Obeying the Celestial Speed Limit

"They've gone into plaid!" - Spaceballs demonstrates the ridiculousness of faster than light travel.

As a genre, science fiction frequently runs roughshod over the hard limits of the universe as we currently understand them. Above all else, science fiction writers consistently forget, hand wave away, or blatantly ignore what is perhaps the most foundational rule in the universe: nothing travels faster than the speed of light.

Of course, science fiction is just that: fiction. All a science fiction author must do to avoid responsibility for scientific oversight is to slowly spell out the name of her genre. And yet, many readers of scifi can recognize and respect scientific integrity. Limitation breeds creativity – and for stories with a scientific focus, what better limitations than the very limitations binding reality?

Why You Should Stick to Sub-Light Speeds

Why is the speed of light so important and inviolate, you ask? The short answer: because it’s a hard limit on the functions of the universe, required in order to prevent a violation of causality,* and one of the foundational constants of existence. (Our universe doesn’t have walls, and may not even have a beginning or ending. Its bounds are defined by math: limits on speed,* size,* and time.*)  A more complete answer must address the fact that humanity has conquered – or slipped through loopholes in – apparent limitations before.

As the past few centuries have made clear, humanity is capable of incredible advances over the course of just a few generations. Our understanding of the universe grows increasingly complete, and we have applied this expanding knowledge towards ever more impressive materials, machines, and structures. We even bend truths once thought to be universal and inviolable.* It is reasonable, then, for authors to assume that this ascension to omniscience will lead to our species’ omnipotence as well. And yet, this is a fallacy: even a perfect understanding of all aspects of the universe would not enable us to break the core rules. And the speed of light is about as “core” as rules get. Even the lesser laws of the universe are typically read-only; sometimes we find a useful loophole to exploit, but we can’t just plug in the universe’s equivalent of the Konami Code and change things as we see fit. It’s worth noting that, as revolutionary as scientific advances can be, its laws are almost never overturned.*

Humanity has generated an impressive array of technological wonders, weapons, and time-wasters. Some of these (human flight, moving pictures, microwaves) would have indeed been inconceivable centuries prior – but most are simply improved versions of previous technologies. None sprang forth from the corpse of some previously-incontrovertible natural law.* Bypassing the speed of light is serious business compared to impressive-yet-incremental advances like cloning vats, nanobots, and artificial intelligence.

But I Read Somewhere That…

Hold on a second: Popular science blogs and optimistic futurists proclaim that NASA is already working on the Alcubierre drive! This Star-Trek-esque warp drive would coerce a ship to travel at faster-than-light (FTL) speed by bending spacetime in front of and behind the ship. The math is sound! There’s just one teensy little problem: it requires the mass-energy equivalent of the planet Jupiter. No, not how much energy Jupiter radiates, how much energy exists in Jupiter as mass.*

But wait! We can modulate the warp field oscillations* to allow a ship to travel at FTL speeds – with only the mass-energy equivalent of a Volkswagen Beetle! It’s still more energy than humanity can currently produce or store, but it’s a simpler scale-up problem, one that we can eventually solve. Now the only missing ingredient is negative-mass matter. Not antimatter, which has positive mass, but matter that has NEGATIVE mass, which most physicists say is an impossibility.*

Other, more exotic solutions are occasionally offered for FTL travel – solutions from both hand-waving scifi authors and math-wielding physicists. These include such phenomenon as wormholes (speculation, but technically possible), hyperspace (no evidence exists and no theories support any version of it), tachyons (again, no evidence for their existence), portals (basically just wormholes plus magic), and manipulating alternate timelines or mirror universes.*  Like negative-mass matter, these all fall into the same category as Santa Claus or the Abrahamic God: impossible to disprove, but highly improbable and suspiciously narrative-convenient.

Creating Your Slower-Than-Light Universe

For the purpose of discussion, let’s say I’ve convinced you. You’re committed. You’ve become the scifi equivalent of vegan: faster-than-light travel is off the table. You’re going to have to cook up a plot with all-natural ingredients. So what recipe can you follow for a plot-friendly universe with this restriction?

For the most part, this is as simple as shrinking the scale. If you’re in space, it goes without saying that you have more than enough, well, space. It cannot be stressed enough that space is impossibly large and impossibly empty. “Impossible” is truly the best adjective: there’s no good way to convey the scale of interstellar distances to planet-lubber humans, and all analogies break down. Interplanetary distances within the same solar system are still ridiculous enough to accommodate any space battle, epic voyage, or megafortress your plot could require.

And speaking of solar systems, you aren’t limited to the one you live in. For example, Firefly’s ‘Verse is a wonderful, only slightly-implausible* multi-star system filled with dozens of terraformed planets and moons. If we’re being a stickler for backstory, you have three options here:

  1. Humans came from Earth via a long journey in colony or sleeper ships.
  2. This is the past.*
  3. You’re writing a space-fantasy, and any concept of “Earth” is irrelevant.

Can you still blend the delicious flavors of space opera science fiction without your characters hopping effortlessly from star to star? Can you still have interplanetary space empires? Can your characters cruise to distant worlds, falling in love with the locals or bombing them into submission? Can you have epic space battles and space piracy? Yes! Not without restrictions, of course, but the curious implications of the cosmic speed limit can inspire unique settings and plot mechanics. Let’s look at these questions one at a time to see how they might work (or already do work) in a slower-than-light space story.

Interplanetary space empires

This is the bread and butter* of epic, space opera style science fiction. And yet, most of these empires are poorly-disguised stand-ins for the Victorian British Empire.* Their spaceships are just chrome-hulled sailing ships with glowing rear ends. Or, if space is for fighting over rather than exploring, ships become thinly veiled analogs of World War II era carrier groups. The oceans explored and fought over by the clippers, cruisers and carriers of the past few centuries are stunningly vast – yet they have nothing on outer space.* There’s plenty of room in our planet’s backyard for empires to sprawl – but they’ll need to work for it, since even the easiest terraforming gig will be a backbreaking and bankbreaking endeavor. But humans have a much better track record with overcoming seemingly impossible engineering difficulties (say, building continent-spanning walls, or city-sized exotic matter generators) than they do with overcoming universal laws of reality like the speed of light. If a slower-than-light empire gets really ambitious, they can be the first to send humans to Alpha Centauri, Barnard’s Star, or Sirius.

I’ll give you a freebie: here’s your custom-generated Epic Space Empire™ in a distant but non-FTL future.

Example: A Non-FTL Space Empire

The barbarian migrations from Alpha Centauri were finally defeated, but the terrifying weaponry of the All-War left every surface sunward of Pluto an irradiated hellscape. A new Dark Age fell. But human civilization rose again from an unlikely place: the innumerable, sun-spurned worlds of the Oort Clouds.

Our Sun’s neglected stepchildren – the Oort Cloud comets – weren’t as large as planets, but they became good candidates for colonization after Prometheus* and Icarus* teamed up to steal the nuclear fires of the Sun for humankind. These miles-wide islands of ice were estimated to be more numerous than the stars of our galaxy. They weren’t Earth-clones with wondrous ecosystems, but they became home to cities, factories, spaceports, and most of the other trappings of imperial power.*

Despite the bleakness of time and place, humans of this millennia-distant Oort Cloud thrived and prospered, powering their incredibly resource-hungry gravity fields, terraforming projects, radiation deflectors, and miniature suns with the Oort Cloud’s plentiful fusion fuel: water. Relative isolation and DNA manipulation created distinctly alien subspecies of humans. Adventure capitalists occasionally braved the deadly Inner System to plunder its wondrous treasures: stone, metal, and ancient artifacts. The far-flung settlements grew into fiefdoms, then nations, then a vast and unruly empire. The Empire didn’t span anything so pretentious as a galaxy; it didn’t need to, to encompass the greater portion of the universe’s diverse human-inhabited worlds.

In the end, almost any story about the Galactic Empire could be told about the Cometary Empire of the Solar Oort Cloud instead.

Travel between worlds

Interplanetary travel can still be wonderfully plot-convenient in a sub-lightspeed universe. If the worlds are the planets of our solar system, travel times for an advanced, yet lightspeed-abiding civilization will range widely. Minutes between inner planets, or hours for visits to asteroids and the wondrous moons of Jupiter or Saturn. For a location as distant as Neptune or Pluto, perhaps a day or two.

If your plot requires a longer trip, there are two simple options.  The most obvious is to banish your characters somewhere more distant: the exotic shadowlands of the Oort Cloud or the twin suns of Alpha Centauri.  But lowering the tech level can be the more interesting choice, since trips aren’t just longer, but stranger. For example, near-future shuttles to Jupiter and Saturn will involve months-long* curved and ponderous trajectories, as slow craft hijack planetary gravity wells for use as slingshots.*

Cassini Interplanetary Trajectory Cassini Interplanetary Trajectory Cassini Interplanetary Trajectory by NASA

Positions of planets in their orbits affect travel times as well, which can be highly plot-convenient.*  For shorter trips, consider setting the story amongst the terraformed moons of a huge gas-giant.* If distant stars are your goal, consider generation ships,* sleeper ships,* or seed-ships.*  And remember, a traveler could age just a few years in a 100,000 light-year trip across the galaxy.* Of course, everything else will have aged a hundred thousand years. Plot hook, anyone?*

Space Battles

Here’s where sub-light speeds can be a huge benefit. So, remember how space is obnoxiously big? Well, if you can travel faster than the speed of light, the amount of space potentially involved in a battle – the distances between ships – is so vast that a spectator would likely never see more than one ship at a time. That is, real battles would look boring. Concentrate everything in an interesting corner of one solar system and slow the ships way down, and suddenly there’s a reason for a battle to be crowded into a small space again. “Small” being defined as “you can probably see at least a handful of them at the same time,” but a viewer might at least recognize what they’re seeing as a space battle.

(Almost) Everything Else

With sub-lightspeed, you can have space pirates,*, exotic locations*, orbital bombardments*, and all sorts of zero-g shenanigans.*

I concede that not every component of the standard space opera formula can be replicated in a rigorously sub-light universe.* Your characters can no longer relish the exploration of far-flung anomalies such as quasars, black holes, nebulae, supergiant and dwarf stars, and more exotic or author-designed phenomena. Even different types of stars and planetary systems – mundane, but interesting in their variety – are going to be missed. A handful of local stars* are fair game for dedicated travelers, but your characters won’t be able to pick a shining point of light and proclaim, “There! I shall travel there!”

Yet if our planet is any guide, it’s not unreasonable to expect that any solar system full of humans will eventually become a microcosm of wonders: a display case for the artifacts and engineering projects of increasingly advanced civilizations. Quasars and black holes may be impossible destinations, but as humans add to the natural wonders of space, your characters will be able to visit enormous light harvesters, reflector dishes, Moon Rushmore, comet herders, ringworlds, and even artificial (if comparatively small-scale) nebulae and black holes.* The speed of light should limit you, but your imagination should not.

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  1. Adam Reynolds

    I should thank you for pushing me over the edge in terms of building a setting with no FTL. I was already bothered by the causality problems with FTL and this sold me on going without it. In a sense it is actually more dramatically interesting as it allows trouble to brew while people travel. It also decreases the number of worlds involved which also makes the importance of a single world much more interesting in terms of the story. That was always a problem in Star Was especially. We have one small out of the way planet(Naboo) that produces the president of the galaxy and no one finds this odd. We also have an invasion of one out of thousands of worlds aligned with the Separatists treated as a major military victory. With the action confined to a single solar system, all of that would be far more plausible.

    One bit I thought I would point out for those that don’t known is that FTL communications and travel are functionally interchangeable with time travel. The problem is that because the signal travels faster than light, it actually travels back through time. Consider a rough analogy of a missile fired from ship A to ship B. Ship B sees the missile being fired using its light speed senors. It immediately tells A to not fire by sending a signal faster than light. A, who receives the signal at FTL, sees it before they pull the trigger and thus doesn’t fire. But that begs the question of how B was able to send that signal in the first place. In effect, it ends up creating the same grandfather paradox as time travel.

    I especially like the ideas of colonies out in the Oort cloud and sky kingdoms on a gas giant. Fantastical and physically realistic are not mutually exclusive. I could easily see a cloudborn Luke Skywalker(or Rey Solo*) with dreams of watching the sun rise over a normal world.

    * Just guessing on the last name. I’ll know next week.

    • Megamoya

      It’s worth mentioning that FTL and causality can be a nuisance in universes where light travels at the speed of time (such as our own). But in settings with Newtonian time (e.g. Star Wars) that part of the problem doesn’t exist.

  2. jim acre

    I recently read a webpage which explained how it might be possible to squeeze something like 36-52 HABITABLE planets into a single star system without a ridiculous number of stars. Each star has something like 4-8 double-planets orbiting it (depending on how big you want the planets), and there are like only 4 stars. If you use gas giants, you don’t get as many orbits per star, but you get more moon-planets per star, yielding a substantial increase in habitable planets per star.

    Also, to get these astounding numbers, he doesn’t use retrograde orbits, and I don’t think he used LaGrange points, except for the gas giant systems.

    Something like this occurring naturally would probably be only slightly less impossible than a natural rosette.

    Sadly, I don’t remember the name of it, or I’d link it.

  3. SunlessNick

    “Impossible” is truly the best adjective: there’s no good way to convey the scale of interstellar distances to planet-lubber humans, and all analogies break down.

    Ben Bova compared (in terms of ratios) the difference between interplanetary and interstellar distances as equivalent to the difference between inches and miles.

  4. GoldJadeSpiceCocoa

    You could have an multi solar system space opera within an STL universe. Just the time is lengthened. If a mayfly tried to hold onto a landmass the size of 100 miles it would be impossible because of their short lifespans. But if humans had 100,000+ years, waiting 40 years is irrelevant, that’s a months time. Compound this with sleeper ships, and the fact when your ship goes at near C, time stops for you a bit.

    If your ship has FTL than fighting is meaningless. Your ship will just get out of there or your weapons would never catch up to the enemy. The only weapon would be FTL missiles. That takes the fun out of our friends Rail Guns, Lasers, and Missiles.

  5. Cay Reet

    Unless you find a way around the problem that nothing in this universe moves faster than light, there will not be any FTL drives. Scientifically, FTL travel isn’t possible.

  6. Oren Ashkenazi

    Editor’s note: I have removed a comment for being nothing but a rude attack on the article and its author. Cay Reet was already here with a logical counter, but we don’t allow that kind of content on the blog.

    • Cay Reet

      Thanks, Oren.

    • Jayden

      I see where your coming from, but having no FTL ruins the fun. You can’t encounter aliens, far off planets or do any of the classic Sci-Fi things. Anyway, FTL could be possible in a way we aren’t even thinking of yet. Remember all the other “impossible” things humans have done, like sailing across the oceans, heavier-than-air flying machines and landing on the moon. Most settings are set far in the future so it’s possible

      • Jeppsson

        There’s nothing wrong with doing slightly softer sci-fi and include FTL if you want to. However, you can’t compare an FTL-drive to an aeroplane or the like. Sailing across the world, aeroplanes and these kind of stuff were never said to go against fundamental laws of nature. We just had to solve the technical problems.

        When was the last time we had firm evidence for an actual law of nature and it turned out to be wrong? And not in the way Newtonian physics turned out to be “wrong” – those equations still work and give correct answers for macro objects in all the contexts where they were originally used. I mean wrong in such a way that it turns out that we had the wrong results all along.

        • Jayden

          FTL drives could be possible. Maybe there’s something we haven’t discovered that can enable it.

        • Pax

          There is one way to make a “no FTL” setting that allows the author to explore lots of weird stars neblulas black holes etc., plus whatever aliens you want and as many of the usual Hollywood shenanigans as you want.

          You only have to follow this little recipe:

          #1 – It is set in a parallel dimension/universe, where the laws of physics are slightly different. Namely, one universal constant is WAY different: The speed of light in your setting isn’t a mere 300000 klicks per second, but hundreds of times higher. It is assumed that all other physical constantsd adjust a bit so that life and biochemical reactions and soolar fussion reactions etc. can all still exist “normally”.

          #2 – Gravity is also a bit different too. Things seemingly drop just as fast as in our universe, but they don’t accumulate as much kinetic energy. Falling down from 3 floors will NOT kill off your protagonists. It also takes a lot less fuel for a rocket to reach orbit or any given acceleration, etc.

          #3 – The galaxy is VERY compact. Average distance between stars is a lot smaller than what we have in the Milky Way.

          If you need to avoid the galactic empire to have millions of city-worlds, just keep the “habitable planets are relatively rare” trope to the max.

          Alternatively, you could make it so that in the setting the speed of light is NOT the top speed. The speed warp, hunddws or thousands tierms higherr, would be the real top speed. Time is linked to the speed of warp ,not t light speed. However, within a solar system or near enough any big mass, the speed of warp tumbles down to light speed. So spaceships first have to normal-STL-speed thheir way out of the solar system then “warp speed” FTL sure but not “faster than the top speed that is actually the real one defining causality”.

          but set in our “real” universe? Nope no can do if you go hard sf then light speed is the absolute limit and what the article poster suggested are all very cool ideas.

  7. LordMunchkin

    Instead of decreasing the scale, you could instead increase the scale but abandon completely human characters. If everyone is near-immortal, who cares if it takes decades or centuries to travel from system to system? If the tech is advanced enough, entire minds could be copied and transmitted across the stars to serve as cultural ambassadors. If there are empires or federations, they might be governed by massive superhuman artificial intelligences. If all this makes you uncomfortable, that is part of problem. In my experience, many people don’t like the idea that humanity is something to be surpassed even though that has been the general pattern of life since time immemorial.

    Going to back to topic of slower than light space opera, I think anything more than an informal alliance is unlikely given the vast distances involved (assuming near-human and contemporary cultures of course). A governing mind that distributes many copies of itself that are regularly updated from the “core worlds” might have more luck. However, if a copy refuses to install a update, now that would make an interesting story.

    In addition, STL space opera opens up the possibility of stories carrying across decades, if not hundreds or thousands of years. Sounds pretty epic to me.

    Finally, I’m not sure if the not author specified this, but adhering to celestial speed limit doesn’t exactly mean you have completely hard scifi. If there anything in physics that precludes the existence of force fields?

  8. rodneyzalenka

    One thing about travel time in-system, with STL (an interesting term…): it need not be bound by Hohmann orbits.

    Even with solar (or EM) sails, powered flight (rather than the chemical “glides” we’re all accustomed to) are possible: picture in-system ships being as festooned with sales as a clipper. One of those could readily cross the Sol system in a matter of weeks (less?).

    That’s not even counting beam-powered flight, by laser or microwave. That, it seems to me, could cut the travel time to days or hours, because acceleration would be constant. (I confess, I’m not sure the inbound _return_ journey would be, but I don’t have the physics to know.)

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