Disclaimer: I am not a theoretical physicist, nor to I portray one in any media.
Time travel as a concept in fiction has been around since 1770 (L’An 2440, rêve s’il en fût jamais by Louis-Sébastien Mercier) but there are instances of time travel in the Mahabharata, and some Buddhist and Japanese tales . Fairies, particularly Irish fairies, are fond of abducting beautiful human youths and after what seems like a relatively short time, a few years, say, they return to the human world, only to discover 300 years have passed and everyone they knew and loved is dead .
In this past year, time travel has made a resurgence in popular media, with no less than three new series created around it: Timeless, Travelers, and Time After Time. DC has been playing with time travel with their series Legends of Tomorrow (in which a time traveler recruits secondary characters from several other DC series to correct historical “aberrations”) and The Flash (who’s really messed up things by traveling into the past to change it).
Science fiction authors return to the concept again and again. Admit it, it’s fascinating. I think we’ve all, at some point, wondered what we’d do if we could have a do-over. Science fiction, as I defined it a couple of columns ago, is built around science. So … what’s the science behind time travel and how do we get it “right”? Hang on to your brain pans, folks.
Down the rabbit hole (or, is that wormhole? Black hole?)
Here’s the thing: there are a number of theories about how time travel might be possible, but right now that’s all they are, theories. A theory is something that has not been tested, reproduced, vetted by scientific peers, and proven. Can there even be a “science” of time travel in its theoretical frame?
Some of the best and brightest scientific minds of our age, like Stephen Hawking, take the position that time travel is impossible.
I have to concede the point, but, as an author of speculative fiction, I prefer to think that time travel is currently impossible, with our present understanding of science and the physical laws of our universe. But, it may not always be so.
Before we proceed, I will also trot out the words of H.G. Wells, author of The Time Machine. In the 1934 preface of Seven Famous Novels (Knopf), Wells writes, “These stories of mine collected here do not pretend to deal with possible things; they are exercises of the imagination …. They are all fantasies, they do not aim to project a serious possibility; they aim indeed only at the same amount of conviction as one gets in a good gripping dream.” 
So one solution, out of the gate, is to realize that, lacking a science of time travel, one is technically able to write whatever one wants. Best, if that’s the case, though, to call it fantasy.
The problem of paradox
There’s this thing, for the sake of equality, let’s call it the grandparent paradox. A young time traveler goes back in time and gets into a fight with someone. The fight becomes violent and, in self-defence, the time traveler kills the person in the past. That person from the past turns out to be the time traveler’s grandparent.
Here’s the paradox: having killed their grandparent, the time traveler can never be born. If they’re never born, they can never go back and kill their grandparent. So what happens?
A related, but distinct, concept is the closed time loop. A time traveler visits their younger self and gives them the secret of time travel so that they can have grand temporal adventures, but where did the secret of time travel come from? Who discovered or created it? The time traveler gave it to themself …
These concepts can be played with in fiction to some success. Looper features a plot in which the protagonist is assigned to kill his older self. Just be careful not to write your protagonist into a paradoxical trap.
The theoretical rules of time travel
The problems posed by time travel need to be answered by a strict set of rules. Here are some of the solutions prompted by the problem of paradox.
The possibility of paradox (changing something in the past that irrevocably changes the future) means that time will protect itself. The past is the past. It has already happened. It cannot be changed.
Time travel into the past may, in fact, be impossible for this reason. Some theorists posit that time travel into the past might be limited by the time traveler’s lifetime, or the creation of the time travel technology.
If a time traveler could make it into the past, they may be limited to observation only . Alternately, any action the time traveler takes is already part of the past and they can do nothing else. Again, some theorists say that the time traveler may be prohibited from changing the past, but they could influence it. How, exactly, the past could be influenced is vague, however.
It’s a matter of energy and size
Oh, yeah. Paradox isn’t the only time travel problem we have to account for. Of the possible theories of time travel, another impossibility arises from the astronomical amounts of energy needed to power such travel. Care to consume a star or a whole galaxy so you can pop back in time and find out exactly what your great-granddad was doing during prohibition?
And how would we accomplish that? Time travel might be possible at faster-than-light speeds (yes, Star Trek was onto something), but that, too, is beyond our current technological capability.
In 1974, Frank Tipler theorized that an infinitely long, very dense cylinder rotating at half the speed of light would allow a closed timelike path to connect many otherwise discrete events in space-time . Let me repeat, infinitely long. How would it be possible to build such a thing, let alone power it?
Time travel might also be accomplished by taking advantage of the gravitational effects of a rotating black hole, or Kerr hole . Now if we could just reach one …
How about a wormhole? Those are so unstable, even in theory, that entering the wormhole would probably collapse it. If we could find one, or create one, in the first place.
What does that leave us with?
The many worlds interpretation  may help. This theory posits the existence of potentially infinite alternate worlds or universes in which everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes.
Therefore traveling into the past, or future, of another world or universe would not result in a paradox. You’d have to be careful which alternate universe you visit, however. Consider a world in which cephalopods evolved to became the dominant intelligent species on our planet. Again, we’re left with the problematic mechanics of how to get there.
Superstring Theory may have an answer. That theory suggests as many as ten dimensions. Here’s an article from Universe Today that explains how the extra six dimensions might allow us to perceive alternate universes.
Then, there’s the holographic universe, theory , which posits that our universe is a consciousness hologram. If we could get a handle on our consciousness, we could accomplish the impossible. Inception, anyone?
In the end, though research is fascinating, it is possible to go down the rabbit hole and get lost. My suggestion is to pick the theory that works best for your story and don’t sweat the details. Seriously, don’t go there.
Because time travel is theoretical at best, don’t get bogged down trying to explain how you’ve made the impossible possible. If you look at The Time Traveler’s Wife, The Fifteen Lives is Harry August, Doctor Who, or any of your favorite time travel stories, the writers never explain how time travel works. They create rules and a consistent and plausible world in which the story plays out, but the time travel works—because it works.
Readers of science fiction are, in some ways, primed to suspend their disbelief. The secret to writing a great time travel tale is in the world building and in creating a complex story with engaging characters readers will root for.
Melanie Marttila creates worlds from whole cloth. She’s a dreamsinger, an ink alchemist, and an unabashed learning mutt. Her speculative short fiction has appeared in Bastion Science Fiction Magazine, On Spec Magazine, and Sudbury Ink. She lives and writes in Sudbury, Ontario, Canada, where she spends her days working as a corporate trainer. She blogs at http://www.melaniemarttila.ca and you can find her on Facebook and Twitter.
 Nahin, Paul J., Time Travel: A writer’s guide to the real science of plausible time travel. Writer’s Digest Books, Cincinnati, OH, 1997. pp 6-7.
 Commonly known as the grandfather paradox. Google it if you don’t believe me.
 Travelers plays with this concept, as does X-Men: Days of Future Past.
 Nahin, Paul J., Time Travel: A writer’s guide to the real science of plausible time travel. Writer’s Digest Books, Cincinnati, OH, 1997. p 131.
 Nahin, Paul J., Time Travel: A writer’s guide to the real science of plausible time travel. Writer’s Digest Books, Cincinnati, OH, 1997. p 130.