Stacked butterfly effect?
i.e. that jumping back to the last days of the ACW would introduce a "you" that has a small (but non-zero) possibility of perturbing the immediate future merely by 'being' where one hadn't been before. (Increased if you go back with the express intention, and resources, to swing the war, but for now let's not ascribe such motives.) But as more time passes from your arrival, the perturbations build up to cause many changes to Today's history. And if you arrive a year or two prior to the war then there's more chance that the people you met in turn met (or didn't meet) other people, or other effects[1] and it's not just post-ACW times that change, but could be the outcome of the ACW itself. Arrive at the time of the Revolution and it's even more likely. Arrive even earlier and even more so. (With all due apologies for this English person's ignorance of a lot of the nuances of your Rebellious Colony's recent history; i.e. the last two centuries or so...
)
If that's the claim of the alleged Traveller, it's one possibility. Without spending all (indeed any!) of my time looking through JT's writings on the subject, would anyone who has care to enlighten me as to whether the new timeline "branch" is supposed to be completely in addition (and parallel to) to the still-thriving original timeline, prune that timeline at the point of departure or to entirely remove the 'false start' branch immediately at the point of arrival, so that there is just one, crooked, branch with the otherwise inexplicable addition of the Traveller at the elbow of that crookedness?
Still, while I'm not averse to acknowledging the existence of time-travellers, I find the overwhelming elegance of a stable time-loop just too obvious an idea to discard as an untested hypothesis. Rather than reject this guy on the grounds of physical impossibility, I'm actually closer to rejecting his ideas because they make the universe look just too messy, if true. Something with a Traveller experiencing something more akin to a Twelve Monkeys situation (not necessarily aware of the fact that what was in his Future self's past is going to be part of his Past self's future, but it's certainly possible) wouldn't arouse anything like the same level of doubts.
But forget about the fading musician of Back To The Future or the silly "you can't touch yourself" (fnar!) of Timecop. (They're not even going to be the same bits of matter, the skin or even bone of Young You and Old You. And even if you leap back just a few minutes, the chances of future-version-of-atom being within a normal atom-to-atom distance of meeting its exact past-version-of-atom partner in a casual touch of hands is going to be minute, unless there's a weird morphogenic field projection thing involved for some reason. Handwavium sorta thing, in several different senses of the term...)
I definitely go for the likes of the guys who go back in time to 'rescue' Hitler from the bunker just before he shot himself (either for real, or as a trick to put him on trial) are going to leave a reasonable facsimile of his dead body (or a volunteer martyr with appropriate cosmetic surgery) in his place to sustain the history that everybody knows (and, what's more, it had always been the case that they did). Or the guy who goes back in time to work out why an ancient culture died out so suddenly didn't realise that he had a modern illness that they couldn't handle (and, possibly, that they had illnesses that his modern body wasn't used to, if they don't just throw him off a cliff for being a stranger who arrived and made the gods angry). The guys with Time Travel ability who are in a fix decide to remember, when they're finally out of the fix, to ensure that their present selves get provided with the tools to get out of said fix. Mysterious strange man who jumped out in front of a character right at the beginning of the story, behaving oddly, then later on seems to have no memory of the incident finally decides to prove he has time travel abilities by disappearing for a moment and coming back with his tie removed, having apparently carried out the action that his odd-self had performed. The archaeological dig students who find ancient and anachronistic remains of a digital watch in the rubble just before the Incident which moves their professor through time are just part of the process that self-sustains that particular loop... So, it doesn't
always make for the better plotline than "Hero[3] goes back in time/comes from future and changes things For The Better Or Otherwise (TM)", but as in a film it's often the journey and not the destination that grabs you, then it could as likely be the re-journey that's the grabs you, and never mind the pre-destination...
I've not actually seen the Sarah Connor Chronicles[2], which I believe is supposed to show a malleable timeline, but when it comes to the Terminator movies themselves, I've long held that
despite the apparent changing of the timing of Judgement Day, part way through the set, this little blip could be explained away by those being sent back (human and machine) having been misguided (or misguid
ing) about such 'details' so as to create consistent loops causing what happened to happen. (I mean, already you have to have had the John's father as part of the loop, for there to actually be a loop. We know that this bit ends up being self-consistent.)
Still, I'd try to be
very careful not to slip off of the levitating walkway and landing on even a single insect, should I ever become a Time Tourist. Pretty much every theory of Time Travel is untestable. Even with Time Travel actually having happened. How do you know that your departed timeline hasn't disappeared upon your leaving? Even if you stick around after your arrival to watch yourself go back again (having either noticeably preserved or averted a time-loop) and find yourself still in existence, how do you know that that trip is your actual self and not a facsimile copy who instead of destroying the timeline he left has just destroyed himself in merely a convincing copy of your actual departure where you
did end up obliterating the future existence of everyone (and everything) not accompanying you? Maybe if one Traveller waited until after the first and then joined them (even pre-dated their arrival), something could be confirmed, but there are problems of false positives (and negatives) even with that, if you think about it.
[1] Your walking along a path eroded it just that little bit more which meant some icy puddle the next winter which caused a packhorse to slip and drop its pack and delayed the arrival of goods in the market by a few minutes, so that another trader had been by already, thus changing the availability and/or asking price of certain goods at a store at the next town along, which meant an expedition setting off into the wilds ended up with a different set of provisions that might have meant the difference (either way) between their being self-sufficient in their trip or having to barter (or otherwise!) with the relevant Native American residents, changing the local political situation, which meant a differing politics on a regional and states-wide level which meant a different Union/Confederate President... Or any number of similar cascades of results.
[2] Or is it "tCoSC", I might be mixing the title up with the Sarah Jane Adventures.
[3] Generic hero or the actual Hiro from "Heroes" hero...