[personal profile] prog
In the last few years I have settled on what I can't help but feel is the inarguably most elegant way to resolve time travel when used as a plot device, particularly in regards to in-story concerns over "temporal paradoxes". These ideas synthesize descriptions seen in works like Greg Benford's Timescape and the film Primer, which, as I think their implications though, strike me as so overwhelmingly satisfying that I can hardly stomach any other flavor of time-travel magic. (I can make exceptions for implementations appearing in the service of worthwhile artistic effect, such as in the film 12 Monkeys or Charlie Stross's novella Palimpsest, but these are rare.)

Basically: Travelling to the past forks the universe at whatever point you re-enter. For this to work, we must assume that the many-worlds interpretation is essentially true. Since that theory is at least as plausible as the notion that you can send something as big and complex as a living human to a particular point in the past, I have no problem starting with this assumption.

Let's say I'm standing beside you in your lab, as you prepare to press the button that will send you back in time 100 years. You have an agenda in mind, and when it's complete you will use another miraculous device to bring you back into the lab. You press the button and you vanish. From my point of view, you never return. As far as I am (and the rest of the world is) concerned, you simply disintegrated.

I cannot pick up a textbook to discover that, say, World War II never happened, nor will my memories get overwritten to match reality however you intended monkey around with it. The world objectively remains exactly as you left it. Soon enough we must reluctantly come to accept you as dead, and life goes on for the rest of us.

From your point of view, you pop successfully into whatever place you wanted to occupy 100 years ago: success! (Let us handwave away how you're able to appear on the surface of wherever the Earth was located within the cosmos 100 years ago, with your personal velocity adjusted to match the planet's rotation and movement through space and all that stuff, to say nothing of how you can push aside your volume of matter as you arrive without disaster. These magics are all part of the unfathomable-science package that allows you to travel in the first place.) You are now free to do whatever you want, without worry of "temporal paradox". Kill your grandfather! Bribe the art academy to let young Adolf enroll after all! Go nuts tearing up as much of the early 20th century as you can, and observe as reality doesn't fall apart, nor does the family portrait in your pocket fade away one sibling at a time, or anything like that.

This is all possible because at the moment you blinked into existence here, the universe split in two -- just as, according to our basic assumption, it probably does all the time anyway. The "trunk" of this split leads to a future where you, 100 years later, press the button in your lab. The "branch" contains a different set of futures entirely, all which account for whatever mischief you have in mind. No matter what you do, your actions are forever sealed off in the reality-sandbox you created through your travel. You cannot in any way effect the "trunk" timeline that I inhabit and observe.

And woe be unto you if you are so foolish to actually flip that switch on your utility belt to bring you back to the lab! While your adventures did not destroy the future I inhabit, they almost certainly did change the future from your new perspective, and who knows what will exist at the space-time vector that you departed from? Perhaps it'll be your lab, with me waiting there -- this would require both you and I and most of the people we know to be born exactly as happened back in the "trunk", and then follow the same intersecting life-paths, note for note, until the moment you pressed the button. But given all the trouble you caused, that seems unlikely to me. More likely is that you won't recognize the place you pop back into, nor any of the people there, and that's assuming that the spot of your original vanishing in the "trunk" isn't occupied by a thick concrete slab or something in your new future -- ouch.

It's an open question whether the nigh-magic involved in future-directional time travel just blindly bumps you down one path of the branching future, like a ball dropped down a pinboard, or whether it causes an identical iteration of you to emerge into 100 years' worth of branched futures simultaneously. Either way, I don't think you-or-y'all will be in for a good time.

Let's take this another way: you have a more benign experiment in mind. You'll press the button and travel only 10 minutes into the past, and your destination will be the middle of the Sahara. Nothing you can do there can possibly affect the course of human civilization at all, especially in such a short amount of time. Again, you vanish, and again, from my point of view, you are gone forever. From your point of view, you pop open a bottle of water and pass the time, admiring the desolation. Then you flip your belt-switch.

Pop! Here you are back at the lab, with me still rubbing my dazzled eyes from your departure. But that iteration of me is a wholly different one than the one in the time-branch that you left behind. You're still in the separate time-branch that you entered the moment you appeared in the desert. But in that branch, back in your lab, I was helping you fasten your wondrous time-belt apparatus, ten minutes before you pressed the button, and nothing you could do in the Sahara could prevent that future from playing out.

So what happened to the you in your new side-branch? Well, they also zapped themselves into the Sahara, creating another branch. Which will create another, and… yes, a whole lot of recursive side-branching happens there. It goes quite deep, but not infinitely; eventually, along the line, something happens -- perhaps a disaster, perhaps merely a change of mind -- that causes a you-iteration to not travel to the desert. And in that world, a time-traveler iteration of you still appears in the lab, water bottle in hand, and if that means that world now has two of you in the same room, goggling at one another, so be it. There's still no "time paradoxes" at any point in the process.

Interestingly, in this latter experiment, from most of my perspective your time-travel jaunt is a complete success, just like in the movies. I see you vanish, and just as quickly re-appear with a pailful of Sahara sand as proof of your travel. But for that one version of me in the "trunk" timeline where you "first" pressed the button, you vanished for good.
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