In illustrating this, we're going to blur the lines a bit between this theory section of the site and the main section, the movies analyses. We're going to draw on events in one of the movies, 12 Monkeys, to show the problem. This movie has been chosen because it is often presented as a perfect example of Fixed Time Theory, and it illustrates the failure with remarkable clarity once you are looking in the right place.
Let's look at the causal chain that is presented. This is oversimplified; at each step, causes branch out to have other effects that have other causes, and we are skipping steps. In causation, it's not a bad thing to skip steps; we do it all the time. When we say that the heat caused the water to boil, we mean an incredible number of intermediary statements regarding individual molecules becoming excited and crashing into other molecules, causing chains of reaction until all the molecules are so excited, but in the end we admit that the heat caused the boiling, even though the intervening events were part of that.
It is also just as true that we can remove intervening steps in the chain without falsifying the results. That is, we can say
To restate for clarity, in the story of 12 Monkeys, the world is destroyed only if it is destroyed; if it is not destroyed, it is not destroyed. It will not be destroyed unless it is destroyed. There is nothing to cause that destruction, so it can't happen. All that causal chain is illusory nonsense. There is ultimately no cause for anything that happens in the film, but itself.
More than one person has argued that there is a flaw in this chain; they maintain that there is no evidence that Dr. Peters released the virus because Dr. Goines changed the security protocols. To their credit, they are correct that bio labs don't really work that way; someone like Dr. Peters would have had access to the organisms that could destroy the world all along, and it might well have been sheer coincidence that he released it at this moment. For my part, it strikes me as overly coincidental that he would do this the day after the protocols were changed, and from a literary and dramatic standpoint it seems to want us to believe that Peters has been waiting for his opportunity which finally came when he was given control of access. Further, although in real bio labs it might well be that everyone who has a doctorate and clearance to work there has clearance to help themselves to such organisms, in fictional works and in the minds of the majority of people there is a belief that the tightest security possible is maintained around such potentially devastating weapons, including that there is one and only one person who has access, who must be made aware any time anything is removed from containment, by whom, and for what purpose. Thus the natural interpretation for the ordinary viewer would be that Dr. Goines' decision to change the protocols meant that Dr. Peters was given unrestricted access, and so able to take the samples.
The only difference it makes here is whether or not 12 Monkeys contains this flaw; the flaw is still quite common in such stories, and this illustrates it well.
Scientists don't use complicated stories from popular movies to illustrate their ideas. They use simplified examples. To illustrate this one, they propose a simple problem of a billiard ball and a wormhole.
Before I address it, let me say a couple of things about scientists and their theories about the metaphysics of time. I don't want anyone to think that I am denigrating science or scientists. The scientific method generally, and especially in the realm of physics, involves the study of observable evidence from replicable experiments. That is, we do this, that happens, we watch what happens, and we draw conclusions. There can thus be no current scientific theory about the effects of time travel, because there has been no successful time travel event to study. Before we have one, let's be certain we know what all the possible outcomes are. This theory is one of those possible outcomes, as are the Fixed Time theory and the Parallel Dimensions theory. Those are all interesting theories with intelligent proponents; the fact that I think them completely wrong does not diminish the efforts involved in considering them. Those, however, are not scientific efforts; they are philosophic efforts. Asking scientists to answer philosophic questions is certainly not entirely unreasonable; they've as much right to express an opinion as any other amateur in the field. They cannot claim to have scientific knowledge until they have scientific evidence, and they cannot have that until they have a successful time travel event, which we do not have. You may ask the physicist for his political opinion, or his view of the economy, or his preferences in pastry recipes; but you're asking an amateur, and should not give his opinion more weight in metaphysics than you would in culinary arts. I'm sure some physicists are quite well studied in culinary arts; the fact that they are physicists does not guarantee this.
In this regard, it should be noted that scientists are divided between theories. The fact that some scientists are quite certain that the Fixed Time Theory must be correct and others are arguing for the Parallel Dimensions Theory should be evidence that they don't know any better than you. So as we examine the billiard ball illustration presented by some scientists, let us remember that it is not Holy Writ. It's not an expert treatise in metaphysics. It's not even really science. It's an illustration of a speculation, and nothing more.
The billiard ball example starts with the idea that a billiard ball is seen to enter a wormhole whose exit point is displaced both spatially and temporally, such that it will come out the other end before (in a strict temporal sense) it entered this end. Then we are told that the billiard ball collides with itself, knocking itself off course. Would the billiard ball prevent itself from entering the wormhole? If it does, it will prevent itself from exiting the wormhole, and so not collide with itself, and not prevent its entrance. This would be a paradox, say the theorists. A paradox is impossible, and thus cannot happen.
The fact that they have no solution for the paradox begs the question. There is an inherent assumption in the argument that because the speaker can't solve it it must be insoluble. This is arrogant, at least; it would be like the third grader telling his teacher that multiplication doesn't work, because he doesn't understand it. The entire theory is at this point founded on the fact that those who propose it have no alternative. Thus they make a completely unjustified leap in logic, and get themselves into trouble.
It is possible, they observe, that the collision between the ball and itself could conceivably knock it on course for the entrance to the wormhole. Since it is possible that it would do this, and since we can't imagine or explain what would happen to time if the ball did not enter the wormhole, that must be what happens. The ball coming out of the wormhole which collides with itself will always knock itself into the wormhole.
Of course, if it knocks itself into the wormhole, that inherently means that it would not have entered the wormhole had it not collided with itself. Certainly you could argue that it might have done so; but it would have entered it at a different angle, and thus it would have exited at a different angle, and so the collision would occur at a different spot. The assumption of the fixed time theory to this solution is thus:
An event whose occurrence is dependent upon its own occurrence cannot occur. That's just simple. If taken as a Fixed Time Theory story, 12 Monkeys must be taken as saying that the Fixed Time Theory is wrong, because this is absurd. That's all it tells us--that, or that if the Fixed Time Theory is true, time travel must be impossible, because otherwise anomalies like this could occur.
The theory expounded on this site resolves this sort of anomaly; the Fixed Time Theory does not. If we have an original cause that has been erased, lost to reality as something that happened and then "unhappened", which does not now exist in any real sense, the replacement cause can maintain the loop without a problem. However, the loop must have had an original cause, or it could never have come into existence.
That's the problem with most fixed time stories. They don't work under the Fixed Time Theory. The only story that works under that theory is this one: time travel proves to be impossible.