Introduction: The expanse
Space is big. Really big. If you're anything like me, you now have the Hitchhiker's Guide to the Galaxy introduction going in your head, so that's one job done. The second one is going to be a lot harder, but we can do this.
Yes, space is immense. The scales involved are always mind-boggling. From where you're sitting right this minute, nowhere on Earth is farther than about 12,000 miles (or 20,000 kilometers) away. In astronomical terms, that's nothing. The closest celestial body, our moon, is around 20 times more distant. That big, yellow ball that starts coming up when proper geeks go to sleep? It's another 400 times farther. Only a few weeks ago, Jupiter and Saturn were so close from our perspective that I couldn't distinguish them without binoculars. In reality, though, they were separated by a few hundred million miles.
It gets even worse when we start looking to the stars. Our sun is one of a hundred billion or so in the Milky Way, and it's in kind of a backwoods part of the galaxy. The next-closest solar system is that of Alpha Centauri, a collection of three stars (one of which, Proxima, might just be visiting) and at least one planet.
The most common unit of measurement for distances between stars is the light-year. Light—more generally, electromagnetic emission of any sort—is the fastest thing we know, and stars are so far apart that even it needs a lot of time to travel between them. Around 4.2 years to get from here to Alpha Centauri, for example. To put that into perspective, if the President of the United States sent a message to Proxima b on his first day in office, he would end his second term six months before the reply arrived. Any aliens that live on Ross 128 b, another "close" planet that might be terrestrial, that watch our TV broadcasts (as in Contact) are getting hyped for the Vancouver Olympics, while those on TRAPPIST-1 e are just now seeing the Apollo landings.
In comparison, humanity's most distant physical creation is Voyager 1. Traveling since 1977, this tiny probe has covered over 14 billion miles, and its signals now take 21 hours to reach home. It's in interstellar space, but just barely; it crossed that boundary in 2012, three and a half decades after launch.
Speed limit
With such an enormous gap between what's out there and where we've been, creators of science fiction have long wondered if there is some way to travel faster than light. That is, after all, the only way to explore the stars on timescales comparable to human lives. Many ideas come to mind, from warp drives to hyperspace "jumps" to stargates. All have their advantages and disadvantages, but they share one thing in common: they're impossible. By the known laws of physics, faster-than-light, or FTL, travel, just won't work.
Alcubierre warp drives, for instance, usually require a substantial quantity of negative energy. That's actually a thing that exists, believe it or not, but the amount we'd need to create a "warp field" worthy of the Enterprise is far beyond even theoretical advances. Wormholes are possible; traversing them doesn't look to be without, again, some kind of exotic energy.
Thus, if you're an author of interstellar science fiction, my suggestion for describing your FTL system is pretty simple: don't. Whatever you do, some nitpicker is going to come along and tell you why it won't work. Instead of thinking about plausible mechanisms for FTL travel, consider how it would affect the story you're telling and its setting.
The best science fiction, in my opinion, doesn't go into great detail about how its core concepts work. It simply takes them as axiomatic and builds around them. Take Asimov's Three Laws of Robotics. We don't care how a robot is programmed to never allow a human to come to harm. What's important is the moral quandary created when, for instance, preventing harm would actually cause more. The story is the meat; the technology is the bones.
It's the same for space travel. Sure, we can have fun with a bit of technobabble now and then. "Invert the polarity of the tachyon field!" sounds great when an engineer is desperately trying to keep the ship from exploding. Overusing such phrases, though, gets annoying. It confuses rather than informs. In the most extreme case—Star Trek reaches this point far too often—it can even become a kind of deus ex machina.
The trick, I believe, is knowing how and when to leave the inner workings mysterious to hide the fact that you, the author, really haven't earned a Nobel Prize for finding a solution to one of the great physics questions of our time. Call it a shameless plug, but in my Orphans of the Stars series, I get away with it by making the narrators too young to understand the engineering details. Thus, I'm free to focus on the constraints of the FTL system and how they shape the story. The interface rather than the implementation, as my programmer side would say.
The first variable
One way to look at FTL in your setting is as a system of variables. The first of those variables is the availability of such technology in the first place.
FTL might not be there at all. If that's the case (as in James S. A. Corey's Leviathan Wakes or the 2019 film Ad Astra), you're necessarily limited to in-system travel, generation ships, robotic probes, or something along those lines. Relativistic speeds actually can get us to the nearest stars in semi-reasonable times, but they're almost as far-fetched as FTL itself. On the other hand, you can make a story dealing with the time dilation that would result: Joe Haldeman's The Forever War is one example.
FTL might be very rare, only possible at specific times or locations. At the extreme, a single wormhole that leads to only one place (such as in Interstellar) has about the same effect as enlarging the solar system, except that it adds a natural chokepoint with its associated ramifications. A wormhole "hub" like that in The Expanse's later entries increases the scale further, but still concentrates the ability for travel, meaning that destinations are fixed and something akin to trade routes will form.
FTL might be easy, but expensive. In this case, we're considering the possibility that there are few physical restrictions on traveling faster than light, but many economic factors making it difficult to achieve. Rare materials such as antimatter may be required, or massive structures that won't fit inside a personal spacecraft. Whatever the case, the effect is that such travel becomes restricted to only those who can afford it. Governments and corporations would thus have complete control over who goes where. (I can't think of any science fiction I've read that operates under this assumption, but I'd love some recommendations!)
FTL might be forgotten. In other words, the technology exists, and it has been built before, but those who discovered it are gone. This scenario veers into "ancient aliens" territory, which is worth its own column; for our purposes here, it's enough to say that forgotten tech can be similar to the "very rare" case above. The Mass Effect games and early seasons of Stargate SG-1 both took this route, with a network of fixed, interconnected FTL endpoints that provide for control and defensibility.
FTL might be commonplace. This is certainly the most "fun" option, the one taken by Star Trek, Star Wars, and thousands of other examples. At this end of the spectrum, FTL isn't restricted in who can use it or where they can go. True, there may be setting-specific rules in place—"doesn't work too close to a planet/star" is a common one—but those are flavor. They don't change the general feel. With this setup, basically anyone can have a ship that takes them to other star systems. This can create an environment most reminiscent of the Age of Exploration, if most locales are uninhabited by humans; the high seas of the 18th century or the Wild West, once ports begin to spring up; or today's interconnected world, when the extrasolar colonies grow to a certain point.
Mixing and matching, of course, is always an option. As an author, it's your job to find what works for you, for the story you want to tell. Contemplation is crucial, because your original idea might be incompatible with the constraints you've created. For instance, if you've decided that FTL requires a huge amount of power that can only come from unobtainium, then it's hard to justify space privateers…unless they've recently raided an unobtainium mine or factory. Likewise, a setting where FTL is ubiquitous makes familiar military strategies useless, for how do you defend against enemy ships when they could pop out of hyperspace literally anywhere?
The second variable
After the "scale" of FTL in your setting, the other major variable you must consider is destination. Where are people going? Where can they go?
In our own solar system, we have at least eight planets. Nine or more if you're a sensible person who counts Pluto, while the most reasonable definition of the term "planet" gives us no fewer than twelve. Add in moons and asteroids, not to mention space habitats like The Expanse's Tycho Station or Halo's, well, Halo, and there's a lot of room to play around.
Problem is, none of those other destinations (except the ones we build) are really suitable for humans. Mars is a frozen desert. Venus is a literal depiction of hell. And those are the two closest to the habitable zone! Everything else is airless, scorched by radiation, covered with ice, or…okay, Titan's its own thing. Without FTL, you don't have a lot of options for sightseeing. That's not to say you can't make a good story. It just won't be the kind we're discussing here.
If we expand our horizons and allow travel to other systems, everything opens up. So far, we've discovered over 4,000 planets orbiting other stars, a number that will only continue to grow. Most of these are fairly useless as far as human outposts: hot Jupiters, mini-Neptunes, super-Earths with surface temperatures hot enough to melt lead, and so on. A few look potentially habitable, in the most theoretical of senses, but we just can't say for sure yet. (If you're curious, check out The Habitable Exoplanets Catalog and The Extrasolar Planets Encyclopedia for the hard data.)
Still, that won't stop us from speculating. If Earthlike planets are rare, FTL becomes somewhat less of a novelty. The sheer distance from one habitable planet to the next means it wouldn't be very cost-effective. Either FTL is immensely fast, on the order of thousands of times the speed of light, or you have a (possibly preexisting) network of connections to the known worlds.
Some theories, however, posit that terrestrial planets are everywhere. After all, their proponents argue, we've got one confirmed and two potential candidates right here. Mars had liquid water at one point in its history; it's possible that Venus did, too. There may have been a point deep in the past when Sol hosted three habitable worlds, each with its own primitive biosphere. To add to that, we have the discovery of Proxima b. Statistically speaking, the odds that neighboring, unconnected stars would both have planets in their respective habitable zones is unlikely unless such planets are very common.
That's a good thing for us, because it opens up a whole world (pun intended) of possibilities. Again, I'd need another post or ten to talk about why not every habitable planet is Earthlike, but simply knowing that we can put them just about anywhere is a boon for science fiction. Common planets means lots of potential for colonies, alien empires, contact situations, and every other kind of interstellar drama or conflict to grace our pages and screens.
Now, you certainly can make great stories in settings with fewer planets. They'll have to be different, that's all. You need to provide a reason for people to make the journey. Maybe it's for rare materials. Maybe it's for common materials we've exhausted here on Earth. Maybe a group or sect just wants to be left alone, and they're willing to go to the trouble of building a deep-space outpost.
As with the nature of FTL, this is a case where you get to make the final call. You don't have to fire up a solar system generator or start studying mean motion resonance, but some thought will help you build a great setting that can stand the test of time even if it gets debunked.
Consider the interactions. That, if anything, is the moral of this story. Think about the implications of having your specific sort of travel in a galaxy with your specific parameters. Tweaking these two variables, the frequency of planets and the availability of FTL travel, will change the overall feel of the story. Hard science fiction needs different settings than space opera. A gritty military tale won't look the same as a swashbuckling romp through human space. If you already know the story you want to tell, work backwards from there to find how best to fit the style you're seeking.
I could go on forever, but that's always the case with me. Thank you for reading, and I hope I've given you something to think about, if not the seed of a new story. Space has always been a passion of mine, and I've never had more fun writing than I did when I first started my very own sci-fi adventure series. To be able to share that joy with the world…I can't ask for much more.
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