Member Rara Avis
Many years ago, I wrote a short story about a mid-thirtyish guy who had just been dumped by his wife of ten years. A mathematician and scientist, Bernard was ruled by cause-and-effect and he convinced himself that, if he could only discover WHY she stopped loving him, he would be able to reverse the damage. At what point in time does love turn to disdain?
To learn the answer, Bernard used recent advances he'd made to travel back in time, not physically (which was impossible), but as a "Silent Ghost" (the name of the story). He could observe, without restriction, but could not in any way influence.
Going back a few months, Bernard overheard his wife talking to a girlfriend, and her remarks about him were much less than kind. Not far enough. Back farther, only to watch as his wife coldly manipulated him to get her own selfish way. Over and over, Bernard traveled into the past, looking for the precise moment, the exact event when his wife stopped loving him. Only to eventually realize she had NEVER loved him the way he imagined she did. The woman he thought he knew, the woman he so loved, never really existed except within his own mind.
Heinlein, needless to say, did it much better. One of his early short stories (circa 1941) was called "By His Bootstrap." The protagonist goes through a series of difficult, often dangerous, incidents, and in each instance miraculously saves himself based on advice from a mysterious, and often cantankerous, stranger. As the story progresses, the protagonist finds it necessary to use time travel to visit the past and warn his younger self of a problem, and by the end of the story we realize that EVERY character we've seen is the same person, at different times in his life. He essentially solves his own problems by telling himself how he already did it.
In other stories, we are introduced to alternate time-lines, where changing the past produces diverging forks in the road and multiple instances of "reality" continue moving forward. Other writers have shown that changing the past automatically changes the present, but without ANY possible way of knowing the change took place. The "you" you remember from yesterday and today didn't even exist until someone changed something in your past, and were it to change again, you would still be reading this and think "you" would never want to be anyone different. In the Superman comics of the fifties and early-sixties, a recurring theme was the Man of Steel traveling into the past - to try to save Lincoln, to prevent the sinking of the Titanic, et cetera - only to inevitably find himself thwarted by silly little coincidences that prevented the past from being changed (he usually caused the very thing he was trying to prevent).
The possible ways to interact through time are pretty much endless, and I think very few of them would require us to simultaneously live two lives at once. We can observe, we can communicate, we can create new "instances" of ourselves, all without necessarily destroying who we are.
In a very real sense, I think changing the past is little different from changing the future. How would your life be different today if you had avoided that certain relationship in your past? Surely, you would be a different you. But if you think of time as a singularity, rather than a continuum, how is that any different than asking what your life will be like tomorrow if you make a change in plans tonight? Surely, you will be a different "you" tomorrow if you go right tonight instead of moving left. Whether you change the past to arrive at a different present, or change the present to arrive at a different future, you necessarily change the intrinsic "you."
Indeed, if you step outside the flow of time for a moment, if you can manage to look at it through eyes not conditioned by human limitations, you quickly realize that the present "you" with which you're perhaps happy is also part of your future self's past. Just as that past self and the events you don't want to change were once your own present.
Past, presence, and future are all relative and ultimately, interchangeable.
Deciding you would never change the past, because you already like who "you" are. is no different than refusing to change the present for the same reason.
And perhaps therein lie the answer to your question (or, at least, my answer to it).
You cannot NOT change your present. Your only choice is to make conscious decisions and shoot for a better future "you," or refuse to make the decisions and accept the defaults thrown in your path. Of course, not making a decision is still a decision, and in any case, the "you" of tomorrow will be different. Perhaps better. Perhaps not. Stagnation simply isn't a possibility, nor should it be part of the deliberation. Liking the "you" of today isn't enough. You have to like the "you" of tomorrow, too, and that always requires choices. Knowing that, at least on some gut level, most of us try to make decisions that will lead to differences we will like. Sadly, we're not always successful.
I think, were it possible to change the past, you would essentially be faced with the same situation. Make a decision or accept the defaults. Erase a negative event in your past and you will have changed the present "you," leading to a new future "you." Refuse to erase the negative event, and you will STILL have changed the future "you." Perhaps for the better. Perhaps not.
Changing the past would ultimately boil down to cost/benefit and be little different than the decisions we already make every day regarding the present. If I return to school tomorrow I could learn more, perhaps fulfill a dream, but the cost would be a lot of hard work and lost time that could maybe be put to other uses. If I eliminate that first marriage of yesteryear I could avoid a lot of needless pain, but at the cost of not having the three children that define my life. Whether I return to school or not should rely on benefits versus costs, though in truth the decision is always affected by other considerations as well. I suspect whether I would change the past marriage would be no different. Benefits versus costs, with a whole lot of those other considerations still thrown into the pot, too. A decision to change or not change, whether in the past or present, WILL change the future me.
Sadly, I doubt many of us would be any wiser in changing our past than we usually are in changing our present. But that wouldn't eliminate the need to try.