originally published November 4, 2013

So Marty McFly shows up back in 1985, the timeline restored and the set-up in place for a whacky epilogue. His parents are now confident and healthy, his brother has an office job and his sister no longer dresses in thrift-store rejects. But wait… why does his brother still live at home? Why does the family end up in the same banal future slum-house when clearly their very beings have been existing in an improved state for the past three decades? And why does it seem like George Mcfly was going grey when he still had the slick-black Brylcreem look in the original timeline?

Unfortunately, every time travel story seems to end up splatting paradox juice all over the walls upon closer analysis. And while generations of brilliant minds nevertheless attempt to rationalize the possibility of temporal jet-setting, we are still shoulders-deep in what-ifs. And despite our fantasies of returning to high school and telling our younger selves not to ask out that hell-wrought shrew that messed up the last part of our senior year, it just ain’t gonna happen.

Besides, there are more serious implications to consider. Time travel is not for the soft-hearted or for those prone to spiraling headaches when confronted with circular trains of thought that derail into themselves. Before you strap yourself into that DeLorean you’d best prepare yourself for the implications of the Grandfather Paradox.

This conundrum of time travel is fairly simple to understand: if you were to travel back in time and murder your grandfather before he had children, what would happen? Simple – you would have never been born. But then you wouldn’t have travelled back and murdered him. Therefore you would have been born. And you would have travelled back to murder him. And so on, until your brain explodes.

Another flavor of this paradox is known as the Hitler Paradox, since one of the most common what-ifs of time travel postulation is the notion of someone cruising back to pre-1933 Germany and slitting Adolph Hitler’s throat before he has a chance to elbow his way into power and mess things up. This would radically change history, which would remove any reason for anyone to travel back in time and commit this act, so no one would have done it.

Also, someone who would have otherwise died in World War II might end up killing your grandfather before he has kids. So all the paradoxes can just link hands and kick our sandcastle into dust.

A Soviet physicist named Igor Dmitriyevich Novikov put his brain to this paradox in the mid-80’s and came up with the Novikov Self-Consistency Principle. This states that anything a time traveler did in the past remains consistent with history. If you show up and plug a few bullets into the kid living in your childhood home, it’ll turn out to have been a different kid, not the younger you. Any event that would actually create a paradox has a probability of zero – it just can’t happen.

If any time travelers showed up on the Titanic, they clearly couldn’t stop it from sinking. Maybe they could save a few people, but only if doing so would not lead to a chain of events that would negate the invention of the time machine, or negate the travelers from climbing in. In other words, the universe won’t let it happen. History is history.

The Huggins Displacement Theory is another universal slap-down on any would-be Hitler-butchers. According to this idea, if you travel back in time you must travel an equivalent amount through space. You go back one year, you’ll be bumped one light year away, which makes the prospect rather unpleasant, especially if your time machine is a convertible. But in theory this should make it impossible for you to get where you need to be to muck up history.

I don’t care for this one. It suffocates any narrative value in a time-travel story. Even if they find it to be true (it is, after all, based on notions of relativity and “light-cone” travel), where’s the fun in it?

According to the Nonexistence Theory, a jaunt back in time to mess up your own existence would simply result in you no longer existing. Kill off your grandpa then hop back in your time machine, and suddenly Bedford Falls becomes Pottersville. You don’t exist, nobody knows who you are or who you were, and it’ll take a lot more than wishing and weeping to make Zuzu’s petals reappear in your pocket.

The Restricted Action Resolution plays along the same strings as the Novikov Principle – it states that if you try to mess up the past, misfortune and pure fluke will step in and steer you off-course. Point a gun at your young grandfather? It’ll jam, or you’ll miss, or an anvil will drop on your head. Show up on a plane to shut down 9/11? As you move to disarm the terrorists you’ll end up tripping over some kid’s poorly-stashed carry-on bag. These notions seem to depend upon a divine being slapping your hand away from the doomsday button, or else a universe with a kick-ass immunity system.

There’s the theory of parallel universes. This one is the snazziest of all the theories because it will let you cruise back in time, kill off Hitler and suffer no dire consequences to yourself or your time machine. You just landed in a parallel universe, of which there might be an infinite amount. Hitler’s gone? Great! Scoot ahead sixty years and talk Shelly Long into not leaving Cheers; let’s see what happens then! Consequence-free timeline-mucking. That’s my kind of theory.

Then there’s the somewhat less optimistic Destruction Resolution theory. Fans of Star Trek: The Next Generation will recall this from the series’ magnificent final episode. If you cause a paradox that makes the universe scratch its head, then none of these theories will apply and existence itself will get wiped clean. Kind of makes you want to stay home and quit playing with Flux Capacitors, doesn’t it?

So this paradox will stay sticky and unresolved. No amount of poring through possibility, through math and physics or through every piece of sci-fi ever written, shot or set to salsa music is going to give us a legitimate answer. Even then, other questions will resurface. Like if Marty disrupted his future by messing with his parents’ first meeting, why didn’t he disappear right away? Why the gradual fade? And what if he’d vanished completely during Marvin Berry’s cover of “Earth Angel”? If he’d never existed, then who the hell pushed George out of the way of that car?

It’s all so confusing. Time for a Tylenol, methinks.

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