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[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.

Date: 2012-09-01 06:11 pm (UTC)
From: [identity profile]
One problem I have in the whole 'traveling back in time splits the universe into two' idea is that in order to split the entire universe you need to somehow gain the mass-energy for that entire universe.

There are ways around this -- First of all you probably don't have to create an entire new universe -- you just have to create part of a universe. In fact, if you travel back in time 1 day you'd only need at most the energy contained in the radius of one light-day.

I also think on the quantum level not only are you 'splitting' small sections of the universe by symmetry you're 'joining' them as well. Once you've sent someone back in time eventually .. although it could take centuries .. or millennia .. anything that said person has done will become lost -- or ambiguous .. as the split universe slowly, on the quantum level .. restores the energy balance by joining quantum sections of the universe more than splitting them.

This is my own personal 'time travel' theory that I haven't fully worked out for myself yet.

I also came up with a way to explain the time-space travel oddities in that 'Contact' movie but have to go to work soon and don't have enough time to type up.

Date: 2012-09-01 11:23 pm (UTC)
From: [identity profile]
My whole thing rests on the assumption that you get your universe-splitting for free by dint of the many-worlds hypothesis being true. In that case, a time-traveler's emergence into their own timeline's past is just another quantum event that results in a world-fork, such as surely as putting a cat in Schroedinger's box does.

If many-worlds isn't true, than yeah, my method is way too expensive to pull off as written. :)
Edited Date: 2012-09-01 11:24 pm (UTC)

Date: 2012-09-02 12:13 am (UTC)
From: [identity profile]
Another possibility that kind of works in both cases is if you have a whole bunch of 'universe time lines' all tightly woven and bundled together kind of like a rope, again with individual threads splitting and merging at various points. Along comes a time traveler and changes a significant event -- from then the entire rope splits with one group being the 'new timeline' and the remainder being the 'unchanged timeline'. Again for symmetry stake it's possible for these large bundles to merge however they may not merge at all -- you could have a slow 'unraveling' of timelines as you progress which would represent an increase in the entropy of the multiverse.

Date: 2012-09-02 05:12 am (UTC)
From: [identity profile]
Sorry, I'm a random person from internet.

"Once you've sent someone back in time eventually .. although it could take centuries .. or millennia .. anything that said person has done will become lost -- or ambiguous ..."

Maybe not! Two points:

1. Okay, questionable physics time. One reason why this probably doesn't happen is that the entropy of the universe is always increasing rapidly, so there are always a lot of extra bits per unit time to store some information about the past in.

The local entropy of the solar system stays more or less the same (otherwise it would gradually heat up or cool down). So that extra information has to be mostly written in solar emissions: light and the solar wind.

So that's a cool idea: the information in the light from Earth is keeping universes separate.

2. What if you write something down and it stays written down?

Well, for comparison, what if you accidentally make a mark on a stone, and it stays there for a thousand years? If the mark was accidental, then it doesn't tell you anything meaningful about the past, because anything could have made it at any point.

On the other hand, let's say you write a letter in 1066 and you date it. If I read a facsimile of that letter in 2012, now I know something meaningful about the past. I know there is a good likelihood that someone in 1066 who wrote that letter. (Of course it might be a forgery, or it might have been miscopied.) And if you describe historical events, I know there was probably someone in 1066 who believed that those events occurred.

So that's a cool idea: writing is not just how we know things about the past, it is the only reason there is a definite past in the first place, together with other types of recording, like the geological record and bones and artifacts.
Edited Date: 2012-09-02 05:14 am (UTC)

Date: 2012-09-01 08:39 pm (UTC)
From: [identity profile]
This seems to be the theory that Doctor Who uses.

Date: 2012-09-03 03:03 am (UTC)
From: [identity profile]
Sometimes. But they're really very loose and inconsistent about it.

An element that they emphasize in the new series, though it was arguably present to some degree in the old one, is that some things about history are OK to change and some aren't, and attempts to mess with the latter category cause poorly-defined cataclysms, ranging from flying monsters eating the universe to history collapsing into a surreal mess in which everything happens at the same time and the calendar date never changes.

The difference between the two kinds of event is only Time Lords understand shut up.

Date: 2012-09-03 06:31 pm (UTC)
From: [identity profile]
You're right. I haven't come across things about history that he's explicitly changed yet. It seems that most adventures of the week are already how things turned out in the timeline we're in, so he's just fulfilling prophecy, in a way.

Date: 2012-09-02 01:07 am (UTC)
From: [identity profile]
If you version A and you version B both arrive at the Sahara at the same precise time, do you fork the timeline into four paths: one where neither arrive, one where A arrives, one where B arrives, and one where both arrive?

Date: 2012-09-02 01:20 am (UTC)
From: [identity profile]
I'm not sure what you're asking exactly. Two parallel iterations of the same traveler making the same trip at the same time will result in two new universe-forks at the point they emerge into their own timelines' respective Sahara deserts. They cannot interfere with one another, being in wholly separate universe-branches to begin with, if that's what you're wondering about...

Date: 2012-09-02 02:29 am (UTC)
From: [identity profile]
However, the second iteration's "respective Sahara desert" won't be separate *until* the first iteration emerges in the Sahara.

Instead, let me try explaining what I mean this way:
When initial time-traveler goes back in time from 2010 AD to 100 BC, that person creates two separate timelines: one where he emerged in 100 BC (timeline B) and one where he did not (timeline A). These two timelines share the same timeline up until 100 BC. Assuming this traveler (A) has negligible effect on the world (by the butterfly effect), then down timeline B, time-traveler (B) discovered time-travel in 2010 AD. If this time-traveler goes back to 99 BC, timeline B will split into timelines B & C, which share the same timeline up until 99 BC. If time-traveler A just sat in the desert and died, time-traveler B will find that corpse, so they can "interfere with one another."

However, instead, if time-traveler B traveled to 101 BC, then timeline C would diverge from A and B before B diverged from A. And even if traveler B sat and died, time-traveler A would never see it. This is the "cannot intefere scenario."

So what happens if traveler B emerges at precisely the same moment that A emerges (but five feet over, say)? The only reasonable resolution seems, to me, to be a four-fork.

Date: 2012-09-02 10:40 pm (UTC)
From: [identity profile]
I sort of feel the opposite of the many-worlds interpretation. It seems like the least interesting way to envision time travel to me. It's inelegant and boring, and it discourages intellectual discipline in conceiving time travel plots. It's too easy. "Oh, you have traveled into the past. No worries; we don't have to think this through. We'll just start over with a clean slate."

I much prefer the kind of time travel portrayed in 12 Monkeys (or try La Jetee, which was more succinct.) Are you familiar with the Novikov self-consistency conjecture? Time travel fiction that draws on the idea that you actually cannot alter the future by going back into the past is so much more interesting. It doesn't mean you can't affect the past environment, just that your actions will naturally result in the same future that you departed from. This is so much more interesting than the many-worlds interpretation.

My all time favorite piece of time travel fiction is The Time Traveler's Wife (the book, NOT the movie). It's so intellectually rigorous. You can't have that kind of rigor with many-worlds, because the only consequences for actions are "oh, new timeline now." Boring.

Date: 2012-09-06 01:56 am (UTC)
From: [identity profile]
Does this connect with the "glancing blow" idea? That's where if you throw a ball that goes through a time-like wormhole and hits itself so it doesn't go in in the first place, the universe finds the solution where it hits itself just enough to change its trajectory to hit itself just enough. I kind of like it though how exactly it would work is not obvious.

Date: 2012-09-02 11:57 pm (UTC)
wrog: (wmthumb)
From: [personal profile] wrog
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.
The underlying assumption here is that the you, as observer of the "original" timeline, continue to exist with your original memories/experiences of the world around you intact. It's possible to imagine sensible (*) rewriting regimes in which, if the timeline gets rewritten, you're just gone, just as if you were flash-incinerated by a nuclear bomb; barring metaphysical life-after-death wankery, there's no you there to ever know the difference.

One of the key points of the Schroedinger's Cat scenario that everybody seems to miss is that the box is never actually opened. If at any point you ever open the box, all that means is that the boundary conditions were not what we thought they were (**), the solutions for the wave-function that describes everything are therefore different, and in particular we won't have that particular superposition of cat states anymore.

People focus on the question of whether the cat is dead or alive but miss the bigger picture, i.e., the fundamental premise that the box is sealed and therefore the dead-or-alive status of the cat will never have any observable effect on the rest of the universe. That is the really weird part. Especially in a scenario where, given human nature, it's inevitable someone will eventually break the seal and open the box if you leave it sitting around long enough. You pretty much have to drop the cat-box into a black hole in order for this scenario to make any sense at all.

Moreover this issue long predates Quantum Mechanics, cf. George Berkeley (early 1700s) and his tree falling in the forest that nobody is around to see. Did it happen? What if an avalanche wipes out the forest the following winter so that we ultimately never have any evidence one way or the other. Should we care? Copenhagen School says, "No."

Suffice it to say, the idea that we have to construct billions of extra universes to accomodate the tree-standing vs. tree-fallen-over scenarios and then justify this using Ockham's Razor seems more than a little ironic to me. Especially when you consider that the whole point of the Copenhagen interpretation was to try to get by with the fewest assumptions.

(Meanwhile, there's Berkeley's own answer, which basically goes, "Somebody has to see it, therefore God exists." Good thing there are no unstated assumptions there...)

Which is not to say that you're necessarily wrong (we're basically deep in the realm of metaphysics, here). It's more a question of better understanding what it is you're actually arguing against and not getting sucked into dissecting strawmen.

Meanwhile, you need to read James Hogan's Thrice Upon a Time if you haven't already. It's still one of the best takes on the rewriting timelines paradigm that I've seen. I should also point out this is one of his earlier books (***).

(*) i.e., not like Back to the Future with its slowly disappearing photos and other bullshit, which I will grant you does not make any sense. Likewise for Bill and Ted and Red Dwarf, though at this point we're basically kicking over strawmen that were only ever intended for comedic value in the first place...

(**) one of the really persistent confusions of quantum mechanics is that people talk about the "collapse" of the wave function, the problem being that "collapse" is a verb, which in English makes possible the inference that this is something that happens, as if this is some kind of event in real time, when what's really happening is that you're changing the topic of discussion to a modified experiment that gets different results.

(***) which in his case means better, unfortunately. Among other things he's got a soft spot for pseudo-science (e.g., Velikovsky) that has only gotten worse with age. Don't get me started...
Edited Date: 2012-09-28 12:14 am (UTC)

Date: 2012-09-05 02:46 am (UTC)
From: [identity profile]
There's also the time travel convention where backwards time travel obliterates the original timeline from the reentry-point forward. It basically ends with the same logic of the multi-timeline concept, but a lot less messy.

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