Space travel is one of the staples of science fiction. Characters move from one planet to another. They set out into the starry unknown in search of adventure, glory, or vengeance—but as a writer, knowing how their starships work has a profound effect on the type of story we’re trying to write.
For instance, our first decision—whether our spaceships can fly faster-than-light or not—dictates the timescale of our story. If we decide to stick with the currently accepted laws of physics, it’s likely our heroes will have to enter some form of cryogenic sleep in order to prevent them dying of old age before they reach their destination. And if their journey takes more than a couple of decades, the world they left will be profoundly changed by the time they return, and some of their friends will have died in the interim.
Good examples of this temporal displacement can be found in Revelation Space by Alastair Reynolds, The Forever War by Joe Haldeman, and A Deepness In The Sky by Vernor Vinge. Peter F. Hamilton and I used it in our novella Light Chaser, in which it became the basis of the story, allowing our “immortal” space trader to re-visit various societies over the course of millennia to see how they had (or had not) changed.
However, if you’d like to move your characters from one place to another on a scale of days or weeks rather than centuries, you’re going to have to invent some sort of faster-than-light drive.
But, just as fantasy writers have to invent rules and limitations for the way magic works in their worlds, so SF authors have to work out a set a guidelines for how their spaceships behave. After all, if a ship can just go anywhere in the galaxy in the blink of an eye, there would be no way to defend planets or bases from attack. Hostile armadas could pop into orbit, unload a thousand warheads, and be a hundred light years away before the first one had exploded. Space battles would be impossible if ships could just leap away at any second. And how would economies function if you could import fresh produce from Betelgeuse as cheaply as buying it from the farm up the road?
Now, before you panic, I’m not asking you to describe exactly how your starship’s jump drives actually work. If you knew that, you wouldn’t have to write a book as NASA would currently be showering you with money and asking you to build one! Instead, I’m suggesting you come up with some limitations. After all, you don’t have to be able to describe the inner workings of an internal combustion engine in order to know that your average car can’t travel at 8,000 mph or operate under water (unless you’re writing about James Bond, of course.)
Classic ways of limiting FTL include putting upper limits on the distance a ship can jump at any one time, and forbidding jump engines from working inside a planet’s gravity well. In Larry Niven and Jerry Pournelle’s classic first contact doorstop, Mote In God’s Eye, resonances between stars mean jump engines only work if activated at a particular point within a system. In Jack Campbell’s Lost Fleet series, in which the remnants of a defeated navy have to travel the long way back through occupied territory, most ships have to use a network of star gates, and only the largest ships have the power to open their own ‘gates’ into hyperspace. In both cases, it becomes possible to blockade a star system by occupying the jump point or star gate—and it can also lead to thrilling chases and battles, as ships try to slog across the system to reach the next gate or jump point.
In my novel, Embers of War, I allow ship to take shortcuts through the ‘higher dimensions’—a place where the usual laws of physics are mutable and the speed of light can be exceeded. I liken the process to a dolphin leaping out of the water into the air. For a moment it finds itself moving through a different medium, where it moves faster because the water no longer drags on it.
However, in order to give my characters time to interact and get to know each other, I’ve had to impose a speed limit on higher dimensional travel. It isn’t instantaneous. In order to make the jump, a ship has to build up speed, kind of like the Delorean in Back To The Future. Then, once it’s in the hypervoid, its engines power it forward at roughly five light years per day. This means journeys can take days or weeks, and regular fuel stops need to be made to keep the engines working.
Whatever you decide, the way your starships move will shape your story, for good or ill. But learning to live with the limitations you impose will help make your story more interesting and authentic, and give your characters more obstacles to overcome.