I’m talking about how to make it work in your stories, not how to build a time machine. Apologies if you are looking for engineering schematics.
Time travel has got to be one of the oldest tropes in fantasy and science fiction. Who wouldn’t want to go back and fix a mistake or just observe the past? Trouble is, basic time travel has, as we all know, fundamental paradoxes. What happens if you kill your grandfather or commit one of the endless variations? Yet, there doesn’t seem to be an end to time travel stories in sight, and, why not? They can be fun. Plus for TV shows, there’s the added bonus of getting to use all those cool historical outfits the studios have lying around.
While I usually groan at time travel, I have to confess one of my favorite Star Trek: Next Generation episodes is Yesterday’s Enterprise. And I still get a chuckle out of Back to the Future.
As with Teleporters or Lasers or the like, an author can simply ignore any issues with time travel. However, in this case, if you are too blatant with the impossibilities it is likely to kill your story with editors and agents, not to mention readers. This is one trope where some amount of hand-waving is required. Let’s look through the methods.
First off, I’m just talking about going back in time. Going forward is easy.
Don’t Pollute the Timeline
This is a favorite of Star Trek but shows up in many other places. It’s basically a way to say you are dealing with time travel paradoxes without actually dealing with them, which makes it fine for certain stories but, for me, anyway, not very satisfying.
Why doesn’t it solve the problem? Because there is truly no way to avoid polluting the timeline. Even if you avoided making contact with any sentient beings, what about that fly that changed course, causing the frog to be elsewhere when the snake wanted to eat, causing the snake to search farther afield for food, biting the naturalist causing her to spend a week in the hospital and miss meeting the man with whom she would have mothered the next Einstein? Or leave out organisms, what about changes in gravitational or electromagnetic fields? You might not think that speck of dust floating in space is all that important but maybe it’s trace as a meteor inspires a poet who writes a poem that changes the life of a person who… You get the point. History is a very chaotic system and tiny perturbations can have dramatic effect. Oh, and don’t forget the microbes time travelers will leave behind.
One way to deal with this is to take the approach that the time travel event was always part of how things were meant to be. There was never an earlier timeline where the time traveler did not appear. This sounds weird but Hawkings has noted that, so far, anyway, we don’t have a good physics reason for why time runs forward. And maybe the universe is cyclical: it runs forward to a certain point then runs backward undoing everything in exactly the same way. If we were living on the backward progressing arc, supposedly we wouldn’t even realize it (we would still think as if we were going forward). Or something like that.
This method has neither aesthetic nor intellectual appeal to me. I have to confess it doesn’t even make a lot of sense to me but I’m willing to concede that smarter people than me think it does so there you go. While your story need not posit the backward replay of the universe to make use of this method, it has similar problems: that of freewill. If things can run backwards as well as forwards, then there is nothing unpredictable in the universe. Using destiny to make your time travel work isn’t any different: something must occur, therefore the characters have no choice in what they do.
There are some physicists who think freewill may be an illusion. But do you really want your story to be about characters who can’t actually make any choices? Seems to remove the entire point of a story. If you use this one, you may want to be very sparing with it and be careful of nudging the reader into the conclusion that your characters’ desires are irrelevant to what occurs because if that’s the case, why care about what happens to them?
Time travel involves a violation of known laws of physics. This is not a problem because your time travel story is speculative fiction. But that doesn’t mean we can’t take advantage of other laws of physics: if the time travel is to a place beyond the temporal event horizon from earth, there is no possibility of a paradox, at least for a single time travel event.
As any astronomy buff knows, looking into space is looking back into time. If you look at the sun, you see the sun as it was about 500 seconds ago. Look at a star and you are looking back at least a few years, maybe thousands. Galaxies are millions of years away. So, for something that is X light seconds away, if you travel back in time less than X seconds, anything you do can’t change anything that has happened on Earth. It’s harmless.
For instance, lets say you had a 10-year-old child. Let’s also say you travel back in time 11 years. If you do that on earth, who is to say something you did might not change whether your child was even born, let alone the course of his life? But instead, let’s say you travel 10 years back in time to Gliese 832, which is 16 light years away. Now, even if the first thing you did was construct a planet-destroying death beam and fired it at earth, it would not arrive until after your ‘present.’ There is no way for your time travel to affect your child or anything else on earth, at least in the present. When that death beam pulse finally gets here, it’s another matter.
Of course, you might be wondering, what good is time travel if it can’t actually affect the past? Well, obviously, this method doesn’t allow you to change events in your local space. But it could still make an interesting story. Imagine if humans are losing a war to an alien race in 2113 but they have the ability to do this sort of time travel. They send humans back in time 10,000 years to several habitable planets 10,000 light years away with the hope that some of them proceed to the point where they can arrive at the home-world in 2113 with the technology necessary to save the world. Kind of a deus ex machina ending but there’s a story there. Or maybe there is no intent for the time traveling humans to save the present, instead, this is an ark-in-time: humanity dies on Earth but lives on, propagating through time and space.
This falls apart if your time travelers turn around and go back in time to the originating world. However, just like it takes infinite energy to go the speed of light, maybe it takes infinite energy to violate the laws of temporal observability: you can try but it won’t work. It’s hard for me to imagine a mechanism that would actually cause that to be but (a) I’m not a cosmologist and (b) this might be obscure enough that this method might satisfy most readers.
Another way to deal with time travel is to say that each time travel event “clones” a universe: when you travel back in time, you create a duplicate of the universe at the time you arrive. Anything you do occurs in a separate universe. You can still kill your grandfather but only the grandfather in this duplicate universe, not the one who sired your father who then made you. No paradox.
Of course, universes do tend to contain a fair amount of energy, as in mind-bogglingly vast amounts. But according to current theory, the universe sprang from perturbations in a vacuum so I am not sure this is a really a problem. The energy would come from outside the universe, maybe it doesn’t really matter in the grand scheme of things. Or maybe it’s the old “how many angels can dance on the head of a pin” conundrum. The answer in the middle ages was an infinite number because god could put an angel down on half of the pin, make a half-sized angel, put it down in a quarter of the remaining space, make a quarter size angel, put it down in an eighth of the remaining space, and so on without end. Maybe when you duplicate a universe through time travel you actually make two halves, like a fertilized egg dividing: same mass but now two half-sized cells. To someone in the smaller universe, things might still look the same.
The main issues with this approach for your readers is how to make the universe cloning seem plausible. Some may also wonder just what can cause a cloning event. For instance, does it have to be a sentient being, because if physical processes can cause it there would be a near infinite and ever-increasing number of universes. Even if it has to be a sentient being, you might soon have a vast number of clones out there. Of course, any person in any given clone would only be aware of his or her universe. And maybe the plethora of many universes provides a setting in itself: your characters might hop between possibilities. While they could not return to their own universe in the past, nothing says they couldn’t go from universe A to B and back to A, as long as they arrived in A later than their departure.
As a side note, you could use this for the issue of freewill instead of time travel: maybe every decision results in a universe fork. While it is true that all possible decisions exist in the set of all universes, any given individual is experiencing the results of his decisions. Not sure it really solves the freewill problem but it’s an interesting thought.
Quantum mechanics tells us that an electron in a hydrogen atom isn’t orbiting the proton like a planet around a sun. It’s actually in a probability cloud, sort of smeared out in time and space. I think QM would say even Jupiter is in a probably cloud in its orbit around the sun, just that given the mass of the sun and Jupiter, and the distances involved, the probability envelope is really tiny: i.e., for all practical purposes it inhabits a point in a space.
What if time travel works this way? Any given time travel event adds to a cloud of timelines. What happens in the now is the sum of timelines in the past. You can go back and kill your grandfather but that will either not really happen in the present because that event is improbable or you will change the now so that you aren’t present.
Maybe to change the past, you have to go back multiple times and nudges events so that the probability sum results in your desired outcome, or at least a different outcome: this sort of time travel seems rife with unintended consequences. This is actually at the core of my next short story (see, I take my own advice to heart. While I find most time travel stories paradoxical, how would I make this impossibility possible? I’m going with superposition.)
Where in the Past Art Thou?
That’s it for ways I can think of to make time travel work but before I wrap, let’s touch on the where of time travel. Let’s say you do find a way to travel back in time. Where do you go? Not only is everything moving in space, it is moving relative to other things. There isn’t a universal coordinate system. If you want to travel back to Earth one hundred years ago, how do you specify where the earth was then, both in its relationship to the sun (and remember our orbit is not perfect, it does vary due to interactions with the other planets, even with passing asteroids) and the sun’s position in the galaxy and the galaxy’s position in the local cluster, etc., etc. Not only would it be very difficult to compute where the earth was with respect to where it is now, how does your time machine spit you out in the intended spot, and at a velocity that allows you to interact with the Earth (rather than, say, impact it at relativistic speeds?)
Let’s say you can do all that: you can put your time traveler in the exact spot and with the precise velocity vector to avoid smacking into something. Do you know where the surface of the Earth was 100 years ago? Things move, not just down but up. Continents drift: 100 years ago America was about 8 feet closer to Europe. Trees grow. It would suck to appear in the middle of an oak tree, wouldn’t it? For all those reasons, time travel might be best executed in a spaceship with enough delta-v to match the destination.
It’s all handwaving
While the Observability Horizon and the Universe Forking methods avoid time travel paradoxes, they have their own problems. In the end, all of these methods simply constitute more obscure ways to explain time travel. Even so, they may satisfy many readers’ suspension of disbelief better than “avoid polluting the timeline”. And for those it doesn’t satisfy, you might get points for trying or at least amuse them as they think through the puzzle you posed.
- Very nice article on time travel on Wikipedia (http://en.wikipedia.org/wiki/Time_travel) I must confess I didn’t think to look here before writing this but lots more ideas here.
- Time Travel Is Possible But Only To The Future, English Physicist Says (huffingtonpost.com)
- Is Time Travel Possible? (arunbabyveranakunnel.wordpress.com)
- The myth of time travel. (berdie75.wordpress.com)