Sub-Lightspeed Setting

"186,000 miles per second: it's not just a good idea, it's the law!"

— Common nerd saying

To the best of our knowledge, the speed of light in a vacuum — 299,792,458 meters per second, or c — is a cosmic speed limit. Nothing with mass can reach or exceed it, as detailed on our page on Relativity. While that's very fast by human standards, the limitations add up pretty quickly once you start to calculate distances in space. At that speed, it takes about eight minutes for light from the sun to reach the Earth, hours for it to reach the outer planets, and years to reach the nearest stars outside our solar system. Consequentially, sci-fi writers tend to come up with some form of Faster-Than-Light Travel to enable Casual Interstellar Travel.

But some writers play this speed limit for drama instead. In their settings, faster-than-light travel remains impossible well into the future, or alternatively is impractical to use for whatever reason. You might have a Subspace Ansible so people can at least communicate in real-time, but that's about it.

When you can't travel faster than light, and interstellar travel takes years or decades at a minimum, it raises all kinds of interesting questions about how a spacefaring civilization would work. A centralized interstellar government likely won't be possible in this kind of setting; every planet, or at least every star system, would have to be able to provide for themselves without relying on Space Truckers to deliver goods to them from elsewhere. Warfare might be far more indiscriminate as well, focusing on destroying infrastructure rather than toppling governments that might not even exist anymore by the time your fleet gets there. Interstellar voyages will likely be conducted aboard Generation Ships or Sleeper Starships, or else fly close to lightspeed to take advantage of Time Dilation.

A setting that meets this criteria will likely qualify as a Points of Light Setting, consisting of isolated bastions of civilization separated by a wide, hostile expanse (in this case, interstellar space). If FTL is eventually achieved and adopted, expect those slower-than-light ships to fall prey to Lightspeed Leapfrog.

Compare:

• Hyperspace Is a Scary Place, where FTL travel is possible, but very dangerous, which may be used to justify this trope;
• No Warping Zone, which is a specific location where FTL is not possible within a wider setting;
• The Milky Way Is the Only Way, where FTL travel is possible but not used to travel beyond the Milky Way;
• Ungovernable Galaxy, where the galaxy is too big for a centralized government to be effective, a likely outcome of this trope

Contrast Casual Interstellar Travel, where FTL capability is accessible to everyone. A common element of Mundane Dogmatic settings and works on the harder end of the Mohs Scale of Science Fiction Hardness.

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Examples:

Anime and Manga

• Cowboy Bebop: Many decades were spent in the construction of the Hyperspace Gate network that reduces travel time between planets down to mere hours/days, depending on the distance. Most of that time constructing the gates was the manual distance traveled just to get to Venus, Mars, Jupiter, and Pluto to set them up in the first place. This leaves Humanity limited to the Solar System since it would take most of an entire lifetime to reach the nearest star.
• Gundam: The Gundam franchise takes place within the solar system (the furthest inhabited part of the Earth Sphere is the colonies around Jupiter). Despite various alternate universes, the travelling distance between locations like Earth or space colonies are often used to up dramatic tension.
  • Mobile Suit Gundam: Several episodes take place during the comparatively dull travel between locations in space. The first few episodes feature the White Base being repeatedly attacked by Char Aznable, and after the White Base heads up to space as the 13th Autonomous Corps they take part in occasional skirmishes during the long travel to battlefields like Solomon.
  • Mobile Suit Gundam 0083: Stardust Memory: FTL doesn't exist in Gundam, but the Delaz Fleet makes use of momentum. While being pursued by the main Earth Federation fleet, they make their way towards the moon, leading the Federals to suspect an attempt to seize control of one of the lunar cities. The Delaz Fleet instead uses the moon's gravity to slingshot themselves towards their true target: Earth itself. The Federal fleet, taken by surprise, burns most of its fuel in a desperate attempt to catch up, but are unable to do so and it's up to the Albion and the defense fleet around Earth to prevent the Delaz Fleet from performing a Colony Drop. They fail.
  • Mobile Suit Zeta Gundam: Paptimus Sirocco, a Titan officer who comes from the Jupiter Fleet ^note  an independent fleet that transports the precious Helium-3 gas needed to power the various Minovsky reactors used by just about all advanced machinery, has spent so long away from Earth due to the sheer amount of time it takes to travel from Earth Sphere to Jupiter that the Titans consider him even more alien than they do their Spacenoid enemies.

Comic Books

• The Simpsons: In "The Geek shall inherit the Earth", Doug and Troy McClure make a sci-fi movie together, in which Doug insists on not allowing FTL travel since he believes it would cause the spaceships to collapse into a black hole. Unfortunately, because he refuses to allow any Acceptable Breaks from Reality and values being scientifically accurate over the movie's enetertainment value, it ends up bombing.

Film — Live-Action

• Avatar: Antimatter-powered human spacecraft can only reach a significant fraction of the speed of light, with the 4.3 light years between Earth and Alpha Centauri (where Pandora is located) taking about a decade to traverse.
• Pandorum takes place upon a slower-than-light Sleeper Starship that has been traveling long enough for the descendants of the earliest waking colonists to evolve into feral cannibals.
• Passengers (2016): The voyage of the Colony Ship Avalon to the colony planet Homestead II takes over a hundred years. Thirty years in, the ship has a Titanic-esque collision with an asteroid which lodges chunks of rock in the bowels of the ship, causing a hibernation pod containing one of the traveling colonists to open. Now he's trapped alone on the ship since it's still decades away from its destination and he can't even call for help since it would still take years to get a response to it.

Literature

• Partial use in Aeon 14. The 'Verse has two major time periods in which it takes place, the 5th Millennium and the 9th Millennium. In works taking place before the Time Skip, humanity deployed a fleet of terraforming ships to prepare the galaxy for humans to settle nearby stars. Interplanetary travel is commonplace, but communication is light-limited, and traveling between stars, as the protagonists do in The Intrepid Saga, takes decades as Human Popsicles. Averted in works set in the 9th Millennium: in the intervening years, experiments with Artificial Gravity led to the discovery of the "dark layer", which allows Faster-Than-Light Travel.
• All Tomorrows: FTL travel is not possible, leading to the Second Empire of Man being held together mostly through deep-space radio transmissions rather than physical travel between planets.
• Axiom's End:
  • The amygdalines have been spacefaring for thousands of years, but remain limited to sublight travel at relativistic speeds. Because of Time Dilation, individual space travelers are said to have a relative age (the time elapsed since their birth) and a subjective age (how much time they've experienced).
  • The sequel reveals that they do have access to FTL communication by means of folding space and transmitting information through. The amygdaline Nikola is able to adapt this technique to transport himself to the solar system, but this was an extremely risky edge case that he never would have attempted if he weren't already a Death Seeker. He also destroyed his research before leaving, and is confident that his kin will never reproduce it.
• The Century Long Journey by Vladimir Tendryakov has Earth communicating with another race 36 light years away. There is no FTL in any form, so they take a person with eidetic memory, upload his brain, and broadcast it to the other guys. The other planet grows a body for him, learns about Earth, teaches him everything about itself, and sends his mind back.
• Childhood's End: When Jan stows away on an Overlord ship he takes drugs to induce hibernation; when he returns to Earth from the Overlord homeworld he discovers that decades have passed and humanity is about to Ascend to a Higher Plane of Existence.
• In the Children of Time series, refugees and explorers cross the stars as "cargo". A jaunt to a neighboring star system can take millenia - which is time for entire cultures to rise and fall, on planets and even ships! Even after the discovery of FTL at the very end of "Children of Ruin" sleeper travel is still being used in "Children of Memory".
• Cluster: Downplayed. Teleportation is possible but prohibitively expensive, so the only practical way to travel between star systems without spending a lifetime on the journey is to have your mind projected into a loaner body.
• The Doom novelization (yes, really) uses completely hard science when it comes to space travel: ships can only approach the speed of light but never reach it, it requires a long time to speed up and down without squishing the crew from insane G-forces, and the horizon is noticeably blue- and redshifted while moving at relativistic speeds. This is used to explain why the aliens invading Mars took the form of demons: the last intel they got on Earth came from medieval Europe and showed that humanity had a superstitious fear of hell, and they intentionally exploited this. The matter of taking hundreds or thousands of years to travel between planets isn't an issue for the aliens because of one factor that is very much not hard science: all intelligent species except humans have souls that manifestly survive bodily death and can be easily revived through repair of the original body or reincarnation. Thus, nobody has to worry too much about their family being dead after a jaunt between another solar system.
• Zig-zagged in the Ender's Game series, which initially has instantaneous communication but no FTL travel. In the original book, this is a double-edged limitation of the interstellar war with the Formics, hobbling Earth's logistics but giving it time between to narrow the technology gap between invasions. 3000 years later in Speaker for the Dead, humanity has settled the galaxy, but anyone who travels between star systems has to abandon their old life and era forever. However, instantaneous travel is invented in Xenocide, promising to change galactic society forever.
• Humans cross the Colonized Solar System of The Expanse with fusion rockets. Even after the Protomolecule ring gates connects the Solar system and other habitable star systems, travel there still takes months.
• The Forever War follows a couple fighting in an interstellar war at relativistic speeds as they go on increasingly long tours of duty and find Earth has changed in the subjective centuries they are gone.
• In Hainish, there's a light-speed limit on travel, but there is a way to transmit messages instantaneously over interstellar distances.
• House of Suns: Countless millennia in the future clone-lines traveling the galaxy at near-lightspeed provide some continuity of civilization for the countless human and posthuman colonies across space. The existence of a wormhole allowing for faster-than-light travel to the Andromeda Galaxy is a major plot point, built by aliens and irreproducible.
• Post-Self: By the 24th century humanity has only colonized the Moon and a space station at Lagrange Point 5 that primarily hosts a massive server storing billions of uploaded consciousnesses. In Toledot the Lagrange station launches two starships with uploaded crews and solar sails. Twenty years later in Nevi'im the ships have only traveled a light-month when one of them encounters an alien craft that has been traveling for five thousand years.
• Proxima: Human-built starships rely on constant thrust to reach near-lightspeed, then spend the second half of the trip decelerating. The crew experience about three years of time thanks to Time Dilation. The Dreamers' Hatches are instantaneous to the transportee, but to outside observers work at just shy of lightspeed.
• Remembrance of Earth's Past:
  • The series revolves around the fact that the hostile Trisolarans of Alpha Centauri have launched an invasion fleet to conquer and colonize Earth... a fleet that will take 450 years to get there, due to the lack of faster-than-light travel.
  • This trope forms the basis of the dark forest theory, which is that meaningful communication is impossible to achieve across interstellar distances, resulting in paranoia and eventual xenocide between alien civilizations.
  • Lightspeed travel is eventually achieved in the third book, but faster-than-light remains impossible. Furthermore, it turns out that the speed of light is permanently slowed in the wake of a lightspeed ship, and these wakes can chain together across space to create permanent No Warping Zones that can span millions of light-years. It's eventually revealed that the universal speed of light was once nearly infinite, but the abuse of lightspeed travel over billions of years has reduced it to its current speed universe-wide, which will only get slower as the wars continue.
• The setting of Revelation Space is (mostly) limited to slower-than-light travel.
• Schild's Ladder and other far-future works by Greg Egan have a galaxy-spanning civilization with all kinds of Sufficiently Advanced Technology but no faster-than-light travel or communication. Most interstellar "travel" consists of transmitting a copy of your mind via gamma-ray beam, often destroying the original, so people only travel to distant worlds when they're ready to leave their current lives forever.
• In the Takeshi Kovacs novels, human civilization spans multiple star systems originally colonized by Generation Ships. The only practical form of interstellar "travel" is to transmit your Ego via Subspace Ansible and buy or rent a Body Backup Drive at your destination. However, it turns out the Martians and at least one other species in their time had FTL travel in the second book.
• The light novel We All Died At Breakaway Station by Richard Meredith centers on a communications station on a lonely world. Faster-that-light travel isn't possible for starships, but interstellar messages can be transmitted much faster than light, provided the light beam link is intact. The Hollywood Science explains that tachyons can travel a photon beam the same way that electrons travel through copper wire. A war between Earth and some aliens means critical intel about the aliens will take many years to reach Earth if Breakaway Station is destroyed.
• Worldwar: The Race's Sleeper Ships take 20 years to get to Earth, traveling at half lightspeed. When humans send a starship to the Race's homeworld, it takes even longer, only able to travel at a third of the speed of light. Subverted when humans discover FTL travel while the first ship is on its mission, and send a second ship that arrives only months after the first one.
• WWW Trilogy: In the Distant Epilogue, faster-than-light travel still has not been achieved five billion years into the future; humanity had to make do with slower vessels.

Live-Action TV

• Altered Carbon: Interstellar travel relies on "needlecasting" your uploaded mind via Subspace Ansible to a Body Backup Drive at the destination system. Human civilization spans several star systems, but they were colonized by Generation Ships, and physical interstellar travel is limited to cargo hauling.
• Firefly takes place in a single multiple-star system. No Faster-Than-Light Travel exists, though ships move fast enough to be able to travel between planets within hours or days.
• Star Trek: Voyager: In "The Omega Directive", it's stated that the detonation of a large number of Omega molecules could destroy subspace across an entire quadrant of the galaxy, enforcing this trope upon any civilizations within. Accordingly, the titular directive orders Starfleet vessels to safely neutralize the molecules whenever they're detected, even if doing so violates the Prime Directive.

Tabletop Games

• Lancer has blink gates, but they're sufficiently expensive compared to near-light starships that some colonized planets are twenty light-years from the nearest gate. Meaning that it can take decades for Union to respond to a crisis.
• Shadowrun: There are some MegaCorp colonies on Mars and manned missions have been sent out to Jupiter, but everything uses sub-light engines so trips take months each way when the planets are at their closest. Also, it's really unpleasant to be outside of Earth's atmosphere if you're Awakened. This is rarely relevant to 'Runners, who generally have no reason to leave Earth.

Video Games

• The titular RimWorlds are isolated because of the lightspeed limit and the vastness of space, in most scenarios your starting colonists are survivors of a crashed Sleeper Starship with no hope of rescue within their natural lifespans. One of the victory conditions is salvaging or constructing enough cryopods to build a new ship.

Web Original

• Orion's Arm: Ten thousand years into the future humanity's descendants have only explored a small portion of the titular galactic arm thanks to a complete lack of Faster Than Light travel. Wormholes exist but each end has to be towed into position at less than 77% lightspeed so they don't count. While Alcubierre Drive can only approach the speed of light with a max speed just short of meeting it.

Western Animation

• Played for Laughs in the Futurama episode "A Clone of My Own". When Professor Farnsworth brags about the engines on the Planet Express ship allowing it to traverse entire galaxies in mere hours, Cubert points out that this is impossible, as nothing can exceed the speed of light. The Professor agrees, claiming that scientists had somehow increased the speed of light some years ago.

Real Life

• Truth in Television, and likely to remain so for the foreseeable future, for reasons explained on our Analysis page on Faster-Than-Light Travel.

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