If you're familiar at all with the concept of time travel, then chances are good that you're also familiar with the concept of the temporal paradox. This is where a time travel scenario leads to a situation which violates the rules of logic and the laws of causality. In spite of what you may have read in books, seen in movies, or been told by a certain goofy, bearded blogger in his last post, the problem with these paradoxes is not that a time traveler might inadvertently cause one, threatening the integrity of the space-time continuum or the existence of the universe. By its very definition, it's impossible for a temporal paradox to happen. There's about as much danger of you causing a paradox, as there is of you eating your own head. It just can't happen. Paradoxes do pose a legitimate problem for time travel, however. As long as it can be demonstrated in any conceivable way that time travel could potentially lead to a paradox, then either the issue has to be resolved, or you're forced to reach the inescapable conclusion that time travel is impossible. The problem is solved at the source. It would be like someone trying to build a house from an M.C. Escher drawing. The problem isn't that the staircases that loop back on themselves or the support beams that connect this corner of the floor to the far corner of the ceiling would destroy the universe if you built the house. The problem is simply that the house can't be built. So, before you go packing your extra plutonium, these paradoxes have to be addressed.
There are three basic types of temporal paradoxes: the predestination paradox, the ontological paradox, and the grandfather paradox. As the title suggests, it's primarily the grandfather paradox that I want to deal with in this post, because it's based on a very common misconception about time travel. Once the misconception is laid to rest, it should take the grandfather paradox into the ground with it, leaving no ghost to haunt us. Unfortunately, it's the resolution of the grandfather paradox that makes the other two paradoxes an issue. Those two are a little trickier to deal with, and quite honestly, at this point I'm not sure that they can be resolved. We'll just have to see where we're at when we get to them.
So anyhow, the grandfather paradox is the basically the idea that a time traveler changes the past in such a way that it negates the cause of the change. The most famous example, and the one from which the paradox gets its name, is a scenario where a time traveler goes back in time and kills their own grandfather when he's a baby. I'm not sure why someone would ever want to do this, but hypothetically, if they did, it would mean that the time traveler had just canceled out his own existence since without his grandfather he never would have been born. This would mean that he would never have been around to kill his grandfather, which means he couldn't have canceled out his own existence, which means that he would have been around to kill his grandfather, which means...Well, I'm sure you can appreciate the can of worms this opens. Although this grandfather murdering scenario is one of the more drastic versions of the paradox, it's hardly the only one. Once you introduce the concept of changing the past, it becomes practically inevitable. One could even make the argument that any successful journey to the past with the deliberate intention of changing anything at all, would automatically result in a grandfather paradox. After all, why make that trip back in time to stop Kennedy from being shot if he's already alive and well this time around, thanks to you?
Some people have tried to solve this paradox by postulating the existence of separate parallel universes for every possible outcome of every event since the beginning of time. Then they cast our grandfather murdering scenario with a couple of different actors from different universes to play the part of our time traveler and voila', paradox solved. Yeah, nice try. Not only does this messy idea have more holes in it than a slice of Swiss cheese, but the actual solution is so much simpler, and we don't need to resort to an absurd multiplicity of countless universes for all the infinite places where I could have set my glass on a table or all the infinite ways I could have put my socks on this morning. To be consistent, every slightest fraction of a micrometer difference would have to be accounted for by these continuously spawning universes. Luckily, we can ditch all of this tedious redundancy once and for all. These parallel universes were brought into the picture to solve a problem that doesn't exist, because, you see, it would be impossible for a time traveler to change the past.
Now, I'm certainly not the first person to realize this. I'm just here to explain it, so buckle up. The first thing that you need to understand is for a time traveler to change even as much as the position of a blade of grass in the past, it would be necessary to assume the existence of two separate time lines: one in which the traveler visited the past, and one in which he did not; one in which he touched the blade of grass, and one in which he did not. This concept is usually presented to us in such a way that the problem with it escapes our attention. First, we have history running its natural course. The cavemen draw some bison on a wall...fast forward...Julius Caesar rules over Rome...fast forward...Benjamin Franklin flies his kite...fast forward...Hitler kills a lot of people...fast forward...We land on the moon, etc. Then one day Dr. Stumblebum builds a time machine in his basement, and he decides to go mucking around in history. He knocks into Ben Franklin, ruining the whole moment with the lightening and the key and forever changing one of our treasured moments of history. See, did you catch the problem? Probably not.
Again, we have two versions of the time line. We have the original version, where Franklin flies his kite in peace. Then we have the "new" version where Dr. Stumblebum shows up and ruins the whole thing. Here's the question, though: When was this new version created? Well, that's easy enough to answer, right? It was created the day Dr. Stumblebum decided to load his clumsy ass into his time machine and travel to the past. It makes sense, or at least it seems to as long as we keep looking at everything from Dr. Stumblebum's point of view.
Let's try instead to look at from Ben Franklin's point of view. It's a cloudy day, somewhere in the middle of the 18th century. Storm clouds are rolling in, and Ben decides this would be a great time to fly his kite. From his point of view, the day is today. What else would he ever consider it? Anyway, he grabs his kite and heads out. Lightening hits the key, Ben craps his pants, and all is right with the world. But, wait a minute? Where was Dr. Stumblebum? Well, that was a different time line, and it doesn't get created for another 200 years or so. Oh, okay then. Wait, hold on. I've got another question. Exactly when is that day he flew his kite ever going to be 200 years in the past from Franklin's point of view? I guess Dr. Stumblebum can't show up from the future until the future happens. When's that going to be? I don't know, but I have a feeling Ben's going to be waiting for a long time.
Still not seeing it, huh? What does Ben Franklin know, right? He's too stupid to even know that it's really 2011 and he's been dead for hundreds of years. So, let's try a different example. Let's say that there's a knock at your door right now. You can go get it. I'll wait. You open the door and a guy in a shiny silver jumpsuit gives you the Vulcan hand sign and tells you he's from the year 2598. What the hell? I guess that must mean it's really 2598, because this new time line couldn't have existed until then, right? But then, where were you when it was really 2011? How did you miss it? Did you oversleep? I tell you, you have one extra White Russian and you wake up and find that it's six hundred years in the future and you're living six hundred years in the past. Ain't that a bitch?
If all this seems just a little too confusing, don't worry. It's not supposed to make the slightest bit of sense. That's the point. You see, once you put time travel on the table, the "present" is no longer a privileged, objective position. It becomes entirely a matter of perspective. To Ben Franklin, the day he flew his kite is the present. To you, as you're reading this and rubbing your temples it's the present. To our silver jumpsuit guy, 2598 is the present. None of us are in a position to prove the others wrong. So, when the silver jumpsuit guy shows up at your door, it's not a new version of today, six hundred years in the making. It's just the same old today it always was and everything's going to turn out the same way it always did. On Feb. 2, 2011 a man from the 26th century showed up at your door. It may be ancient history to him. It may be the present for you. It may be the future for Ben Franklin. But no matter who's point of view you look at it from, it's still the one and only version of this day that it's ever going to be.
There is no new time line. There is only one time line, and it happened the same way it always did, and it can not be changed. If Dr. Stumblebum gets into his time machine and travels back to the day when Benjamin Franklin flew his kite, then he was always there that day. He can't ruin Franklin's experiment, because he didn't ruin it. The kite flew just like it was supposed to, and we all read about it when we were kids. For some kind of change to take place in the time line, it would have to happen from some point in time that the universe itself considers the present, and I get the feeling that the universe is staying out of this one.
So does that mean that this isn't really, really the present? Not necessarily, and in fact, we'll be considering that very thing in the next post. For the moment, what this means is that once you grant the premise that someone can move around in time, backwards and forwards, then time has to be viewed as a line that one can travel on, and the position that we call the present is only one possible point on that line. If you can land in the 18th century and meet a man that thinks the date is today, or land in the 26th century and meet another man that thinks that date is today, then the mere fact that you're the one with the time machine hardly puts you in a position to argue with them. When in Rome, it's today in Rome. As for your grandfather, you can't kill him because you didn't kill him. The day you tried only happened once, and you clearly failed. If you can figure out when you'll ever be able to make a new version of that day, be sure to let me know.
(This post is also available in extra cheesy version.)
