The past decades of sci-fi movies coming out of Hollywood have built up a sci-fi mythology. One of the elements of this mythology is the completely illogical idea of time travel.
The great Saint Thomas Aquinas has written about this topic in his Summa Theologiae in the first part, question 25, article 4: “Whether God can make the past not to have been?”
He writes as answers to the objections:
On the contrary, Jerome says (Ep. 22 ad Eustoch.): “Although God can do all things, He cannot make a thing that is corrupt not to have been corrupted.” Therefore, for the same reason, He cannot effect that anything else which is past should not have been.
I answer that, As was said above (Question 7, Article 2), there does not fall under the scope of God’s omnipotence anything that implies a contradiction. Now that the past should not have been implies a contradiction. For as it implies a contradiction to say that Socrates is sitting, and is not sitting, so does it to say that he sat, and did not sit. But to say that he did sit is to say that it happened in the past. To say that he did not sit, is to say that it did not happen. Whence, that the past should not have been, does not come under the scope of divine power. This is what Augustine means when he says (Contra Faust. xxix, 5): “Whosoever says, If God is almighty, let Him make what is done as if it were not done, does not see that this is to say: If God is almighty let Him effect that what is true, by the very fact that it is true, be false”: and the Philosopher [Aristotle] says (Ethic. vi, 2): “Of this one thing alone is God deprived—namely, to make undone the things that have been done.”
Reply to Objection 1: Although it is impossible accidentally for the past not to have been, if one considers the past thing itself, as, for instance, the running of Socrates; nevertheless, if the past thing is considered as past, that it should not have been is impossible, not only in itself, but absolutely since it implies a contradiction. Thus, it is more impossible than the raising of the dead; in which there is nothing contradictory, because this is reckoned impossible in reference to some power, that is to say, some natural power; for such impossible things do come beneath the scope of divine power.
In other words, changing the past is logically impossible and not just impossible according to the laws of nature, which God can suspend.
Reply to Objection 2: As God, in accordance with the perfection of the divine power, can do all things, and yet some things are not subject to His power, because they fall short of being possible; so, also, if we regard the immutability of the divine power, whatever God could do, He can do now. Some things, however, at one time were in the nature of possibility, whilst they were yet to be done, which now fall short of the nature of possibility, when they have been done. So is God said not to be able to do them, because they themselves cannot be done.
Here is another way to understand the problem of changing the past. If God could change the past, then God would go through the following three phases:
- The “old past” exists.
- God changes the past.
- The “new past” exists.
Since God is immutable (incapable of change), this is impossible.
There is another very important point. If time travel is even theoretically possible, then this would mean the end of the Faith, because we would need to add caveats to dogmas. When we pray in the Creed that Jesus was crucified, we would need to add: “as long as a future time traveler doesn’t travel back in time and hinder the crucifixion”.
The Faith is absolute and there is not even a theoretical possibility that any of it could be altered.