Time Travel and Resolving Paradoxes in Fiction

By
Scott Wagland

I’m sure it’s happened before. You’re watching a movie, or reading a book, and a character travels through time. While there they do something that must surely change the fate of the world, and their own past, and you can’t help thinking to yourself “How could that happen? It doesn’t make sense; surely it’s a paradox?”

So how do you craft the story in such a way that this can’t happen, or at least so that anything that might appear so can be explained away ‘logically’. The easiest way is to make sure that, despite travelling through time, the one place they can’t travel is their own past. Does that make sense? Travelling back in time without travelling into your past? Well, let’s put it like this.

The past is written. It cannot change. But you can create a new past. The time traveller can go back and create a new timeline, but their own past remains unchanged. In a sense the time traveller does not so much travel backwards, but travels forwards in a backwards direction. In the first figure you can see how the time traveller goes back, creating a new history, different from his own. But it’s not his past. His past will always be the black line. He ‘sees’ the black line, the backwards loop, and the red line in his past. Everyone else would essentially see this from side on, as represented by the right hand side of the diagram. Their past would not contain the upper part of the black line, nor the backwards trip. So where does this leave our paradoxes?

The traveller in this method can only change alternate versions of his past, never his own. He could never kill his grandfather of the classic grandfather paradox, he could only kill an alternate version, removing the ‘but if my grandfather never had children I wouldn’t exist to go back in time’ conundrum. To the non-time travellers who only see the timeline of the right hand side, it may appear paradoxical, but to the traveller, who ‘sees’ the whole timeline, there is no problem. Besides the fact of course, that in the alternate world, he has appeared as a grown person with no parents to be found anywhere.

Also note, as a corollary, that you can never encounter something from your ‘future’. Just like how you cannot travel into your own past, no-one else can travel from ‘your’ future back to you. You can, however, encounter someone travelling back from an alternate timeline’s future.

But what of forwards time travel? This is essentially just ceasing to exist for a period. Whether by a machine that propels you ‘forwards’ or ‘removes’ you from time, or by cryogenics, or near light speed travel, the traveller essentially does not interact with the surrounding world. This leads to some of the more common “how did that happen?” questions when not taken into consideration. When travelling forwards to a future time, or returning to the ‘present’ from the past, the time traveller skips the intervening period. They are not there to interact, to age, to alter the course of time (except by their absence). You should not be able to travel forwards and ‘see’ yourself as an older person, since you weren’t there to age. You can’t return to the future with a new ‘past’ where everyone has been interacting with you in the changed timeline – you weren’t there, and you already have your own past.

There is, however, another possibility – there’s another ‘you’. The ‘you’ of the new timeline, that grew up with the changed timeline. Travelling back in time creates an alternate version of yourself, provided you haven’t sufficiently changed the past enough to prevent the alternate you being born (by, say, killing your grandfather). This is not your timeline – it’s his. The traveller is essentially a refugee from their original timeline, not truly belonging. They could take the place of the new them, however. If the events have not changed sufficiently, the new ‘you’ could attempt time travel and disappear. The less morally inclined could ‘dispose’ of them, relying on being genetically identical (if the time changes didn’t interfere with their conception) to bypass questions regarding their lack of knowledge of the new past. This was well demonstrated in “Back to the Future” when Marty returns to the now ‘Lone Pine” mall in time to see his other self travel back in time. He then assumes the identity of his other self, but there was a Marty who grew up with successful parents and then disappeared. If Marty had returned in time to stop the trip, there would have been two versions coexisting in that timeline. (It should be noted, however, that his ‘fading’ while in the past violates these principles, and the notion of being removed from the timeline for the forwards trip completely disappears at the start of the sequel).

Other paradoxes can be explained using multiple trips with this technique. Take for example the problem in “The Terminator” of the soldier who travels back in time being the father of the man who sends him back. There is no way this could ‘logically’ happen – at least the first time around. When we introduce multiple trips, and ‘loops’, the paradox disappears. In the original timeline Sarah O’Conner’s life is never endangered, and she hooks up with random guy. She calls the boy John. War ensues, John sends back Kyle Reese, who then fathers an alternate John, who receives a slightly different upbringing and has different genes, and this John then sends back Kyle Reese, who fathers an alternate John, with identical genes and upbringing (but still an alternate version, not the same person, just like everyone else). The story on screen cannot be the ‘first pass’. The history of Skynet can be explained in a similar way, with the original Skynet being built much later, decades or more, but in the next pass being built earlier with the ‘future’ technology. It would take until at least a ‘third’ pass for the timeline to fit the story.

The biggest problem, however, is having two different time travellers arriving in the same timeline by two different ‘jumps’. Each jump ‘creates’ it’s own timeline. Essentially, whoever jumps ‘first’ would succeed in altering the ‘past’. In some ways whether the timeline continues after someone travels back in time is irrelevant. No movement is possible between the lines, the traveller themselves seeing a continuos flow in their ‘past’, from the end of one line to the beginning of the new one. If the old timeline does continue, the traveller would simply disappear forever, and no change would take place. If anyone ‘jumped’ back to find the first traveller, they would create a new timeline, where they are the only person who has jumped. Multiple people could travel, but only via the one jump. The only advantage to having the original timeline end when the jump occurs is in attempting to salvage the idea of conservation of mass and energy in not creating a whole new universe (although the duplicated mass of the traveller themselves (and machine if applicable) needs to be accounted for), and of having ‘one’ timeline, even if it loops around a lot.

Furthermore, it should be noted that the continual causal flow (maintained by the ‘travelling forwards in a backwards direction’) can seem to disappear. Usually the time traveller themselves is the only one who knows the whole ‘history’. If, however, a different person in the ‘new’ timeline also decides to travel back, the ‘knowledge’ of an old timeline can be lost completely. In the third figure you can see that only the ‘blue’ time traveller has any knowledge of the ‘green’ time. If they were to die without passing on that knowledge, or yet another person goes back in time, the knowledge of the ‘green’ time will be forever lost and unrecoverable. Also, if the ‘red’, the ‘yellow’, and ‘blue’ time travellers were to get together, each would claim to be from a ‘different’ future. Lastly, note that “time traveller” is a generic term, it can be a person, an animal, a machine, or anything capable of changing the past, either by action or merely by it’s existence (physically being there is a change).
As far as the notion of having both people in the past and people in the future/present racing against the same deadline simultaneously…that’s just stupid. Same with the “when I escape from here I’ll go back in time and put this thing here to help me escape” type stories.

In summary, in this model time a traveller cannot change their own past, they can only create another past. Causality always flows forwards, although any observer who does not travel in time is only able to observe their own past, never the alternatives. The timeline will always appear to be consistent and continuous to the traveller, although it may appear disjointed to other observers.

Is time travel possible?

While the above concerns causality, consistency, logic, flow, and consequences of time travel, ultimately it’s fiction. In one sense this author does believe in time travel, just not any form of ‘practical’ time travel.

Forwards time travel? That’s easy. You’re doing it right now. Jumping forwards? Plausible. In a sense, cryogenics is ‘forwards’ time travel (if it actually worked…the current technology leads inevitably to the degradation of neural pathways as well as extreme cell damage. Even if the cell damage could be reversed and life restored, the information that makes up ‘you’, your personality, would be lost). An extremely fast (near light speed) round trip to parts distant could see you age extremely little while decades or centuries pass on Earth. Some outlandish method of ‘freezing time’ or ‘stasis’ could one day be invented, or even a ‘jump’ forwards, ceasing to exist in between. All of these, however, would be one way trips. (And no, you can’t leave 5c in your savings account and emerge rich. Why? Inflation for one. You might end up with billions due to compound interest, but it’ll probably only be worth the equivalent of 5c…maybe 7c; and if it worked, lots of people would do it, and again, your money would be worth less. It’s also a sure bet that a new tax would be invented to seize such funds. After all, what government is going to let that money go, when it could be seized and spent now, and if you want to complain, too bad, that government left office three centuries ago).

Often mentioned is travelling or jumping ‘back’. Back to what? What is ‘back’ there? The atoms that made up the past aren’t there anymore, they’re here, making up the ‘now’. Could you get the atoms, the photons, all of it, back to where it was?

One example I was once given was of a container with a partition down the middle, separating two different gases. When the partition is removed, the gases mix, in a random fashion, bouncing off each other. If you reversed the motion (velocity, momentum) of all the particles, would the gases eventually un-mix, however briefly, after the same amount of time they moved forwards? No, they wouldn’t. Not if you just reversed the velocity. But what if you reversed everything?

Another similar example would be a break on a pool table. If you reversed the velocities would the balls spontaneously form a prefect triangle and spit the white ball back up the table to the baulk line? No. But what if you reversed everything? So that the sound emitted and the internal heating caused by the collisions was turned back into kinetic energy, friction with the felt increased speed, photons flew out of the eyes of the players, bounced of the balls to be absorbed by the lights? The scale quickly becomes apparent – you couldn’t just reverse the balls motion, you’d have to reverse the universe, since everything in the universe interacts with everything else. If everything reversed, then yes, it’s possible that the break would perfectly undo itself, sending the white ball up to hit the cue back in the players hand.

But here is the practical problem. To reverse time, you’d have to reverse everything. Everything. Including yourself. You could not exclude yourself, since you need to be there to ensure everything else flows backwards correctly. With the pool table example, what would happen if you reversed everything, with the exception of one ball removed from the table? The others would no longer ‘un-break’ themselves. By getting the rest of the universe to run backwards with one part exempt, the ‘past’ would change. Or to put it another way, you would arrive at something, but it wouldn’t be the same as your past.

Another thing to consider is the randomness of the universe. In the normal, ‘forwards’ direction, the universe is random at the quantum level. If you were to reverse ‘everything’ (including you), would it all undo itself perfectly to arrive at your ‘past’? Are quantum decisions ‘random’ forwards, and deterministic/causal backwards? Or does the uncertainty and random character act in the backwards direction as well, so even if all physical laws were reversed, the outcome of reversing the flow of time would still differ from what we would have considered the past? For now we’ll assume that the universe is random in the forwards direction, but deterministic backwards (it ‘rewinds’ perfectly), and look at another interesting view of time. One way to explain relativity and time dilation is to imagine a giant clock, with you sitting on the arm as it sweeps around. To measure time you have a trundle wheel. Every time the wheel clicks, one second has passed. If someone else is travelling faster than you (with respect to your frame of reference), or in a deeper gravity well, time slows down. This person would be sitting closer to the axis, and as the hand sweeps around less clicks would be heard in any given period. For example, if you were at the tip and they were halfway in, you would here one of their clicks for every two of yours. The tip of the hand would represent a theoretical object that was at rest and experiencing no gravitational field. The axis would represent the speed of light. The axis does not move, and anything at the axis would record no clicks (and anything with an actual size would overlap this infinitely small point). Moving in or out along the arm represents acceleration or a change in the strength of the gravitational field, and this movement is not instantaneous.

This model simplifies many things and ignores some issues with relativity, but it does illustrate issue with the flow of time quite well. Our observer, sitting on the arm, only has one tool to measure time, the trundle wheel. What the observer cannot know is at what rate the arm is moving. “Time” to the observer is clicks of the wheel, each click is one second, whatever can happen in one second in their frame happens in the interval between clicks. The angular velocity of the arm does not change this. The entire universe exists on that arm along with the observer. To measure the angular velocity of the arm (its revolutions per minute if you prefer that analogy) you’d need to be able to see an external reference point/frame. Outside the universe…

But to an external observer the hand could be going fast, or slow, it could be speeding up and slowing down. Those in the universe would never notice since they can’t see any external reference points, all they know is one click is one second. The hand could even stop -or go backwards. Here we get back to the earlier points. If the universe undoes itself perfectly, we’d never know if it ran backwards. But the universe is random in the forward direction, and if you ran the hand backwards then forwards repeatedly you’d always end back at the same point, but different when it runs forwards (different is a relative term here, from trivial changes over short time spans (would you notice different positions of three molecules in you coffee?) to large changes over longer periods).

Here we see two possible applications then to time travel. One is that you could cause the hand to go backwards for a certain amount of time, and simply hope that random events cause the ‘present’ you’re so disillusioned with to never happen. If not much changes, you’ll invent your machine again and try again (never knowing you’ve done it before). If it changes sufficiently, you’ll never bother creating the machine or using it.

But that relies on ‘kicking’ the hand back a certain amount. A question that arrises is that if your machine reverses the flow of time, wouldn’t the machine unmake itself, or at least undo the process that reversed time, and time would flow forwards again? Where it would be made again, only to unmake itself again, the hand on the clock oscillating back and forth over the same tiny period of time over and over, stuck for eternity. Fortunately here the randomness of quantum physics saves the universe. Some quantum events are incredibly unlikely. The odds, say, of all the protons in the machine decaying simultaneously are ludicrously high, on the order of the happening once in the lifetime of the universe times billions. Other less unlikely things that stop the machine from working could happen as well. The point is that it would replay that period of time over and over until some highly unlikely event becomes probable, and the machine fails to work. To the inventor, the machine simply fizzles and nothing happens. To the external observer, the hand did its crazy dance for however long until it finally overcame its obstruction.

As long as we’re in this universe we’ll only ever perceive time to flow forwards, at a constant rate in our inertial frame.

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