Right from the beginning we are given to believe that this is a fixed time story.  Jim and his companions (Chuck and Matty) successfully open one end of a wormhole.  The notion of the machine is that it can open one end of a wormhole wherever and whenever it is operated, but it cannot open the other end.  Yet something comes through the wormhole, which means that the other end must have been opened.  Yet the other end is in the future.  Thus from the perspective of the experimenters, it means that the future end of the wormhole will be opened.  They thus conclude that they have to repeat the experiment in one week so they can send the object to the past which they in the past already received.

This would have given us a lot of intriguing temporal problems to resolve.  For example, how did the object--a dahlia--get received in the original history?  However, we ultimately discover that Jim's wormhole machine doesn't connect points along the same timeline, but rather connects points temporally distinct in divergent dimensions.  These are eternally existing parallels, each unique, but many of them similar enough that the traveler might never notice the differences.  That means that the story is not actually time travel, but as we saw with Source Code, a movie can still have fascinating temporal ideas despite actually being a divergent dimension story.

Under fixed time, everyone knows that you can't change the past--that's pretty much the definition of the theory.  However, what is overlooked in this is if time is fixed, you can't change the future, either.  That matters.

It matters because once Jim realizes that something came through the wormhole from the future, he gets the notion in his head that he must arrange to send that object from the future, at any cost, in order to validate his experiment.  He bargains ownership of his machine against being able to send the object, and then risks his life to prevent the events leading to its loss.  He thinks and acts as if it all depends on him making the future happen, as if he could fail, and the experiment would fail with it.

The problem is, if this were fixed time in a single dimension, it would be impossible for the dahlia not to be sent from the future.  If Jim died immediately after receiving it, and his companions abandoned the project, somehow someone would open the other end of the wormhole eventually and send that flower to the past.  It is fixed time, and while we do not know the future in any detail, the fact that dahlian KMC 0-14 arrived in the past means that someone sent it from the future.

Of course, it's not fixed time.  Since it's multiple dimension theory, then whether or not Jim sends the dahlia becomes irrelevant.  The one he will send is not the one he received; it is a duplicate from another universe.  If he sends his dahlia, it will go to some other universe, and he will never see it again--he will simply be fooled into thinking that the one he received was the one he sent.

If it were replacement theory, of course, then failure to send the dahlia would be a disaster, creating an infinity loop.  In that case, Jim is right to make every effort to reproduce the exact same departure from the future that resulted in the dahlia arriving in the past--but he does not do so, because he has very wrong notions about what happened on that trip.  Ultimately he does repeat the same trip much the way it originally happened, but he believes that he is changing it, and is actually intending to do so.  That would be disastrous in replacement theory--but since we know this is multiple dimension theory, it would be foolish to pursue what would have happened.

We know this is multiple dimension theory based on several details.  The most blatant one is that they tell us so, explaining that the differences were too small for Jim to have noticed, such as that the bartender was a different person (but who notices the bartender?).  However, the most important one is that Older Jim, having traveled back in time and trying to get as far from his younger self as he can, winds up at a hotel where he discovers that he has already registered.  There are thus three of him in that world at that time.

It is not impossible for a time traveler to triplicate himself at some point in the past; it would require two distinct trips to the past which overlap each other and his own life in the past.  If on Friday you traveled back to Monday, then the self that traveled to Monday decided on Thursday to go back to Tuesday, there would be three of you alive on Wednesday.  However, that did not happen in this case, and it gives us an interesting point about travel in multiple dimension theory:  either the universes are linked in some way that controls destinations based on origins, or they are not.  For there to be three versions of Jim in this one universe, assuming that there has been only the one wormhole opening, we have only two plausible explanations:

1. The universes are linked in a temporal pattern, such that if you go back fifty minutes you land in the universe that is fifty minutes behind yours, and in one universe the experimenter set the time of the jump to a slightly longer or shorter time than in the other such that both travelers landed in the same world; or
2. The landing point is random, the wormhole connects more than two universes, and there is nothing to determine where you will exit once you enter, in this case two versions of Jim exiting in the same world.

We only ever saw one Jim enter the wormhole, but apparently two of them came out in that world.

The ending also confirms for us that these are divergent universes:  Jim leaps into the wormhole and comes out in a universe in which Jim had previously been killed in an explosion in a lab, something that had not happened in any other of our universes.  That is a separate problem, though, which we will examine below.

The lighter should have clued us that something was amiss, but it was underplayed and is a trope.  Perhaps the most famous example is the watch in Somewhere In Time.  The first time we see the lighter, Younger Jim picks it up from Abby; he asks about it, and she says it's his--because the first time she sees the lighter, he, Older Jim, gives it to her, asking whether it looks familiar and telling her it's hers.  The film gets away with it because each of them in turn says to the other, "It's yours," meaning "I got it from you," but each understands the other to mean "It's my gift to you."  The lighter has no origin.  It gets passed from Abby to Jim to Abby to Jim perpetually, and ought eventually to deteriorate to dust.

However, this not being a time travel story, that's not what happens.  Just as with the watch, there is a history of the world in which one of them obtained the lighter from another source.  It doesn't matter which one, as long as Jim has it when he travels to the past, and gives it to an Abby who has never seen it.  It spends a brief time in this universe before the next Jim takes it to another universe.  The more suprising aspect is that neither of them encounters a version of the other who recognizes the lighter and says, "Oh, I have one like that."  Different iterations of them would be different, but a unique lighter is unlikely across too many universes, particularly given that Abby smokes in most of them.

It will still eventually decay to dust, but we have two reasons to think so now.  One is that it is existing through those several days a vast number of times.  The other is that there is a very high probability that it has a temporal duplicate--which we will cover below.

There is another problem, reminiscent of The Philadelphia Experiment (the series):  how do the two ends of the wormhole identify each other?

If this were replacement theory, the problem woudn't happen yet.  On June 1st one end of the wormhole would open but nothing would come through.  On June 8th, the other end would open, the flower would be sent through, and as (presumably) the only two wormhole ends in existence the two would connect, the flower exit in the past, and the experiment would be a success.

However, when on July 1st they again open a wormhole, it will connect with one of the already opened ends.  That means either it breaks the original connection, ruining that experiment and undoing itself in a grand infinity loop, or it joins with those, creating a three-way tunnel and a 50% chance that the flower will go the wrong way.

Fixed time only complicates this, because all the wormhole ends ever opened anywhere in time or space become potential paths for the first one, and either it will connect to one at random, or they will all be interconnected.

Our multiple universe scenario makes it worse again.  Just having two open ends in each of a myriad of universes creates an incredible maze of tunnels--and the high probability that someone leaving from June 8th would arrive on June 8th in another world:  50% of the opened ends are connected to that time, and there is no indication that the openings are distinguishable from each other, being created by exactly the same process.

Even if every end connects uniquely to one other end, it is still likely to connect two of the same time.  Yet we know that this is not so:  since there were three Jims in the world when the wormhole had only been opened once, two of them must have come from different worlds through the same opening.

We thus have one of those systems that works once, maybe, if you're lucky.  Generally, though, it's a crap shoot--and that introduces another problem, which we might call Clumping.

Take any die--a six-sided die will do, but more sides means more interesting results.  Roll it a number of times equal to the number of sides on the die.  Any Yahtze® player will tell you that your odds of rolling each number exactly once are very slim--the long straight of five in a row is very difficult.  That's because the fact that you've already rolled a number on one of the dice does not impact the odds of rolling that same number on any of the others.

Suppose that this wormhole machine has been activated in only one thousand worlds--a drop in the vast bucket of possible universes under its theory.  That means that there is one chance in a thousand of a traveler who enters the wormhole to emerge in any one of those locations, times a thousand travelers, but we know that we're not going to get one-to-one correspondence.  Some of those wormholes are going to feature quite a few travelers emerging, easily a dozen, while many are going to be failures, with no one emerging.

Of course, at least some of the worlds in which no one emerges will never repeat the experiment in which someone enters, but others will take the chance that they can change the past.  Some of the worlds in which multiple persons emerge will recognize that there is a problem, and cancel the second experiment so that no one enters.  Yet even if five hundred persons enter headed for fifteen hundred random targets (one thousand in the past, five hundred in the future), you will get clumps.  We got one in one of the worlds we see, as Jim finds himself dead in a hotel room before heading to a different hotel room to die while his younger self is preparing to make the trip oblivious to the circumstances of his doppelgangers.

And keep in mind that a thousand worlds conducting the experiment is a tiny estimate.  By the theory of the film, billions of such worlds would still be a severe underestimation, and the more worlds are involved, the more marked clumping becomes.

The moment the first wormhole experiment is successful, the Jim conducting the experiment, whom we will call Younger Jim, experiences some kind of mental feedback, and collapses.  He has spells periodically thereafter.  What we don't know for that entire week is that at that moment his temporal duplicate, whom we will call Older Jim, has emerged from the wormhole and run out of the building without being recognized.

The movie rather cleverly follows the movements of Younger Jim through that entire week, as he attempts to make happen what as we noted must, by his theory, be inevitable.  We then see the events that cause him to enter the wormhole (with the dahlia), and leave the building, and then we follow him through the second week, as he attempts to work out what has happened, what is happening, and how to prevent Meisner from stealing his very valuable time machine.  However, the dahlia is dying, and apparently he is also dying.  His companions, who are aware that there are two of him but are keeping that secret from his younger self, are giving him medicine which will keep him from feeling the dying process but will not keep him alive.  He discovers that he dies more quickly when he is closer to his duplicate.

His friends propose that this is evolution's way of preventing there from being two of him:  the one who traveled from the future will die.

It is an interesting plot device which drives a lot of the action and leads to a significant part of the ending, but it is not particularly plausible.  Is it because the two creatures have the exact same genetic identity?  Why then don't identical twins suffer this problem, or cloned animals?  Is it because they contain identical molecules?  We have noted before that the human body is constanty replacing itself, and even in a week Jim will not be comprised of the same matter as he was the week before, particularly on the exterior.  Does Jim's mind work on a specific frequency, and so it is interfering with itself?  While that is plausible, the effects would be the same at both ends, and they are entirely different, as only Older Jim is dying.  Is it that Jim has a soul, and only one soul of the same kind can be in the universe?  Not really something evolution would recognize or alter.  As we explained in TimeCop and again in Mr. Peabody & Sherman, there is absolutely no sense to the popular trope of a time traveler causing a disaster by coming in contact with himself, let alone simply being in the same place in a universe's timestream.  While it is important to the plot, it damages the credibility of the story.

There are two other points to note about this.

First, the shock that Jim receives when the two versions of him touch the opposite ends of the intercom is another symptom of this foolishness.  It is, of course, a necessary plot device, so that Jim can talk to himself without Younger Jim learning that Older Jim exists, by having the electrical jolt scramble the video.  It is still foolish.

Second, it is passing strange that this temporal feedback sickness affects the dahlia and Jim, but not the lighter.  To what is it that the universe is objecting?  Why should the lighter not also be an affront to reality?

In fact, in the final universe Jim arrives to discover that his doppelganger in that universe already died; but why should that make a difference, given that his doppelganger's body is still there?

As important as it is to the story, temporal feedback sickness is the weakest piece.

As we mentioned, at the end of the movie Jim leaps into the wormhole and comes out in a universe that is significantly different.  In the role playing game Multiverser there is a standing joke that "there is no fiction."  After all, if every imaginable world is real somewhere, any effort to tell a fictional story is actually recounting the real history of another universe.  In the other universes we saw, a girl named Abby is writing a novel about a scientist named John Bane, inspired by a story she read about Jim Beale.  In this world, when Abby sees Jim Beale, she says he looks exactly like the real scientist John Bane.  Apparently, then, in this universe Jim Beale exists, but is named John Bane.

That raises issues about the whole temporal feedback sickness problem:  was this John Bane actually a duplicate of Jim Beale, or was he different enough that he was someone else?  If that's so, why are they not all different enough to be someone else?  After all, if everyone one of them is just a tiny bit different, then none of them are exactly the same.  Of course, there would be worlds in which they are exactly the same, but these would be less common than those in which they are slightly different.

In any case, we are in a universe that is observably different, and Jim begins noticing the differences immediately.

The first difference he notices is that Abby is not standing outside smoking a cigarette.  Jim had originally thought Abby was the original time traveler; that was not true.  He probably also noticed that he did not follow himself outside, as he had done in previous universes.

He eventually finds Abby at her favorite bar, and launches a fresh relationship with this divergent version of Abby by telling her about his time travel story.  She is very interested in time travel, and he has her notebook in his pocket, recording a version of his story including the revelation that it was multiple dimensions, not time travel, that he had demonstrated.  We are given the opportunity to imagine the happy ending.

However, there is a genuine mystery regarding how he got there.

We should first observe that in this world John Bane died.  In every other world, John Bane was Abby's fictional version of Jim Beale, but here either he is Jim Beale or Jim Beale didn't exist.  Maybe he just died, the day the wormhole opened, but that's not the impression we get; from Abby's attitude, it was a while ago, not that he just died before she had the opportunity to meet him but that he had been dead for some time and no one knows whether he ever built the machine.  Besides, if he died in an explosion related to opening the wormhole, Jim would at least have heard the explosion even if he had already made it out front before it happened.

That's the second problem.  From Abby's statement, one would think that the death of John Bane/Jim Beale was the end of the project.  However, the way the wormhole works requires that there be a machine at each end opening that end of the wormhole.  If John's project was terminated before the date of Jim's arrival in the past, then even if the machine were fully constructed there would probably be no one to operate it, and the wormhole would never be open in that world.

Compounding this, the fact that Abby was not outside suggests that Meisner was not there.  It is within the realm of possibility that Meisner didn't bring Abby, perhaps does not know Abby, in this universe, and that he had already left; but her absence is suggestive of the notion that he was not there to observe the experiment, and raises the question of how they funded it and how they obtained the needed radioactive material.

The conclusion seems to be that the wormhole must not have opened in that world, and that means Jim could not have arrived there.

We don't really expect time travel fiction to go too deeply into how the time machine works, but when it does, well, the method is subject to examination.  We are told very little about how this machine manages to open a wormhole; the last I heard the best method proposed would require a planetary mass of strange matter (which has only been found in microscopic volumes to date).  Jim, however, says that he found a means of obtaining the tremendous amount of energy required by tapping the rotational energy of the earth.

The idea is correct that there is a tremendous amount of energy in the rotation of the earth.  It is kinetic energy, in the form of momentum.  It would be very clever indeed to find a way to tap it--one could, in theory, build a giant rubber wheel affixed to an object in space which by touching the surface of the earth turned generators or other energy conversion systems.  The problem, though, is that the earth isn't actually generating energy; it's slowly consuming it--very gradually losing rotational velocity against the very minimal friction required to keep it spinning.  John speaks of "the energy which a planet makes," but the planet isn't making energy--it is slowly consuming stored energy left over from its creation.

Ultimately, though, if you were to tap that energy, you would have to slow the rotation of the planet by an amount equivalent to the amount of energy you took from it.  It doesn't replace that energy.

How much energy are we discussing?  He never says, and that's good.  The mass of the planet is huge, and its rotational velocity close to a thousand miles per hour at the equator.  However, the impression we are given is that the amount of energy required to operate the machine was great enough that nuclear power would not be sufficient, so given the accuracy with which we currently track the movement of the planet, the use of the machine would undoubtedly cause a measurable decrease in rotational speed, an increase in the length of the day.

It's not that it couldn't be used as a power source; it's that we would have a lot of objections to doing so.

The story is compellng and well-told despite its temporal difficulties.  That everyone is fooled into thinking that a multiple dimension wormhole is actually a time travel wormhole is not at all implausible, and a lot of what we might call the "hijinks" of Jim mistaking Abby for a time traveler and believing she is in on the plot to steal his idea is very credible.  The major problems arise from the notion of duplicates getting sick and dying simply because they are duplicates, which is unfortunately a necessary plot driver.

The ending is confusing, because we can't really explain how Jim got to that time and place if Jim is not already alive in that time and place.  It stretches credibility again, but is glossed over so we miss it.

Overall it is an enjoyable movie with a complex relationship between its main characters, that raises some interesting points about dimension travel but loses them by chasing less credible notions.