One of the staples of sci-fi is the ability to visit other worlds. This means if you’re writing sci-fi you have some big questions to ponder. The number of things to consider is bewildering, but there are a few very clear forks in the road that might help.
Space travel – yes or no
First is kinda obvious. Are you allowing space travel at all or not? If not, then your story is planet-bound which simplifies things a lot.
Not so fast, though. There’s nothing to say that planet has to be Earth. You could be setting a story on another world, which opens up a wealth of possibilities as well as begging some questions. If the inhabitants are human, how did they get there? Is this a world where people landed, populated, and then lost the ability to reach the stars? If so, are they aware of their history, do they have legends, or are they oblivious to their origins?
If yes – within known science or not
If you opt for spacefaring ability, the next logical choice is whether or not you’re sticking with known science. Either way, there will be more questions to ponder regarding abilities and limitations, but at least you’ve narrowed down the possibilities.
Within known science
Technically, this includes us right now. We can get into space and we’ve sent people to another world. However we are at the bottom of the ladder in terms of capabilities. Fictional futures might contain a broader range of technologies and travel than we see today.
To reach space, you need to overcome gravity in such a way that you won’t simply fall back down again. This usually means either putting yourself into orbit, or escaping altogether from the planet’s gravity. There is a difference, but both involve achieving great speed very quickly. Getting into orbit is not simply a matter of firing yourself straight up – you’d only fall straight back down again. You need to achieve enough height to clear the atmosphere and a large sideways speed to put you into free fall around the planet. To escape the planet completely you need an even greater speed (escape velocity), either from orbit or directly from the ground. Launch mechanisms used or theorized within known physics: Chemical rockets, nuclear rocket, ground-based linear accelerator, ground-based laser propulsion. Most systems involve a source of energy to eject propellant to move your ship. The linear accelerator is different in that it uses external magnets to accelerate the ship directly without need for propellant.
Propulsion: Once in space, there are technologies other than chemical rockets that can move you around, but which simply don’t have the oomf to get you off the ground: Ion drive, solar sail, Bussard ramjet.
A more exotic way of reaching orbit is the space elevator. This is on the fringes of known science. It doesn’t break any physical laws as such, but we know of no materials strong enough to withstand the forces on the cable.
Similarly, you can extend rather than break physics by extrapolating today’s capabilities with any of the known systems. For example, what if you had a vastly more efficient fuel, or lightweight fusion reactors to power your craft? Speculation like this extends the possibilities for your story world, for example bringing colonization of other moons and planets more into the realms of possibility.
Slow interstellar travel
None of these options within known science allow you to circumvent the speed of light. That is a hard limit, which places tough constraints on your story world. Travel within the solar system is possible, but slow. Travel to other stars will take years at best. More likely decades or centuries. This means that story action will take place in isolated bubbles of reality with limited interaction between star systems and ships.
You can turn this slow interaction into a feature that drives how the story plays out. C.J. Cherryh does this to great effect in the Alliance series.
You can choose to focus on one location, such as a ship in transit, knowing that the entirety of the story will take place without any external interactions. If this feels a bit limiting, many stories are set on vast generation ships traveling between stars over hundreds of years. This opens up a world of possibilities for the occupants – establishing their own culture and conflicts within the confines of their artificial world, have they retained any memory of their origins, is the voyage running according to plan or has something gone awry with leadership or systems?
You can also dodge or minimize the time lag by stretching biology a bit, with extremely long-lived people to whom a few decades is no loss, or putting the occupants into suspended animation so you can pick up the story at the destination.
Beyond known science – Faster Than Light
Because our current understanding of the cosmic speed limit places such tight constraints on stories, the sci-fi genre allows some creative latitude to breach the barrier. This then allows true interstellar adventure, with meaningful interactions between different worlds in story-manageable timeframes.
This requires some hand-waving to explain away technology that is not even theoretically possible today. Most FTL options don’t directly contravene the speed of light barrier, but try to circumvent it by taking the ship out of normal space. This doesn’t directly violate known physics, but it assumes the existence of as-yet-unknown mechanisms. Popular avenues include: hyperspace, subspace, wormholes, folded space, warp bubbles.
You should have some underlying theory that sounds like it might be plausible at some future point in time, given a technological breakthrough yet to come. You should also think through the implications and any rules for your FTL system. For example, can your FTL drive work in atmosphere, near a large mass, or only between certain points in space? What are typical journey times (instant, hours, days, longer?)
You can get creative with these rules, depending on how much you want the rigors of travel to play into your story. The important point here is that this should not be a free-for-all. Your worldbuilding should have some inner consistency and at least a nod to science.
More exotic approaches
Beyond these sci-fi staples, authors have got creative in extending stories beyond one world.
One approach that worked a few decades ago was to simply ignore the speed of light limit. E.E. Doc Smith did that in the Lensman series by assuming making things inertialess (itself a real stretch, but with constant interesting story implications) gave matter a free pass through the light barrier. Whereas in the Skylark series he simply stated that physicists had got it wrong and the speed of light wasn’t a hard limit after all. With the advances in physics since then, this approach might get a rough ride with science-literate readers.
Larry Niven’s Long Earth series never left Earth, but assumed an endless range of neighboring parallel Earths that people could step to.
The there’s the tongue-in-cheek bloater drive in Harry Harrison’s Bill, the Galactic Hero. This drive simply bloated the ship and all its contents to light years in diameter, then collapsed it again at one edge, supposedly moving it from A to B without actually having traveled, and therefore not bothered by the cosmic speed limit. Yes, this was a satirical story.
In a similar vein, you have The Hitchhiker’s Guide’s Infinite Improbability drive.
At the end of the day, what you can get away with is limited only by how well you carry your audience with you. People will suspend a lot of disbelief when they’re hooked on a good story.
One last thought – unless you’re going for hard sci-fi and rigorous science, as a general rule the less you try to explain, the better. Going too deep into the mechanics of your handwavium just exposes you to being picked apart. Better to put it out there as quickly as possible and crack on with the story.
Ian S. Bott, Writer and artist 