But they don't really answer it; they just reject it out of hand.
Here we will propose some events, and the way the Fixed Time Theory addresses them, and perhaps find it wanting.
I am going to suggest that perhaps I might wish to commit suicide. I have in fact often thought of suicide over the course of my life; I've never attempted it. I wish I could say that I've never attempted it because my thinking has been entirely the theoretical ponderings of a happy and well-balanced philosopher. In fact, suicide is not easy, and it is threatening. There are few means, if any, which are certain and painless, and for many the consequences of failure are great. I am perhaps one of those who is alive today not because I am afraid to die, but because I'm afraid to survive.
I could eliminate much of the trouble with suicide by a simple method I've imagined. I could travel back to the beginning of the last century and locate my grandfather, and kill him before he ever has the chance to sire my father. This would not be a terribly difficult task. I know where he lived; he was my grandmother's next door neighbor. He was also seventeen years her senior, so I have plenty of time. They were in a small town in rural Mississippi, so there's not too much danger of losing him in the crowds. If I've got access to a time machine, a few weapons of mass destruction should not be problematic. I can take Mississippi off the map, if need be. That should finish him.
Note how perfect this plan is. If I succeed, not only do I end my suffering, I will have eliminated all that suffering I've already experienced. Suicidal people at least since Job tend to see the misery in their lives overbalancing any good that ever happened; the good is merely a deception, a moment when the reality was disguised. Thus for the suicidal person, there is nothing worth saving of the past; to wipe out the entire existence is the best course. I can do it. I can make it so I was never born.
Further, the consequences of failure are minimal. Probably no one would ever know I made the attempt. I can't suffer brain damage, disfigurement, crippling. Even social disdain is unlikely.
I don't have to worry about who might find the body; there will be no body in this century. I need not fear the pain, as there will be no pain. In all, it is the perfect suicide plan.
To plans like this, fixed time theorists will only say, you will fail. Something will go wrong. After all, they argue, clearly you are alive, so clearly you have already failed to kill your grandfather. Perhaps your weapon will misfire. Perhaps you'll accidentally kill the wrong person. Maybe you'll be apprehended before reaching your target. It is possible that you will relent upon seeing him, and be unwilling to cause that pain to someone else. Maybe you'll just miss. Somehow, you will happen to fail.
This sounds so reasonable. After all, they aren't saying that I can't do it, only that I won't. I have failed, so I will fail. We don't know what will go wrong, but something will go wrong.
Obviously, I am not the first person to have thought of the idea of suicide by time travel. I will not be the last. Many will read the idea on my web site; far more have read it elsewhere, or encountered it through other media. There must be thousands of people already who wished they could kill a parent or grandparent before the chain of conception which led to them was initiated. If we assume that the universe is unending and populated by uncounted lives, there must be billions of creatures who have an idea not much different from this.
It is also generally true of technology that once it is discovered it grows more common. Once cars were toys of the rich; now in many parts of the world they are basic necessities. Television began in corporate hands. Today your local church or civic group could, for not too much money, own its own LPTV station, if it could program it, and many cable companies have public access channels. Not so long ago state medical boards were trying to control rising health care costs by limiting the number of CAT scan machines; today such machines are considered essential to the operation of all hospitals and emergency medical facilities. Who even owned their own computer forty years ago? Thus it is reasonable to suppose that once a working time machine is built, it will be just a matter of time before time travel becomes widely available.
We must now contend with the twin facts that billions of people want to undo their own births by killing their grandparents and that time travel is within their grasp.
This suggests that there will be at least millions of people traveling back in time, taking with them advanced weaponry that makes modern thermonuclear devices look like firecrackers, trying to erase their own existences by destroying their ancestors, with no regard for the collateral damage whatsoever.
The fixed time theory tells us that every one of these highly motivated, capable, prepared, and equipped individuals will just happen to fail.
People who defend this notion get offended when I tell them that what they're saying is God will not allow it to happen. I don't mean it to offend; it is what they are saying. Of course, they're trying to be scientific, so they don't say God. Usually they say Nature would prevent it. Yet it is a strange thing, this Nature that does not allow paradox.
Somehow Nature must be able to perceive when Paradox is going to occur. After all, no one has said that time travelers wouldn't be able to move about in the past as freely as they do in the present; it is only claimed that any action which would create paradox would be prevented. Thus Nature must somehow be able to spot actions which are likely to create paradox, so she can prevent them. I get letters all the time from people who can't spot actions that are going to create paradox. I even see such actions in movies that were very expensive to make and paid someone a lot of money to try to produce a credible story that wouldn't contain paradox, presumably. It's not always so easy to see when an action might create a paradox, and I'm surprised this Nature, whoever she is, is able to do so before the fact.
This Nature also must have incredible power, to be able to prevent all those events. Millions of capable individuals are going to have their specific intended actions thwarted by her efforts, and it will be done so subtly there will be no hint it was anything other than happenstance. A gun misfired. A bomb did not go off. You killed the wrong person. The police caught you first. Nature is a very clever lady, if she can do all that.
The fact is, if you're talking about an entity which is intelligent enough to spot potential paradox before it occurs, and capable enough to prevent it, you're describing some notion of a deity. It might not be the God I, as a theologian, would expound, but it is a very powerful and wise deity who obviously has some benevolence toward creation to take such action. I don't care what you call it, you mean God. You mean it won't happen because God would not allow it.
I am, as I say, a theologian by training. I'm no expert in the field, but I have taught undergraduate studies. I believe in God, and in divine intervention, and in providence. However, I'm far too knowledgeable to think it safe to assume that we can do any fool irresponsible thing we want, and God will protect us from the consequences. It doesn't work that way in our lives as individuals, and it's never been that way in the world at large. People thought God would not allow a disease to be so deadly and devastating as the Black Plague was. Some thought God would not allow man to fly, and others that He would not allow us to reach the Moon. There were those who thought the atrocities of Hitler were beyond what God would permit, and those who honestly believed that God would not let us split the atom. All of these people were wrong.
Now we have a theory that is no more than, God will not allow us to alter history. I don't see how this is any more likely to be the limit than any other thing someone believed He would not permit.
I suspect there may well be divinely-appointed limits to what we can do; I also recognize that we have no way of knowing what they are. It is irresponsible to suggest that we do know something so occult.
In the end, there are only two possibilities. Either God will prevent us from changing the past, or one of those suicide bombers will get through. If it's the former outcome, we will probably find time travel itself to be impossible. If it's the latter, the Fixed Time Theory will crumble in a heap as the world scrambles for some other theory of time which resolves the disaster that has been created.
What is the point?
The Fixed Time theory does not resolve paradox. It ignores it. It makes the unfounded claim that paradox cannot happen.
There is no evidence that the past is immutable. That is at best a hypothesis and at worst wishful thinking at this point. We cannot assume that it is impossible to damage something we do not fully comprehend. If the past proves mutable, we must be prepared for the consequences of changing it.