Most of the plot of the book didn’t make it into the movie. Not that the book was better than the movie, but it was much, much longer.
The novel’s radically different, and I find it an enjoyable read, whereas the movie strikes me as ample evidence that aliens would be justified in wiping all of us out. In the novel, after kicking the Psychlos off Earth, the humans use the teleporter to send a camera off into space, so many lightyears from the Psychlos homeworld, so that they can see what happened to 'em.
Ok my take is this:
We’ll assume these aliens have been watching us for a while since the dark ages lets say. So they have records of the horses being brought over by Columbus. So someone goes back in time and changes that. Therefore the records change and they have no idea it was any different. The records show that the horses were there before Columbus.
But now if the aliens had no contact with Earth I would have to say that they would not detect a change. In order to see something has changed they would need something to compare to, and if they haven’t been watching there is nothing to compare so they can’t notice a change… Make sense? If they had not had contact with humans a change in our timeline would have no effect on an alien culture. This means no contact at all, no redirected asteroids or stray satilites.
We also must consider whether the universe is fundamentally the same after time travel occurs.
There are two possible resolutions to the causality paradoxes in time travel (of which I am aware):
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Many worlds: This universe is only one of the many possible universes. Any time travel to the past would, in order to not violate causality, allow for a shift to an alternative universe which was totally consistent from its perspective but inconsistent from the universe from whence the time traveller came. Aliens would not have any dissonance of not knowing whether horses had been introduced earlier or later because the world lines would be totally consistent in the universe. They would witness the horses being introduced earlier and that would be the end of it. The only people who would know for certain that something had changed would be the time-travellers themselves who could remember from the alternate universe that the horses had alternatively been introduced later. Aliens see a totally consistent universe, albeit rather bizarre.
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Self-consistent solution. For some reason the universe conspires to prevent you from killing your grandfather before you are born. Assuming you have excellent record-keeping abilities, you cannot go back in time and introduce horses 1000 years before Columbus because if you did then you would “know” that you did but you “know” that you didn’t therefore you didn’t. However, let’s say the history says it wasn’t Columbus at all but some magical mystery explorer who appeared and introduced the horses. That magical mystery explorer might just be you, the time traveller, predestined to travel back to the past and give those Native Americans the horse. Still, the only people who realize that time travel has occurred are those that time travel themselves. Aliens see a totally consistent universe, albeit rather bizarre.
I am inclined to believe #1 is the answer, but that is simply a WAG at this point. As you can see, either way the aliens will not know from simply observing history that time-travel occurred. I am unaware of any other solutions to time travel paradoxes that are consistent with the physicality of worldlines in our present physical universe.
Reminds me of a SF story I read once a long time ago. The hero was the son of two nomadic time travelers, and when asked “when were you born?”, he could only say what time period his mother happened to be in when she went into labor.
Nothing about aliens, but you ought to check out Pastwatch - The Redemption of Christopher Columbus by Orson Scott Card.
Very interesting way of dealing with alternate universe vs time travel.