The Grandfather Paradox Explained
Let's say you go back in time and for some stupid reason decide to murder your grandfather before he has a chance to impregnate your grandmother with your mom. No biggie, right? So you cease to exist, and all's good. Well, if you no longer exist, then who the hell is the murderer that went back in time and killed him in the first place? We have a paradox.
In a story about time travel, you REALLY DO need to deal with this, because it is paradox, and any time you have someone travelling back in time for any reason at all, I'm going to be asking what would happen if they killed one of their ancestors? Even if it's by accident. Google defines a paradox as thus:
A statement or proposition that, despite sound (or apparently sound) reasoning from acceptable premises, leads to a conclusion that seems senseless, logically unacceptable, or self-contradictory.Time travel opens that can of worms all the time, every time, no matter how you tell it. Okay, so you have a few options in how you're going to explain this one away. This is also a list—not by any means exhaustive—of how most authors tend to deal with it.
Don't Deal With It At All
This is by far the favorite method. You have time travel, your characters can go back in time and change things, and they change the past and yet somehow everything in the story's okay. Don't mention the paradox, and plenty of your readers won't even realize the logical self-contradiction you've introduced. This is acceptable for time travel romances and action adventure stories where people are travelling to ancient times for the ambiance and such, where no one cares about the logic of it all, but if anybody has to "fix" something in the past, the more curious readers will be frustrated. Because let's say John went back in time and "fixed" the broken past, then returned to the present. Now, in this different version of reality, he had no reason to go back in time and fix anything in the first place, so presumably it doesn't get fixed, in which case he would have had to go back and fix it after all, but then he wouldn't because he would have in fact fixed it, ad infinitum. It's a paradox and you can't escape it. When you don't deal with the paradox at all, there's a gaping hole in your time travel story. So, let's look at some—slightly—better ways of addressing the paradox than just sticking your head in the sand.
Create A Self-Consistent Loop
Harry Potter, book 3. Yeah, you remember. It was clever how everything tied up in the end. Harry and Hermione were able to travel back in time and save two lives, without ever letting their past selves know they had saved the lives, so their past selves reached the present and made the perfectly logical choice of going back in time to save two lives, even though their future selves had already saved those lives. If you'll remember from the movie (I don't remember it from the book) Hermione even had to toss a rock at herself to keep things consistent. But here too, the logic falls apart. What would have happened if they were sloppy and their past selves saw them? Their past selves would then do something different, and theh all the circumstances leading up to their future selves would have been different, presumably leading their future selves to not be so careless...but then we're back with the old paradox. Well, you might say, they weren't so careless. Yes, but they COULD have been careless. Shoot, what's to stop someone from CHOOSING to be careless just to test the paradox? I mean, I would. Just to see what would happen. No matter how this works out, the past and present can never be truly self-consistent in a broader sense because a character can always deliberately screw things up. We don't fault J.K. Rowling for this, because she's J.K. Rowling and Harry Potter and Hermione would NEVER deliberately screw things up. But this brings up another common answer to the paradox.
The No Free Will Version
You know how this one works, right? Even though we have the illusion of free will, the past, present, and future is actually already set in stone. So even though we travel back in time, that travelling back in time has already been accounted for in the past because the universe "knew" we were going to do it. Maybe we try to change something and then it turns out all along that it's because we changed that thing that the present is as it is today. This one might be even more flimsy than the don't deal with it at all method. Here's why: you can argue we don't have free will, but only as long as we can't see the future. If we can see the future, then we're automatically given the choice to disobey that future. Here's an example. If I know it's my destiny to call so-and-so at exactly 9:27 PM, surely I can resist calling until 9:28, even if just out of spite? Plenty of people would test that, believe me. Now, if there's some kind of cosmic force that urges my hand toward the phone at exactly 9:27, then you've got something else to explain, and "no free will" isn't good enough. Now you've got a cosmic force that forces people to obey their destinies. Which could work, but you do have to add that part in. Then again, you can always get by with...
Go So Far Back It Doesn't Matter
Authors do this by sending you back thousands or even tens of thousands of years. No grandfather paradox because your grandfather didn't exist yet, right? Wrong. Now you've got a whole bunch of ancestors you could murder. But what if you have...
Exceedingly Careful Characters
These are the guys that travel back in time and say stuff like, "Don't change anything," or "Touch as little as possible." It's a fun as hell concept, but it doesn't work that well because of the butterfly effect.
The Butterfly Effect
If you've seen the movie with Ashton Kutcher, you might know what this is. If you haven't seen the movie, or you didn't get the movie, here's what the butterfly effect is: in a nonlinear system, small changes will accumulate and become vastly larger changes down the road. What's a nonlinear system? The universe. Here's an example: I met my fiancee at a college party, got only her name, and left the party with no intention of ever seeing her again. A week later, I was at another party and wasn't really feeling it and decided to go to a different party. I walked into the second party just as she was walking out. We exchanged contact info, the rest is history. I've been with her almost six years. Can you imagine how vastly different the last six years of my life would be if I had gotten to that second party just one minute later? In other words, one minute, maybe even one second, shaped the rest of my life. You might think this is cherry picking an example, and it is. You're right. But these kinds of time moments are happening all the time, and if you change just one of them, things will turn out very different for EVERYBODY. Oh, and the farther into the past you go to change something, the greater the effect is going to be. Travel back in time a day, and you might be able to hide in a closet and not change much. Travel back a year, and today's headlines won't even match anymore. Travel back to the birth of Christ, and the USA never comes to be. Travel back to the dinosaurs, and humans never evolve. Get it? If you change ANYTHING, it's over.
Only The Main Character Remembers
This is a technique used by a lot of authors, and it's kind of necessary for a lot of stories. Your hero goes back in time, changes something, then comes back to the present. Everyone else seems to be different, but he's the same. Somehow he still remembers the "old" past, while everyone else got the memo about the "new" past. Back to the Future had a version of this. It's a fun scenario, but in no way is it logical. In fact, it's so illogical you can't even really talk about it in terms of a paradox. So I guess you dodged that bullet. Well, if we're going to talk about it, I guess it does kind of make sense. The main character's past is retained, so him changing the past is also retained, but then you run into the issue of why doesn't he know anything about this new present? To those around him, he would appear to have suddenly lost his memory, and there would have been no causal event, because they didn't see him travel back in time and change things. In their version, no one traveled back in time. Yet in his version, he did, and you've got a whole new slew of logical inconsistencies. Paradox not solved.
Okay, So What Should You Do?
As you can see, we've gone over quite a few good methods already employed by the best authors and screenwriters out there, and they've all done an inadequate job of addressing the paradox. There are a few others that don't bear mentioning, like the time tends to correct itself camp, and other equally wonky explanations. But all this begs a question...can you address the grandfather paradox in a way that makes logical sense? Or do you have to skirt the issue? I think there are good methods out there for dealing with the paradox, and they're outlined below:
You Can Look But You Can't Touch
It's kind of like the watered down version of time travel, where you can go back, but no one can see you. Or maybe you can only look. This kind of time travel works, and there's no paradox, because you can't send any information back in time to change it. The Light of Other Days is a sci-fi novel with this kind of time travel. But it's also no fun.
We're starting to get more clever now. Michael Crichton did this in Timeline, but then blew it. I love him no less because of that. The idea is there's an infinite number of universes at different stages of time, and you can travel to another universe that is identical to our universe just days/months/years in the past. This one works also, because you can change stuff, and it WILL change the future of THAT universe, but not the universe you're from. So no paradox. Incidentally, in Timeline, a man traveled to a different universe that was a few hundred years behind ours, dropped his glasses in the sand, and then we dug them up in an archaeological dig in our universe. It's not supposed to happen like that with parallel universes. That's the tiny little part that Crichton messed up, but it was an essential suspense-builder in the beginning.
The Duplicate Past
So far the best I've seen, and also a fun one. Guy goes back in time, hangs out a little bit, accidentally kills himself...but nothing happens. He still remembers his version, and his past version, well, is dead. This one's similar to the only the main character knows method. But here's where it gets fun. You go back in time, talk to yourself, and let's say they change their mind about travelling back in time. Since you still have your past where you did travel back in time, nothing happens to you, but they don't step inside the time machine when their time comes, so now there's two of you. Oops. It's kind of like every time you travel in time, you make a duplicate of the past that you can change, but you can never get back to the one you left, even though you remember it. Watch the movie Primer for an example of this kind of time travel.
The Feedback Method
We're getting tired of all these wishy-washy ignore the issue solutions, so let's just take the paradox head on, grab the bull by the horns. Let's look at what's actually going on. Someone's travelling back in time and changing stuff, which, if it's a good time travel story, should have ripple effects that propagate instantly to the future and reflect itself in the main character, in a kind of feedback loop. If we really think about how this might work, it's pretty easy to play this one out. As I'm changing things, my past is changing. What if I decided to kill my past self? I'd never get there. I might make it so far as my house, but my reality would be constantly reshuffling. Besides being crazy disorienting, I might not even physically be able to take a swing at him. It would settle into different realities but not others, depending on which ones were self-consistent. You might not even be aware of not being able to do certain things, because the moment you took a step in the wrong direction, you would have altered something that calculated out to a different scenario. Here you're not trying to explain away the paradox at all, just let it run it's course. It's a lot to think about, though, and it's easy to get wrong.
The Quantum Physics Limitation Method
This is where stuff gets a bit heavy. You've got a pretty good understanding of quantum physics—and you know it's already rife with paradoxes anyway—so you craft a quantum physics-esque explanation that handles the paradox. Most readers won't understand it, but you can rest at night knowing you honored Neils Bohr and Erwin Schrodinger. Here's how a rough line of thinking might go as an example: According to the Heisenberg Uncertainty Principle, there's a limit to the amount of knowledge that you can pass back in time—and the accuracy of actions taken in the past (the farther back you go, the more inaccurate they are)—and it just so happens that this limit disallows paradoxes. You've got a time machine, and you can use it all day, but the uncertainty principle guarantees that nothing you do will ever constitute a paradox. Yeah, good luck with that one. Still, it'll be a hit with hard sci-fi readers.
Grab what you like from each method and put together a special case. Maybe a duplicate past scenario with a little feedback. Or a parallel universes version with an exceedingly careful character. Or a no free will version with a quantum physics explanation. The rules are really nonexistent here, because you can make your reality however you want. In dealing with the grandfather paradox, what's most important is not how you address it, but that you do in fact address it in a way that's satisfying. The worst, and I mean WORST thing you can do, is ignore the paradox altogether.
With all that said, you've probably guessed by now that I'm working on a time travel novel of my own. If you want to see how I handle the grandfather paradox myself, subscribe to my newsletter by entering your email address below and you'll get a message when it comes out:)