoubtedly more French soldiers would have perished, and again we would have put the life of Francois in jeopardy.

So time can survive Johnston's trip without the rescue.  The next trip is the rescue--but we have a few odd details to examine first.

"What would be written here?"

Archaeologist Kate Ericson asks that question concerning the inscription on the side of the unearthed sarcophagus, which tells them that their friend Marek and his wife, the Lady Claire, were buried there after a long and happy life.  Marek himself had uncovered the relief on the top of the sarcophagus, of a knight missing an ear holding hands with his lady, shortly before traveling to the past, and had wondered who might be in it.

It is not impossible for Marek to uncover his own sarcophagus.  He does not find it in the original (AB) history, but when he leaves from point B to point C to create the CD history, he remains in the past with Claire and is buried in the past.  His future younger self then uncovers the sarcophagus, and sees the relief on top.

There are, however, two side issues which are problems.  There are other problems that arise because of the sarcophagus, but they are less directly related.

The first issue is awkward.  When Marek is in the past, Decker slices off his ear.  At that moment he finds new strength from the realization that he is the one buried in that sarcophagus, alongside his wife, the lady Claire, and with that renewed strength in that hope he defeats Decker.  But the first Marek to arrive in the past could never have seen that carving, and so the loss of the ear could not have given him that kind of hope and encouragement; it could only mean that he had suffered a severe injury and was, we might say, behind on points.  He needs to succeed despite the injury in that first altered history, or he will never marry the girl.  In the third history, after he has seen the relief, he succeeds because of the injury, as it now has an entirely different meaning for him.

The second problem, though, is perhaps the biggest complication in the movie.  For any given version of Marek, there can be only one history of the world before he travels to the past.  In some of those histories, Lady Claire was hung from the battlements and died, inciting the French to overthrow the English.  In some of those histories, Lady Claire was rescued by her knight, who became her husband and was buried with her in a sarcophagus with their images carved on the lid.  Unless we think that the reported history is a mistake, we must conclude that the Marek who finds that sarcophagus cannot know the history in which the Claire buried in it was killed on the battlements at La Roque.  That coffin only exists if she and he are in it, and if they are, she died his wife sometime after the battle, not hung from the wall.  Thus the Marek who finds the sarcophagus has never heard the story of the Lady Claire being hung from the walls.

That is problematic, because there is probably no reason for him to have heard of her at all absent that event, and thus if he saw the sarcophagus he does not know who she is when he hears her name, and does not think to warn that she must be protected, nor worry that he has changed history by rescuing her.  Indeed, the same is true for the professor, whose knowledge of history must be from the world with the same history as Marek.  Thus neither of them would think that Claire is significant in any history in which she was buried in the sarcophagus rather than hung from the wall.

Thus you can have the story in which Marek knows that Claire was hung from the wall, as long as it is the Marek from the timeline in which the sarcophagus never existed; and you can have the history in which Marek sees their sarcophagus, as long as no one in that history knows anything about a Lady Claire hung from the battlements.  You just cannot have a Marek who knows that story and sees that sarcophagus, because the story and the sarcophagus cannot both exist in the same timeline.

And the answer to the question, "What would be written here had Marek returned with us?" is quite simple:  "here" would not exist.  Had Marek not stayed in the past, the sarcophagus would never have existed, and Marek would not have found it, and there would be no place for such an inscription to appear.

The archaeology team is drawn into the time travel story by the discovery of two artifacts in a newly opened area of the dig site which tell them that the professor was there in 1357.  One is his signature on a document; the other is a lens from his bifocals.

The signed and dated message is interesting, and it was a bit of a gamble by the professor to place it where he did.  He would have known that that area had not yet been uncovered, but hoped that it would be soon; he could not have known that it would be discovered on the equivalent of the next day when there was a cave in, and might have waited months before it was found.  But he was playing long shots, as he really did not have many other options.

The more immediate problem is his glasses.

Before we address this, there is a mistake of history in the script.  When Francois is headed for the past, they take his glasses from him, stating that eyeglasses did not yet exist.  This is not true.  They were already in use in Italy, and their use was probably known beyond that country by this time.  Certainly bifocals did not yet exist, although it is likely that the professor had no-line bifocals (no line appears on it in the shot) which would not be easily distinguished from regular lenses.  Peasants would not have owned glasses, but that is a separate issue.  It was inaccurate to say that they did not exist.

That raises the question of whether Professor Johnston left his glasses in the document box to help certify that it was his message, or because they made him conspicuous in that time and place.  Whatever the reason, though, he apparently left them there.

He did not, however, leave them broken on the floor.  This gives us a complicated bit of anomaly to resolve.  The professor placed his glasses in the box, and in what is the new original history for our time travel team, the glasses must have been found intact in the box when the cave-in occurred.  This gave them more evidence of the presence of the professor in the past than they have in the movie.  They then travel to the past, and Kate and Chris go to the monastery seeking the tunnel to La Roque.  Chris moves the glasses, and Kate sends the monks to get Arnaut.  Arnaut's troops troupe through the area, knocking the glasses to the floor and breaking them.  It is thus in the second (CD) history that the lens is found separated from the glasses, as we see in the movie.

There is another problem with this.  It is certainly reasonable that the glasses were found in the box in the original history, and reasonable that coming back to rescue the professor Chris caused the glasses to be broken on the floor of the lower level of the monastery.  Why, though, did the monks never pick up the pieces?  There is no reason to believe that they left the monastery; indeed, the wall which Kate broke was subsequently patched by someone (or she would have seen the tunnel when she was down there in the future).  How could the lens have lain undisturbed on the floor for however many years the monastery was still in use?

Finding the lens on the floor is thus less credible than finding the glasses with the documents in the box.  It makes for better drama for the team to find only a lens, as opposed to an intact pair of glasses, but it would have been better in terms of plausibility to keep the glasses intact in the box.

When the cave-in occurs, Kate and Marek descend to the newly opened area, and in addition to retrieving the documents and the eyeglass lens notice the smashed relief on the far wall.  Kate wonders who might have smashed it, and Marek declares that it obviously was not done by an archaeologist.  In the end, though, it was, because she realizes that she herself smashed it in order to get to the tunnel hidden behind it; she then does so, revealing the tunnel.

Here we have a predestination paradox in an unusual form.  Kate has in essence given herself the information of where the tunnel is.  She smashes the wall to reach the tunnel because she knows that in the future that wall was smashed by someone, and it suddenly occurs to her that she smashed it in order to reach the tunnel.  However, she never found the tunnel in the future, so she did not know where it was.  In the past, the clue that tells her its location is her memory of the smashed relief in the future--which can only be smashed that way because when she was in the past she knew where the tunnel was from having seen that.

The normal resolution for an uncaused cause is to find an original cause that was replaced by the actions of the time traveler.  For informational paradoxes, as we saw in Star Trek 2009 and previously in Star Trek IV:  The Voyage Home, all that matters is that once the information is carried from the future to the past, it is revealed in a way that guarantees it will be known to the messenger in the future.  The problem here, though, is determining how Kate was able originally to find the tunnel in such a manner that she smashed only the relevant part of the relief, so that her future self would see the relief and realize at the right moment that it was hiding the tunnel.

At some point in the film, Chris comments that the team has six and a half centuries of advancing knowledge on which to draw in solving their problems.  That probably provides the answer now.  Kate has reason to believe that there is a tunnel leading to La Roque, and it most plausibly connects as near to the castle as possible.  That tells them which wall to search.  The other clues include that someone will have done something to hide it, and that the wall will be solid except over the tunnel entrance, where it will echo slightly if tapped.  Given a bit of time they can find it--and they have time.  Arnaut does not have to be there before the tunnel is open on the other end.  The danger is rather that he might arrive too soon and kill one or both of them before the blast occurs.

Thus we have an original history in which the wall in the future was undamaged, and then an altered (CD) timeline in which Kate and Chris search for the presumed tunnel, find the echo, and smash the wall, and then Kate sees it smashed in the future, and then a second altered (EF) history in which Kate realizes that she smashed the wall because she knew the tunnel was there, and so smashes it again.

The Lady Claire was captured in Castlegard when she led the time travel team there, but Marek freed her and delivered her safely to her brother Lord Arnaut's armies across the river.  Marek winds up with mixed feelings, when on the one hand he thought he damaged history by saving her and on the other hand he sees her again captive on the wall of Castle La Roque.  Professor Johnston tells him it was inevitable, because it is history.  Marek then determines to change history.

The professor is mistaken; it is nothing so dramatic as an immutable past.  It is instead a simple matter of not understanding the events in the original history and confusing their own changes with the causes of those events.

We are not certain where Claire was prior to her first interaction with the time travelers, when she yelled (in French) that they should run and hide, and Marek wound up crawling into her hiding place alongside her.  However, it seems she had been missing for a significant time and was attempting to get back to her brother Arnaut without being caught by the English.  Lord Oliver later suggests that Arnaut used his own sister as a spy, and it may be that this is what she was doing.  At Marek's request, after he apparently saves her at the risk of his own life, she agrees to take him and his friends to Castlegard.  There she is captured, and we are fooled into thinking that this is the cause of her having been in English custody during the battle.

It is important to consider what would have happened had Marek not intervened.  Claire had escaped her English pursuers and secured a hiding place near the river.  She would not have been found before the French raiding party put an arrow in one of the English knights, diverting her pursuers in another chase.  From there she undoubtedly would have crossed the river and reunited with her brother's forces several hours earlier than she does in the film--but she would then have stayed with them for the remainder of the day.  It is Arnaut's intention to attack that night, and the English spy will still overhear this and report their position to the English command, who will raid the army at much the same moment.  Claire will flee and be captured as she is in the film.  The difference is after the time travelers intervene she is captured twice that day.  They have nothing to do with the second incarceration, which was the only incarceration in the original history, and the one that put her on the wall at the critical moment.

Thus the professor was mistaken.  In fact, they can change history, and they did change history--they just didn't happen to change the part of history they thought they did.

Because the time travelers are in a war zone, we expect that people will die.  We are tempted to think as some fictional time travelers have done, that these people are all long dead anyway, and indeed if they are in a war zone they have a good chance of dying.  Yet several of our time travelers contribute to the death toll, killing people in the past who might otherwise have lived.  Kate, Chris, Marek, and Gordon are each directly responsible for the death of at least one soldier, and indirectly for several more.

This is not as small a matter as we would make it.  We do not know which soldiers would have lived and which would have died.  English soldiers who lived long enough would have returned to England, and probably would have married.  We mentioned previously the complication created when someone does or does not marry in a changed history, but it might help to clarify it.

Let us suppose that Aaron (a name chosen for convenience) would have survived had Kate not killed him, and would have returned to England to marry Barbara.  With Aaron not returning, Barbara instead marries Carl; but since Carl marries Barbara, he does not marry Deborah, who instead marries Edward, who would otherwise have married Fannie, who now marries Greg, and so on.  This means that the children of Aaron and Barbara, of Carl and Deborah, and of Edward and Fannie are never born, and instead we have the children of Barbara and Carl, Deborah and Edward, and Fannie and Greg.  Those children are genetically different from the ones they have displaced, and they will in turn propagate children who would not have existed, who will marry others who would not otherwise have married them, and in a very few generations a very large number of people are replaced by entirely different people.

The same thing happens in reverse.  We might think that no harm is done when Marek marries Claire, because Claire would have been dead had he not saved her, and therefore she would not have married anyone.  However, according to the sarcophagus, Claire gave birth to Catherine, and Christophe, and Francois, who were thus injected into the French population.  Whomever Catherine married did not marry whoever he would have married had she never been born, and the displaced bride undoubtedly stole a groom from some other bride, who in turn prevented some other couple from connecting, and so just as the deaths of English soldiers changes the population of England, so the births of the Marek children have a similar impact in France.

It is difficult to believe that not one of the key people involved in the time travel would not have ceased to exist.  The Johnstons and Marek are Scottish, but there was some intermarriage with the English over the centuries and they might well have English blood.  Francois was French.  In one sense the Americans are most at risk--we cannot be certain from the names Ericson, Doniger, Kramer, Gordon, or Decker what their national origins are in the ethnic melting pot that is America.  Any one of them could have been descended from one of the English knights slain by them in 1357, or to a line disrupted by one of the French children born to Andre and Claire Marek.

And it is not merely the soldiers killed by their own hands who matter.  They changed the shape of the battle.  Most of the English died either way, but for the French the death toll would have been very different, changing the population because the wrong men lived and the wrong men died.

What matters, though, is that people who are effectively the same as those involved in the development of the time machine and the travel to the past are born and become so involved.  Odds are high that it will collapse at this point, but these are probabilities, not certainties.  If Francois comes from another part of France, and none of our team are affected by the genetic changes in the population, history is preserved.  It is improbable in the extreme but not impossible, and so we can preserve history by accepting that the improbable does happen sometimes.

There is an odd quirk in our timelines.  We have already noted that the Lady Claire was not hung from the battlements, and thus that the time travelers from the future would not have known anything about a Lady Claire being so treated.  Yet there is another highly significant change in the history of the battle which somehow is not mentioned in the history known to the archaeologists.  That is, not only would they never have known about Lady Claire, they also should have known this instead.

This which they should have known would be that the French commander Lord Arnaut entered Castle La Roque through a secret tunnel connecting from the monastery, and so was able to surprise Lord Oliver's forces and open the gate from the inside to admit the rest of his forces.  Kate's tunnel became a significant part of the history of the battle at that point, and would have gone from something she believed existed to something everyone knew had been there.  Why is the tunnel not part of the story?

It may have been intentionally omitted.  There is good reason to think it would have been.

Neither the English nor the French were aware of the tunnel until Kate found it and had the French commander Arnaut bring troops that way.  The English did not see how the French entered their fortress, and would only be guessing; and few if any of them survived to tell the tale.  However, once Arnaut is in command in Castle La Roque, he has good reason to keep the tunnel secret, and to swear his men to secrecy concerning how they entered the castle.  He clearly would not want it to be noised abroad that his impenetrable fortress has a back door, lest the English (or any other enemy in these turbulent times) decide to use it against him.  On the other hand, if at some future time he again loses the castle, he may wish to use that back entrance again to retake it, and he would not want the enemy to know where to place a guard.  Thus we can be fairly certain that he not only swore his men to secrecy concerning their secret entrance, he also hid the inner door well, and probably also arranged to close the passage in the monastery as well.

That leaves the question of what history would be recorded, and unless Arnaut has a clever writer of fiction on his staff he will probably keep it to the simple notion that his troops charged the gates, overwhelmed the English, and so breached the walls to retake the castle.  Any mention of a secret weak point would be conveniently overlooked, and the castle would retain its security and its formidable reputation as an impenetrable defense, with the added advantage that the next attacker will believe that it can be breached by assaulting the gates and walls because he believes it was done that way by the French in 1357.

So although it is unlikely that the hanging of Lady Claire would be part of the future history, the secret tunnel will probably not be mentioned.

It is likely that some reader will write to the effect that the book makes the story completely clear.  To this, the simple response is that the book is not the movie.  This is an analysis of the movie, and the differences between it and the book are more than superficial.  Still, someone will say that it all makes sense because the book explains that time travel is accomplished by traveling to a parallel dimension, and thus that any changes made in the past are actually made in another universe and do not impact the travelers' own past.  It can be argued that parallel dimension theory is not time travel, but it might be that this does not matter if the story is not really about time travel.  Yet there is still a problem.

The problem arises because the travelers do several things in the past which change their own futureProfessor Johnston leaves his signature and his glasses in the lower level of the monastery for Kate and Marek to find, Marek leaves a message on his sarcophagus for his friends, and Kate and Chris smash the relief in the monastery basement to reveal the tunnel.  You cannot have it both ways, that is, have a world in which you are not in your own past but you can change your own future.

The proposed solution for this, though, is that there is an infinite number of universes and while the time travelers left from ours to another identical to ours, identical travelers from another universe came to ours and made exactly the same changes, and in every universe identical travelers departed to every universe so that in every universe exactly the same things were done.

Here the problem is that you have undone the supposed advantage of having multiple dimensions.  To illustrate, let us suppose that the man Kate kills is her own ancestor, such that she undoes her own existence.  Parallel dimension theory is supposed to resolve this because Kate will never be born in this universe, but she will be born in her own.  This creates two distinct histories, each of which represents half of the infinite number of universes.  In one half, Kate is born and travels to one of the other half, where she kills the ancestor of her duplicate and so prevents her existence in that world.

That is not overly complicated, except that we have the pre-existing problem that we know that changes made by the time travel team impact their selves.  That means the Kate who killed her ancestor came from a universe in which the time travelers made changes to history, but she cannot have been one of them, and therefore it must be a different group of time travelers who made the trip and did different things in the past.

In short, even an infinite number of identical universes does not undo the problem of the grandfather paradox; it only complicates it, creating two distinct infinite sets of universes whose histories are different from each other but which still interact with each other.  Travelers from one could find themselves in the other, and as history diverges the pasts could become unrecognizably different.

The notion of an infinite number of universes is attempting to use an incomprehensible concept to hide a flaw.  There might be such a multiverse, but if so the first trip to the past shatters it, and each subsequent trip shatters it further, causing them to diverge into many different universes.

As we have noted, there have been more anomalies than we can count, based on trips to the past by many ITC personnel.  The most serious of these was Decker, who made a life for himself in the past, but there were also multiple trips by Gordon, Taub, Gomez, and Baretto, and probably unnamed others.  The last trip was that of Professor E. A. Johnston, the Scottish archaelogist in charge of the dig site, and the one who gave Greek fire to the English to use against the French.

We have assumed that all of these trips have resolved to N-jumps, because otherwise our story ends at the moment the traveler steps into the time machine and destroys all of time.  We are thus going to recreate the history of the world as it exists following his trip to the past, that history which would have been known to the time travelers in the trip we actually watch.

History might record that the English Lord Oliver had the help of a Scottish magister, who gave him a highly flammable weapon to use against the French trebuchets.  Yet despite this, La Roque fell, and the English were largely slaughtered, although the French took heavy losses.  As part of that battle, Lady Claire, French Lord Arnaut's sister, was hung from the battlements and died.

Before he was captured, that magister, Professor Johnston, left a note and his glasses in a document box in the basement of the nearby monastery.  There will be changes to the population of the world, but no one not born will know it and no one born will realize it.  We assume that it does not affect Professor Johnston himself, because then he would not have been able to make the trip, and history would have ended in an infinity loop at that moment.  Thus it is also at least probable that Chris is born.  The other members of the team are whoever they are at this point; it is not that important whether they would have existed absent his trip, because there would be a team there.

Whether or not she existed in any previous history, in this one Kate is there, and Chris finds her attractive; she is looking for the tunnel from the monastery to the fort.  Marek is there, too, but he does not find a sarcophagus.  The history lesson is much as we hear it, including that Claire was hung from the battlements, and her death so enraged the French that they assaulted the castle and took it with heavy losses.  It might include the fire weapon used by the English, but since fire was in common use and it and its inventor were destroyed in the battle, it could be that the French are unaware of this and did not record it (the English recorded no details of the battle, as they did not survive).

The cave-in appears to be as natural as can be expected, and it leads to the discovery of the document box in which are the call for help bearing the professor's signature and his intact glasses.  This motivates Chris to call ITC to determine whether his father is safe, and the archaeology group travels to New Mexico, learns about the time machine, and leaves from point C to travel to 1357 in search of the professor.

Our team's trip is complicated because ITC security officer Bill Baretto brought a grenade.  It is not that he has the grenade in the past, but that he returns it to the future, where it destroys the time machine.  This strands the time travelers in the past.  It also creates a race at the future end to make repairs before the time ends.

This incidentally underscores an illogical aspect of the story.  The company sends people to the past carrying "markers", recall devices which have a limited lifespan, which must be pressed to bring the travelers back to the future.  The team is given six hours on theirs; the professor was given only one hour.  Why wouldn't all such devices be set for their maximum possible lifespan?  The professor's problem might have been resolved more easily if he'd had six hours--or six days--on his device.  If you want someone to stay for an hour, you should set the device for at least twice that, and it would be preferable to set all of them at maximum.  There is no evident detriment to doing so, as the users can activate them at any moment until they expire.  It is obviously a plot device, but it is a foolish one.  It would have been better to say that such devices have a six hour lifespan, and that the professor got separated from Gordon and did not return before his expired.

Baretto, though, does return; and this creates a significant temporal problem.

When Baretto leaps to the future, he leaps over the six and a half centuries of history that must pass to reach that time (as we observed with the glasses), in which the effects of actions in the past create the future.  Thus before the time machine can be destroyed everything the travelers will do will have been done--including activating the recall devices.  Thus at the moment when they first press the recall devices, the machine has not yet been destroyed, because Baretto has not yet arrived in the future.  Thus we must consider how the device works.  There seem two possibilities, both of which lead to untenable results.

The recall devices might be self-contained devices which stimulate the wormhole to draw them into itself.  Once they are in the wormhole they emerge from the other end, along with the signal they sent, and the machine at the other end teleports them from within the wormhole to its own platform.  The first problem with this is that it is stated that they cannot pass through the wormhole intact, but only as a stream of electrons to form at the other end.  (This raises the issue of how they would be transformed between matter and energy in the past.)  The second problem is that if that were so, the device would call the wormhole, and our travelers would disappear into it.  They would reach the future end instantly, but it is not clear whether they would be caught there in limbo, or would return to the past--but they would know something had happened.  If they were in limbo, the matter would in theory resolve:  once the machine was operational, it would find them waiting and materialize them.  If they returned to the past, they (like the previously sent packages, in reverse) would vanish from the past for a while and then reappear where they had been.

The alternative is that the device sends a signal to the time machine, which sends something to the past which finds them and draws them to itself.  This is a much bigger problem.  It means that whenever the device is used it first does not work at all and then erases the fact that it did not work.

That is, from 1357 I send a signal to 2003.  The signal will be received in 2003, and then the machine will send the beam to convert me to an energy stream to pass through the wormhole.  However, before it can be received in 2003, 2003 must arrive--which means that when I pressed the button, nothing happened.  I was stranded in the past because I cannot leave until the signal reaches the future, which means until the future arrives.  I will die in the past, whether killed where I am or later of old age.  Then in 2003, the signal is received and the machine sends its power back to 1357--another anomaly, changing history at the moment I sent the signal by removing me from time.  No version of me now exists who was ever aware of the device having failed, and no message I may have sent to the future after that moment was ever sent, so no one knows that this is how it works.

That, though, gives us multiple overlapping anomalies.

Whenever anyone leaves the past for the future, a new anomaly is created because the return trip has to be initiated by the machine in the future.  That is, for a traveler to travel from 1357 to 2003, the machine in 2003 must receive a signal sent from 1357 and send a something to 1357 to convert the travelers into energy.  That means that a trip to the future is also a trip to the past, and a trip to the past always creates a new history because it always changes something in the past.  It also means that anyone who travels from 2003 to 1357 will die there, because the signal from the recall device cannot be received by the machine until all of history reaches 2003, but then in 2003 the history in which the machine never responded is erased and replaced with a new history as the machine sends something to the past to retrieve the travelers.  The travelers who survive past 2003 are never aware that the device failed.

It also means that we have multiple overlapping anomalies.  History is altered when the team arrives, but then only minutes later Baretto presses his device, and because it cannot work until 2003 it fails.  No one understands why it failed; perhaps no one even knows that it failed, because he has already armed the grenade, he has already taken three arrows, and he is going to collapse and be killed in his own explosion.

It is difficult to guess the impact this will have.  The English already have cannon, but it does not appear that they have yet devised small explosives.  Decker knows they exist, and that ITC rules do not allow them to be taken to the past; but he has become paranoid and will assume that Taub's failure (as he thinks) to kill him has caused the project to loosen the rules and send better equipped assassins.  The other knights will wonder at the explosion, but it is sufficiently like a cannon blast that they will guess it was a similar but smaller weapon that malfunctioned (early cannon were known to explode at times).  They might report to Lord Oliver that the French have hand-held cannons.

The grenade was probably not near enough to harm anyone else, and so it is likely that Marek will again attempt to lure away the English soldiers, and wind up with Claire.  From there, the story unfolds much as we see--the recall device fails when they first escape, and they do not understand why.  Then they get separated, spend the rest of the movie trying to get away, and Marek gives them the last recall device, which they take into the center of the field and again attempt to activate.  Again it fails, because the signal will not be received for six and a half centuries and no pickup beam will be sent until it is, and the world has to live through that time before it can arrive.

We do not need to think too much about what happens to them.  The knight is almost upon them, and it is unlikely in the extreme that any of them would survive, unarmed and untrained as they are.  Chris, Kate, and Professor Johnston all die in the battle, and only Marek survives.  He chose to stay in the past anyway, but now he cannot return to the future nor guess why his friends never made it back, whether they failed to activate the device in time or it failed to work.

We do not know whether he will leave a message on his sarcophagus, or what it would say.  Of his closest friends at the dig, only the physicist Josh Stern has survived, and Marek might think he not being an archaeologist would not return to the dig or finish unearthing the sarcophagus.  It is irrelevant, though.  In the future, minutes after the team is sent to the past a signal is received and the machine activates, sending a beam to the past to pick up the DNA-matched traveler within range of the device, and it finds Baretto and his grenade, removing him from the past and creating a new history.

That history is different only in that Baretto vanished and there was no explosion in the past.  Our time travelers will again do what we see in the movie, and will again die on the battlefield but for Marek, who will live and die in the past and leave a sarcophagus probably without a message on it.

We rushed through the impact of several anomalies because we were focused on the problem created by the fact that Baretto cannot travel to the future until a signal comes from the future to retrieve him, and neither can the other travelers.  In doing so we overlooked the fact that they are in the past changing events even as the device has failed them.

The problem is that before we can reach the moment in the future when Baretto is recalled, we have to resolve the anomaly created by the departure of the seven-man team; and they have made changes in history which will impact them, and so have put themselves into a brief sawtooth snap.  As we mentioned, the professor's glasses were intact in the document box and the relief on the monastery wall was intact, because Kate and Chris did not break the wall nor call for Arnaut to come.  Now, though, they do both, and now in the future Kate will see the broken image and Marek will retrieve the lens, not the glasses.  This means that when the team leaves, they have different knowledge and are different people who will take different acts.  Most specifically, instead of looking for where the tunnel might be Kate will realize that it is behind the wall which she broke to find it.  Arnaut will be called sooner, but not too soon, as this is the history we see in the film.

Also, Marek will save Claire from hanging from the battlements, detonating the contained oil to do so.  They will marry and have three children.  In the future, he will uncover the lid of his own sarcophagus, and will tell tales of the battle that do not include the death of Claire, who no longer matters to the story as told.

Thus the differences between the 1357 timeline we see in the film and the final resolution of this one are only that no one, not even Baretto, was taken to the future, and no one who came from the future attaches any significance to the Lady Claire.  It is only after this has resolved that time can advance to the moment the signal is sent and Baretto arrives in the future.

That event does not change the past significantly.  Whether after taking three arrows Baretto is killed by the grenade in the past, or vanishes from the past to die in the future, does not significantly impact events in 1357; he is gone either way, and they are not going to look for him.  The real impact is in the future.  This anomaly resolves easily enough, because nothing will change the fact that the machine sent the beam at the moment it received the signal and brought Baretto and his grenade to the future.  We have an N-jump, and can move forward to the next anomaly.

Every round trip to the past is actually two trips to the past.  In order for the time traveler to return to the future, the machine in the future must retrieve him.

This is a more complicated problem than initially appears.  Let us suppose that Frank Gordon, William Decker, Vincent Taub, Bill Baretto, and Jimmy Gomez--all ITC employees known to have made multiple trips--travel together to February, 1357, explore for two hours, and then hit the recall button to come home.  At that moment, a signal is sent through the wormhole to the machine in 2003, and the machine immediately activates, sending a beam back to 1357 to disintegrate the quintet for travel.  That, though, means that the signal has crossed six and a half centuries of history, and that history must exist in order to reach the machine.  It can only be the history that follows from the presence of the time travelers in the past.  This means that when they push the button, nothing happens, and they are stranded in the past for the rest of their lives.  No one else will ever arrive from the future, because another minute cannot actually pass in the future until that entire intervening history is established.  Certainly in one sense the transmission of the signal is instantaneous; in another sense, it takes over half a millennium to reach its destination.

The matter is simple enough to correct.  Six and a half centuries after the signal is sent, it is received, and we have another trip to the past, a trip in which a beam is sent back to dematerialize the travelers and bring them to the future.  History is changed, as the travelers are removed from the past and brought back to the future, and they never are aware that it was different or that they had been stranded in the past at all.  So ultimately the only history anyone remembers has the travelers hitting the recall devices and being instantly recalled.

That is, provided the machine still exists in the future.

The problem is that every time anyone hits the recall device, he finds himself stranded in the past, and has no way of knowing why that is so.  Our quintet will assume that either the machine has failed and help will come as soon as Steven Kramer gets it working, or that the company has abandoned them in the past and they have to make lives for themselves.  Eventually they will have to do the latter, because they are now part of history; but if they are part of history, they will become involved in the lives of those around them.  It is doubtful that not one of them will marry and have children, or at least prevent some other couple from doing so, and thus they have the aforementioned genetic impact on the world.  They will be looking for work, and will probably use such advanced technical knowledge as they possess in whatever they pursue, which may impact the advance of technology to some degree.  It seems highly improbable that not one of our five travelers will do anything that will change history drastically enough to eliminate at least one of our five travelers; the butterfly effect is very much against them.  Once they do, they are caught in an infinity loop, and the future will never reach the moment when the signal is received.

We have considered five men on one trip, but the problem is that this happens to every traveler on every trip.  The dismay Frank Gordon displays when the recall devices don't work is repeated on every trip, every time a recall device is pressed; it is simply forgotten if time corrects itself.  Those five travelers have made up to ten trips each; that's fifty chances that someone prevented the time machine from being created on schedule or eliminated one of the people who had to be part of one of the trips.  The interaction is such as to make success incredibly improbable.

Yet it is not impossible.  Saying that the chance of success is negligible is not saying that the probability of failure is absolute.  Were we looking at the time machine before the first object was sent to the past, we would have to say that the odds were against reaching the moment our time travelers make their trip.  We are, however, looking at the moment of their departure, the incredibly improbable sequence of events being a fait accompli which will reproduce itself if the final time travelers do not disrupt it.  There is an incredibly complicated interaction of overlapping anomalies, but in the end we cannot say that the story is inherently impossible, only highly improbable.

We have resolved most of the events of the major anomaly of the film.  We have watched the time travelers repeat their visit in 1357 until they have it right, but for one remaining detail:  the return trip.  As noted, in order to return to the future they must send a signal to the future and then be drawn to the future from the future.  The time machine must send something back to get them, creating another anomaly.

This time the fact that it is a two-way trip is made the more obvious, because company owner Robert Doniger is on the platform and travels back with the beam, to be left in the past.  That makes it clear that when anyone travels from the past to the future, something travels from the future to the past, and history is changed.  It happens dramatically this last time, but it happened somewhat less dramatically every previous time.

It is again important to recognize that even though the time travel team leapt over six and a half centuries of history, that history still happened, and they did not reach the future until after it happened.  The trip to the future is in one sense akin to being placed in stasis for all that time, and being released at the right moment in the future.  The events have to stabilize before they can continue.  In this case, though, the stabilizing will be a simple matter, because the changes, although meaningful, are not significant.

The first change is that three people from the twenty-first century are not lying dead on a battlefield in the fourteenth.  Only one such person is there, and he is not one of those three.  Once the battle is over, though, the French will burn or bury the bodies rather indiscriminately, and no one will pay much attention to the fact that an unknown man in strange garb is among them, save perhaps Marek who will know that his friends made it home.

That gives us the second change, which is that Marek now knows that his friends are in the future, and he composes the epitaph for his sarcophagus with some certainty that upon their return to the future they will also return to the dig and find his somewhat cryptic words there.

We have already considered the problems created by the fact that different people live and die in the battle, and Marek and Claire add three children to the French genome.  These, too, now resolve, as everyone does what they did before the time travelers were swapped.

Thus as we said at the beginning, everything in this movie is possible in one history or another, but it is not all possible in the same history.  Some of it is a bit confusing, most notably that once Claire's sarcophagus is found no one knows she was ever in danger of hanging from the wall, but since Marek neither rescued her nor married her because of that it is not a problem for the resolution of time.

The movie based on Crighton's book has a few rough edges, but comes through with approval and does not necessarily destroy time.