Subspace Ansible

Space is big. No, really big. You might think it's a long way to the chemist, but that's peanuts compared to space. The only way to have snappy dialogue between characters in different star systems (hell, even to the moon) is with faster-than-light radio. This Subspace Ansible (a.k.a. FTL Radio) is also necessary for spaceships using Faster-Than-Light Travel to have two-way conversations, since actual radio waves are light (of a non-visible frequency) - and are therefore slower than the ship.

There are several types of faster than light technologies in fiction; however, Subspace Ansibles need to use one that doesn't require sending the ship's engines along with the message. So, they typically use the "shortcut" method: sending ordinary radio waves through an exotic Subspace or Hyperspace that is smaller than real space.

If the setting has both subspace and hyperspace, then typically subspace will allow nearly-instant communication, but can't be used for travel. Even in Star Trek, which uses subspace for both, real-time conversations take place between characters who are days of FTL Travel apart. This allows plots to be written as if Space Is an Ocean. On the other hand, just as not all FTL methods are equal, neither are all FTL comms. A humble 10c is technically FTL, but it'll still take about 5 months for a message to reach Earth from an Alpha Centauri colony. Raising the speed of transmission to 365c turns the travel time down to 4 days, but still an eternity for any poor spacers calling for reinforcement, and God forbid you need to call for help from anywhere further. Having a determinate speed rather than that of plot can shape a story's events.

In some cases, a Subspace Ansible may exist even if Faster-Than-Light Travel does not. There might be some attempt to justify this, proposing that technology exists to bypass the limits of relativity for information but not matter, but often, it is simply a matter of necessity: while a single-star-system Space Opera can get by without Faster-Than-Light Travel, it takes several minutes for radio waves to travel the distance from Earth to Mars, the problem of communication remains pressing even if Faster-Than-Light Travel can be safely shelved.

Presumably, an FTL Radio is also what allows ship sensors to work faster than light, viewing objects that are light-minutes away - or even light-years! - in real-time. However, any attempts to explain it are indistinguishable from a Hand Wave (unless tachyons are involved; then it gets complicated).

The term "ansible" for this kind of near-instantaneous subspace communication system was coined by Ursula K. Le Guin in her 1966 novel, Rocannon's World. "Ansible" is a derivation of "answerable", i.e. "messages will be answerable in realtime". Many other science-fiction writers picked up the name after Le Guin.

If Psychic Powers exist in a setting, they often work instantaneously at any distance, and function as a Subspace Ansible.

Literature

 * The ansible appears in most of Le Guin's Ekumen novels. In The Dispossessed she tells, among other things, the story of its invention.
 * It's interesting to note that, in the world of the Ekumen, there is no faster-than-light space travel, but there are FTL weapons. It's explained by saying that nobody can survive faster than light speed but radio waves and unmanned ships have no problem. In Rocannon's World, there are even FTL weapons that have people inside them, to be launched in event of emergency; they're the equivalent of suicide bombers. There is, however, survivable near light speed travel.
 * Later into the cycle the problem of the Faster-Than-Light Travel gets solved, but as the means to do so are both psychic and mystical, the results are... varied.
 * Ender's Game is a major example of another writer taking the name from Le Guin. A character mentions that there is a formal name for their FTL Radio, but "somebody dredged the name ansible out of an old book." The working principle of an ansible link is something about "subatomic philotic links" that form between two subatomic particles, which can be stretched infinitely when properly separated. FTL travel doesn't exist though, so the ansibles are really the binding force for the entire interstellar civilization (to the point where one colony declares itself to be in rebellion and does the unthinkable by destroying their own ansible, or so it would seem...)
 * These links are also both The Power of Friendship and what holds the universe together.
 * AND how the Buggers/Formics communicate with each other and their drones (they EVOLVED to possess FTL communication!) It makes sense if you read the explanation of how these links work and what they really are.
 * Turns out the Piggies use them to communicate too or at least
 * Also, the super-AI/alien-hybrid/space-god, Jane
 * James Hogan uses a similar system (and also worked in the computer industry—for DEC during the PDP minicomputer days) in his Gentle Giants series of books where both communication and FTL travel are accomplished through rotating black holes. Toroidal black holes. A similar concept is used in his book The Genesis Machine, in which a Cold War era book about a machine that is initially used to pull information from remote places in real time without there being a device on the other end to transmit the data is turned into a superweapon to stop the Cold War in its tracks—by causing a nuclear exchange and destroying the missiles before they hit the ground.
 * Richard Morgan's Altered Carbon features "needlecasts", but no FTL travel. Interestingly, since Brain Uploading is routine, people may travel from planet to planet by downloading into a local body.
 * The information-only aspect is a power limitation, not an inherent limitation of the functionality. The information is transmitted by energy; the best matter-energy conversion reactors they have can hold open a needlecast portal for at most 15 to 30 seconds, and transmit only blinking light. They find a stable Stargate-style portal created by the precursors, extrapolate the energy costs, and realize that a hiccup in the portals matrix would crack the planet open like an egg. Hazards of children playing with adult toys.
 * In Time for the Stars by Robert A. Heinlein, telepathy is used for instantaneous communication. Eventually, scientists studying the process learn how how it works well enough to create Faster-Than-Light Travel as well.
 * Similarly, in E. E. "Doc" Smith's Lensman series, the title Lens enhances the Lensmen's telepathic abilities, thus allowing instantaneous communication between them.
 * There are also a variety of "waves" and "rays" that can be used for FTL communication and sensors by people who aren't Lensmen.
 * Likewise, in the Talents universe of Anne McCaffrey, Psychic Powers not only provide interstellar communication, but interstellar travel by psychokinetic teleportation.
 * In the Charles Stross novels Singularity Sky and Iron Sunrise, FTL communication is possible through the use of quantum-entangled particles, however: a) the communication is only possible between paired transceivers, and b) taking "qbits" through an FTL jump destroys the entanglement. This means that useful interstellar communicators have to be shipped the long way 'round, making them very expensive.
 * Notable subversion in James Blish's short story "Beep". Ansibles are common and cheap to use, if you can stand the loud and annoying beep that accompanies every one. Due to quantum effects, the title beep contain every message that ever was or will be sent, ever, and they can be heard if slowed way, way down and appropriately filtered. The government's primary purpose is to ensure that the events described in the beep come to pass at all costs, to prevent a paradox from prematurely ending the universe.
 * The Fatline in Dan Simmon's Hyperion uses modulated neutrinos (or some such Phlebotinum) to send messages instantly across interstellar distances. At the end of the second book
 * Alan Dean Foster uses the Subspace or Hyperspace version in his Humanx Commonwealth series. "Space plus" is hyperspace and is used for Faster-Than-Light Travel - it can also be used for communication but at the same speed as starships. "Space minus" is subspace and can be used for near-instantaneous communication, but at a very high energy cost. Later in the series, it's revealed that Sufficiently Advanced Aliens figured out how to travel in space minus as well.
 * In his remake of Design for Great-Day, Foster's Solarian Combine is a kind of galactic Hive Mind created as a natural extension of intelligent beings learning to live and think in harmony. Said thought processes apparently travel instantaneously, ignoring the speed of light. Of course, they've learned to circumvent this with starships as well; the Combine's ships can traverse a galaxy in a matter of hours, and intragalactic jaunts are considered to be fairly trivial, if not entirely routine.
 * Though it never actually comes up, the Discworld series has a bit of fun with this in a footnote. It points out that a Disc philosopher decided that the fastest thing in the universe is monarchy, noting that regardless of distance, the instant a monarch dies, the heir immediately succeeds them. He further proposes subatomic particles—kingons, or perhaps queenons—but his rough-hewn plans for artificially generating them by torturing a minor noble and using them to send messages vast distances were cut short when
 * The first of The Stainless Steel Rat novels by Harry Harrison feature psychics used explicitly as communicators over interstellar distances. Later novels don't seem to rely that much on conversations at distance.
 * Some Death World novels (which, supposedly, take place in the same universe) feature portable psychic locator beacons. Also, every ship is equipped with a psychic transmitter/receiver, making actual psychics obsolete.
 * Heightened stellar activity can interfere with FTL communications in the Wing Commander universe, as shown in the novel Action Stations (aka "Pearl Harbor In Space"), but otherwise, the only time there's significant time lag for communicating across interstellar distances is the human steps relaying transmitted messages to/from the comm system and the people ultimately at either end of the line.
 * Elizabeth Moon's Vatta's War series featured the operation of these  as a major plot point. Notably, interstellar FTL comm is possible, but not local FTL comm, due to the ansibles being full-scale space stations.
 * FTL communication is possible in Empire From the Ashes but very cumbersome, not available in sub-lightspeed craft and requiring exotic synthetic materials that starships aren't equipped to make. The mutineers sabotaged Dahak's and stole the only spares, so Dahak was forced to throw together a mundane lightspeed device instead in his attempts to phone home.
 * The popular Star Trek Expanded Universe novel Spock's World by Diane Duane tells the history of the planet Vulcan, and points out that since Vulcans are psychic to varying extents and telepathy is assumed to be instantaneous, telepathy was their first subspace ansible. The limiting factor is that only a small minority of Vulcans are strong enough telepaths for this to be practical.
 * In Infinity's Prism, there's a Star Trek: Voyager story set in an alternate timeline where Voyager didn't manage to cross Borg space at the end of season 3, and instead ends up building a new Federation analogue in the delta quadrant. The holographic Doctor becomes a distributed AI with one node sent back to the Vidiians to cure the Phage. That node's updates via subspace take a day (each way, I think) to make the trip to the Doctor's central processing core located near Borg space.
 * The Hitchhiker's Guide to the Galaxy itself has the "Sub-Etha" (occasionally written as "Sub-Ether"). In later books, it's even implied that a sort of Internet has been built upon the technology (it's used to automatically update the Guide to the latest edition).
 * FTL communication in The History of the Galaxy books is achieved using enormous spherical orbital stations, which use powerful generators and transmitters to punch through hypersphere to reach other worlds. During the heyday of the Confederacy of Suns, these were networked to create the Interstar, the future version of the Internet. As communication is vital between worlds, no one in their right mind would dare destroy an HF (Hyper Frequency) station, so they aren't even armed. A major plot-point of one novel involves a previously-unknown alien race launching a sneak attack on humanity by taking out the Interstar hub, isolating the human colonies. Ships also have their own HF generators, but those are usually not very powerful.
 * The Star Wars Expanded Universe has gone so far as to make its prevalence an Invoked Trope in The Corellian Trilogy. FTL communications are so commonplace that they're relatively easier to tap into, and so people sometimes use lightspeed communications over short distances (outer planetary orbits, tops) for security reasons.

Anime and Manga

 * In Macross Frontier, the Vajra are capable of interstellar communication via their instantaneously across the galaxy, without any sort of fold interference or delay. In fact, abusing this
 * The SolarNet in Cowboy Bebop.
 * The ability of the Emilys from Soukou no Strain.
 * Galaxy Network in Starship Operators.

Film

 * According to Avatar's wiki, the Pandorapedia, the Venture Star has faster-than-light communication technology with a low-bitrate device using quantum-entangling. This is a case of All There in the Manual, as this Ansible was never shown or mentioned in the film itself, though it is probably how Quaritch was able to confirm "corporate approval" for . Actual faster-than-light travel is notably averted.
 * In The Fifth Element radio broadcasts and telephone/video conversations are instantaneous.
 * Star Wars, of course. People communicate in real time through holograms, despite being separated by several light years. "Disturbances in the Force" can also be sensed instantaneously.

Comic Books

 * In Legion of Super-Heroes comics, communications are relayed through the same Stargate system used for FTL travel, making it possible to have a real time conversation between Legion HQ on Earth and the Legion Outpost in deep space. In the post-Zero Hour, pre-Infinite Crisis Legion, when the Stargate system goes down, people can still travel in "old fashioned" warp vessels, but the only means of communication is Titanet, a relay of Saturnian telepaths.

Live Action TV

 * Star Trek, as mentioned.
 * However, when the plot required it in the Original Series, they'd make mention that their messages would take several days to reach a Star Base.
 * They touched briefly on the logistics of subspace radio in the TNG episode "Aquiel," where the plot took place in what amounted to a subspace repeater station, an idea later confirmed in Enterprise.
 * Communicating over long distances is important for the crew of the USS Voyager, as they're stuck on the other side of the galaxy (and presumed dead), so even subspace communication won't work. After several failed attempts, they eventually succeed via Lost Technology and Applied Phlebotinum (as well as physically moving closer in the mean time).
 * Babylon 5's tachyon relays are a complicated bit of Applied Phlebotinum to justify this. It's established in-story that they're expensive and limited in bandwidth, with the result that most civilians have to rely on (hyperspace) snail-mail.
 * The Stargate Verse uses stuff like this on occasion, and it is explicitly stated that radio waves can travel both ways through wormholes, which work only one way for matter streams.
 * Besides simply sending radio signals through stargates, plain old FTL radio is used, the range of which can be determined by one episode which involves relaying an important message to Atlantis by sending someone with a subspace radio to the outer edge of the Milky Way in order to contact the Daedalus, halfway between galaxies.
 * Also, in the episode "Tangent," Teal'c and Jack are stuck on a glider that is on its way out of Earth's solar system at a speed where it won't reach its destination for over a century, thanks to a realistic space travel speed. The further from Earth they get, the longer the delay in the radio signal between the glider and Stargate Command. By the time they're rescued, the immediate transmission informing the SGC of that rescue is delayed by almost ten minutes.
 * The Ancient Communication Stones, which allow one to not only communicate, but INSTANTLY transfer your entire consciousness from point A to B. Distance does not seem to be an impediment - from thousands of galaxies away one can report to Stargate Command.
 * When the Tollan were first encountered, one of them used an FTL communication device to send a message to the Nox. When Daniel asks Omoc how the Nox are supposed to get it within any reasonable amount of time, Omoc grudgingly (due to the Tollan unwilingness to share technology) takes a tree branch and bends it so its ends touch. Daniel guesses that he's talking about space folding, only for Omoc to disappointedly drop the branch and reply "no".
 * Used with subtlety on Firefly. The orbit-to-land transmissions lack any delay at all, which is impossible if they were ordinary radiowaves. The Movie has several conversations between Mal in deep space, and somebody else on a far away planet.
 * Used without any justification Defying Gravity. It's far more jarring than in Firefly, because the rest of it is trying to be on the hard side of the scale.
 * IIRC, they Handwaved that in the first ep mentioning that the recent invention of some new instant communication device was what made the entire mission feasible.
 * Of course, the show only starts trying to be hard sci-fi. With the introduction of, all bets are off.
 * In Eureka, after Fargo and Henry build an FTL drive, the government starts planning the Atraeus mission, the first manned flight to Titan. One episode involves a scientist trying to build a Subspace Ansible for real-time communications and telemetry with the ship. Given that this is Eureka, this ends up causing a big problem (although, not by itself).
 * Later on, after Sheriff Andy is accidentally transported to Titan, Jack is able to communicate with him via real-time video.
 * The Mighty Morphin Power Rangers communicators, which are connected to the Command Center's teleportation system, can communicate across incredible distances.

Tabletop Games

 * In a typical Warhammer 40,000 take on an otherwise innocuous trope, FTL communications within the Imperium are the responsibility of Astropaths, psykers who send telepathic messages to other planets... through hell...
 * And frequently, the messages either arrive too late or not at all or are completely incomprehensible due to the receiving psyker not being able to work it out via space tarot cards or throwing chicken bones or... Well, there's one example of an Imperium ship going to its death responding to a distress call sent by its future self.
 * Eclipse Phase's Quantum Entangled Communicators provide instant communication regardless of distance, but like those in the Charles Stross example rely on pre-linked, finite "qbit reservoirs".
 * Since the vast majority of what's left of posthumanity is still in the solar system most people just deal with a few minutes or hours of lag with radio, laser, or (speed-of-light) neutrino communications. Or farcast a fork of their ego if they absolutely have to have a real-time conversation. QE comms are mostly used by spies or to contact the few exosolar colonies that have been established.
 * GURPS: Ultratech has two versions. The first uses quantum entanglement (so it's impossible to intercept) and the second is generic FTL Radio.
 * The BattleTech universe has Hyper Pulse Generators, which send messages between star systems instantaneously. It's been stated in canon that sending a message via HPG can often take anywhere from a few days to weeks, but that is mainly because the cost of transmission is so high, the operators will wait for a large number of messages to be sent at once.
 * Just don't mention the TV series, which had a realtime video conference using said HPGs.
 * That would be a HPG relay, set up by using two HPGs at once. It has been used in the novels and is noted to be very expensive.

Video Games

 * It is never explained how exactly communication works in the Descent: Freespace games. Seemingly, instant interstellar communications do occur in that universe, which means FTL communications. Command can also communicate with you as your ship is traveling in subspace, though the messages break up and distort like a bad telephone connection.
 * Similarly, the Wing Commander series explicitly uses FTL radio with no particular explanation given for the differences between how it and jump drives work.
 * The film shows there are limitations to FTL radio, as Admiral Tolwyn needs to send a message to the Tiger's Claw, but it is too far away to reach, so he instead transmits the data to Paladin's ship to be delivered by hand.
 * Mass Effect allows faster-than-light travel by giving physical objects zero or negative mass or through the use of large and fairly rare mass relays that create long tunnels of space where everything has zero or negative mass. Communications often take place through large numbers of comm buoys deployed throughout explored space, which transmit messages via photons passed through miniaturized corridors of mass-free space. Time lag in real-time communications occurs if transmitter bandwidth is overloaded, though high-level government operatives and military personnel get access to high-end, high-speed channels that allow near-instantaneous communications. Long-range interstellar calls are implied to be highly expensive, judging by one overheard conversation on Noveria.
 * The second game also features a quantum-entanglement transponder which allows the Illusive Man to contact Shepard instantly while conveniently avoiding the relay network. While this is efficient, as it bypasses the limits of bandwidth and time delay, it is highly inefficient for mass production, as it's basically a two way radio with a truly astronomical price tag - according to EDI "each quantum pair costs almost as much as a comm relay." It only communicates between the Normandy and the Illusive Man's hideout. If they wanted to contact somebody else this way, they would need to do more quantum entanglements.
 * The existence of Subspace Ansible also Handwaves the fact that Harbinger is capable of "ASSUMING DIRECT CONTROL" of individual Mooks, despite being a Reaper whose physical form is drifting thousands of lightyears outside the galaxy.
 * Fridge Brilliance: Done by implanting every Collector with part of an entangled pair which the Reapers can surely manufacture more easily than Cerberus, being Eldritch Abominations so far above us technologically.
 * The third game has quantum entanglement communication units practically everywhere, after the Systems Alliance takes apart the Normandy SR-2 and finds out all the enhancements Cerberus made to it. It seems as if they're all connected to each other, which wouldn't make sense given EDI's comment about the device in the second game (i.e. a single communication unit can only connect to its partner).
 * The plot of Commander Keen 4 starts with Billy Blaze building a FTL radio and accidentallly picking up a transmission from a group of aliens planning on destroying the galaxy.
 * EVE Online uses pairs of particles which are synchronized with each other: Manipulating one affects the other as well, no matter the distance between the two particles. This was used to create a galaxy-wide network (not unlike the Internet) where latency due to physical distance is not an issue. Carrying one half of a particle pair on a spaceship allows one to "phone home" from any part of the galaxy.
 * This is based very loosely on the concept of quantum entanglement. It is possible to create pairs of particles both of which are both in superpositions of quantum states (e.g. spin up and spin down at once) and can be separated by a substantial distance. Observing the spin of one of the particles forces it to collapse into one state (either spin up or spin down, but not both) observations of the other particle are then guaranteed to see it in the other state. The particles seem to have needed to communicate faster than light to make this coordination. Unfortunately, in the real world, this does not help us communicate faster than light—when we collapse the state of the first particle we can't force it to enter one state or the other, so it's simply random noise which happens to be correlated with random noise somewhere else in the universe, but not a communication channel.
 * The Homeworld verse is implied to have FTL communication (the Veer-Rak alerting the Kuun-Lan which is in another system that Hiigara is under attack at the beginning of Cataclysm, for example) but it's never explained. FTL sensors however, are a bit sketchy: the manual of the first game explains that the Mothership has a cobbled-together short-range sensor that can see if the ship is passing near a high-mass object in realspace but they have to drop out of hyperspace to actually see what it is. Homeworld 2 has Advanced Sensor Arrays buildable on Hiigaran super-capital ships that can detect enemy ships in hyperspace.
 * Every (playable) race except the Liir (see aversions with FTL travel below) in Sword of the Stars.
 * Humans the the Zuul actually use regular radios to send and receive messages. They just leave relay buoys near node space points that re-transmit the messages through to the other side. Given this All in The Manual description, this method is far from instantaneous.
 * The Tarka actually start without one but can quickly research it in order to be able to send orders to their ships while in flight.
 * The Hivers use tiny gates hooked up to their Portal Network used to send and receive messages. While ship-sized gates require being in the gravity well of a planet-sized object, the mass of the ship is enough for a communication gate.
 * All races seem to have FTL sensors, though, which the manual doesn't even attempt to explain. Also, one of the prerequisites for interstellar trade is FTL "Broadband", which carries the side effect of letting you observe allies' combat.
 * In Dead Space 2 the player sees a recorded conversation between Isaac and Nicole, which happens in real time between earth and The Ishimura.
 * In Descent 2 has a real time FTL communication connection that somehow keeps working even when the ships warp drive malfunctions and deposits the character in the middle of nowhere with the ship broken.
 * In the Alien Crossfire expansion to Sid Meier's Alpha Centauri, the Progenitors have FTL travel and communication technology. However, since their ships were destroyed, the survivors have to re-discover the latter and use it to build powerful FTL transmitters to alert their fraction's fleet. Doing so results in automatic victory for the builder, as the massive fleet that arrives shortly after the message is sent is assumed to crush every other side. It can also be assumed that, after everyone on Chiron undergoes Transcendence, the various parts of Planet (including humans) become parts of its Hive Mind. The final interlude mentions a ship sent by Planet to rebuild the devastated Earth. This would imply Planet can instantly communicate with its parts across light years.

Web Comics

 * In Schlock Mercenary, the main means of communication is the Hypernet, a refinement of the ansible idea as a packet-switched system that relies on tiny wormholes. It carries all communications in digital form, from real-time video to text mail, and it's explicitly said that the lag time in the sender's electronics is more significant than the infinitesimal transit time. The cartoonist knows a thing or two about packet switching; his former day job was for Cisco Systems.
 * Which is why ships that expect a battle deploy remote probes, and sometimes pre-launch interceptors, missiles with sensors, and so on - big sensors are better, but ability to win time even from light speed lag often matters more. Also, teraport normally starts with sending a bunch of miniature probes first to see whether they'll go "splat" on Teraport Area Denial or see something unpleasant on the other end, though it's mentioned only in rare cases when the probes were expected to make it, but didn't, or when the ship runs out of probes and have to improvise.
 * The same applies to area defence: if you're worried about relativistic projectiles, what you need is some ships with enough of (gravitic) juice to deal with such threats, plus lots and lots of drones to detect the problem while you still have time.
 * Also, hypernode jamming or even lasting disruption is possible, though at relatively short ranges. Once the jamming area was appraised by looking which opponents reacted after light from an event could reach them, and which ones before, and thus are able to use information from sensors somewhere closer.
 * The Starslip technical manual asserts that FTL communication is actually an incredibly sophisticated computer that anticipates what the other party will say. "Modern" systems are so good, it can predict and initiate a call at the same time it is placed, light-years away. FTL Travel is almost as preposterously handwaved.
 * Only at first though (and hell, the strip is NAMED AFTER their FTL system), the detailed explanation becomes a major plot point and the titular Starslip Crisis when the future declares war on the past to make them stop using it.

Web Original

 * In Orion's Arm messages can be sent faster than light via wormhole. One side effect of this is that some entities use systems of wormholes to cheat the normal limitations on processing speed which makes them far more powerful than they could possibly be otherwise.

Anime and Manga

 * The time taken for messages to reach to the other recipient is practically the entire point of Voices of a Distant Star. There is FTL Travel in Hoshi no Koe.
 * There's also, briefly noted, some form of subspace communication. But it's only there for irony—a newspaper shown for a moment has a report on the next generation of starships, the ones built after Mikako's fleet leaves. The new ships will have this form of FTL communication; the fleet Mikako is in, however, does not.
 * And, to make the irony even more cruel, -- Noboru is a comms officer on one of these new ships.
 * Communication across large distances is virtually impossible in the Super Dimension Fortress Macross universe; it is actually a plot point in Macross Frontier, where even relay buoys must contend with "fold interference" that delay or disable communication entirely. However, the Protoculture and the Vajra have their ways around that (see above.)

Film

 * Dune.
 * Alien. The Nostromo and other ships are out of contact while on the frontier, and are thus on the spot, for taking on the terrors that wait on whatever God forsaken planet the crew finds themselves on.
 * However, the sequel, Aliens, explicitly uses FTL communication, as the Sulaco is expected to report back to Earth on a daily basis, and the lost contact with Acheron is noticed after a matter of days, not decades. Since this communication is not explicitly accomplished by means of FTL drones or somesuch, the presence of an ansible is implied.
 * In the Extended Edition opening, one of the colony higher-ups on LV-426 says that "[i]t takes two weeks to get an answer out here", implying that the speed of messages is no more than one week either way (as the delay may be partially due to Executive Meddling or other delays on the end of the replier).

Literature

 * In Poul Anderson's Technic History novels and short stories, there is no "interstellar equivalent of radio" and all messages have to be carried by courier spaceships.
 * Stephen Harper's Silent Empire Quadrilogy exists in a universe with FTL flight, but no FTL communication. This necessitates the use of psychics for communication. This universe has a thriving slave trade for the same reason.
 * Ditto in The Gap series by Stephen Donaldson. Courier drones are the order of the day—although the Amnion do briefly try to use some sort of experimental "symbiotic crystalline resonance device" as an FTL Radio.
 * The CoDominium universe and The Mote In God's Eye by Larry Niven and Jerry Pournelle.
 * Honor Harrington has courier ships to carry messages between star systems, but because of the delay, there's no way to control battle groups in multiple systems with military precision. Early in the series, The Kingdom figures out how to create a comm system using gravity pulses, but this only works within a system, and initially is roughly as fast (in terms of bandwidth) as Morse code. The technology undergoes multiple improvements, and in the later books is capable of carrying video.
 * At one point, a Manticoran diplomat intentionally exploits this trope just to issue a calculated insult to a Solarian Admiral, by keeping his video feed on and using the lengthy comm delays to kick his feet up on a nearby piece of furniture and catch up on some casual reading.
 * David Feintuch's Seafort Saga: while FTL travel existed in the form of "N-Waves" propelling a ship, even human-level computer AI couldn't run a ship (robotics not being advanced in Feintuch's 'verse). Physical mail was carried by the ships traveling to extrasolar colonies, while ordinary radio was used in-system.
 * H. Beam Piper's Terro-Human Future History. News and communications travel with FTL starships—taking months to get to their destination, and months to get an answer back—but no FTL radio exists. This has a major impact during the System States War, in his novel The Cosmic Computer, where planning and controlling fleet and army movements has to be done for a theater of war thousands of light years across. In fact, the Terran Federation's "Manhattan Project" is to create a super-powerful computer capable of evaluating not only military but social and economic factors and effectively predict the enemy's future actions, so that the Terran generals can determine just where to send troops and ships.
 * It can be said that Piper and other writers who have FTL travel but no FTL radio setups in their works often do so to justify why the protagonists must handle the issue at hand and can't just call back to base for help/relief/support from more qualified personnel.
 * They come the closest in the Empire story "Ministry of Disturbance", where a scientist discovers a method of teleporting a particle. Unfortunately, the only story following "Ministry of Disturbance" took place several thousands of years after that one and on a backwater planet, so Piper never managed to explore the consequences of this new technology.
 * In Peter F. Hamilton's Night's Dawn Trilogy, it is possible that certain alien races have developed FTL communications, but humans certainly have not. Information is couriered around by manned starships.
 * In his Commonwealth series, constantly-maintained planetside wormholes allow very rapid transportation and communications from one end of the Commonwealth to the other. The most common method of traveling from planet to planet is by train, spaceships having been made redundant.
 * In Dune, the Spacing Guild is responsible for getting information from place to place. In the books by Frank Herbert's son, two Navigators manage to contact each other FTL with a device one of them cooked up, but that never caught on, mainly due to the unfortunate side effects (namely, fatal cerebral hemorrhaging after several uses).
 * Technically that was a Navigator and his (human) twin, elsewhere it was shown that two Navigators could communicate directly mind to mind in realtime. And the navigator half died of pressure induced hemorrhaging, while the brother actually fried his brain from the inside out.
 * They never even try to perfect the technology, so that there are no side effects. Probably because instantaneous communication would mess with the Guild's profits (and storytelling).
 * Those two navigators also died, by the way.
 * Lois McMaster Bujold's Miles Vorkosigan novels use wormholes as shortcuts through space; unfortunately, there's no way to send messages through a wormhole without recording them and putting them on a ship. High traffic routes have regular courier ships shuttling the mail back and forth through the wormholes. On low traffic routes, your mail may sit for weeks for someone to come along and carry it on. Once on the other side, a message may be beamed at the speed of light to the next wormhole (if there's a permanent station there) or it may be carried on a ship the whole way.
 * The lack of FTL communication plays as a plot point a few times; in The Vor Game, Miles uses the time lag between communications to play with the head of his compulsive plotter opponent,
 * In Isaac Asimov's Nemesis, the humans left on Earth eventually discover faster-than-light travel, but point out that there is no way to send radio waves through hyperspace. This leads to difficulties communicating with the Nemesis colonists.
 * Averted in C. J. Cherryh's Alliance Union and Chanur Saga series. Communications between star systems can only be done via FTL courier, and FTL travel is only possible between the outside edges of star systems. When a space ship arrives at the edge of a system and travels inwards it takes a long time for light-speed messages to reach the inner system (or visa-versa), with the time shortening as the ship hurtles forward at a considerable fraction of the speed of light.
 * In The Lost Fleet series, the delay in receiving electromagnetic signals plays a large role in the various spaceship battle tactics shown. For example, at the opening of the first book, Geary uses the massive delay in communication with the enemy to buy the time he needs to rearrange the title fleet for a Tactical Withdrawal. Even then, the fleet barely makes it.
 * In Hellspark by Janet Kagan, there is FTL travel but no FTL communication. Messages have to be physically transported, and there are severe penalties for interfering with a ship carrying mail. Expeditions planning to spend time away from the regularly-travelled routes may take unmanned message drones, but these are expensive and hard to replace, and are kept in reserve for emergencies.
 * In Timothy Zahn's The Conquerors Trilogy, there are two possible FTL speeds for ships allowed by physics, but faster-than-light communications are impossible.
 * L. Ron Hubbard's Battlefield Earth features the Psychlo Empire, a civilization linked by the miracle of teleportation. Here's the thing, though - the rules of teleportation make opening two "links" to the same planet dangerous, so the Psychlos set up a strict schedule of when their worlds can link up with the capital. This means that the disparate worlds of the empire have to function on their own for a year or more, until they reach the hours-long window to exchange communications and supplies and personnel. Aside from the inherent problems with this scenario, this means that once the protagonist bombs the Psychlo homeworld into a new sun, there's no way for the rest of the empire to know about this until they try to open a teleport link to it and instead get a facefull of atomic fire.

Live Action TV
"Dylan Hunt: I hate physics."
 * Notable in its absence in Andromeda, where communication over interstellar distances requires a living courier.
 * It's mentioned that they attempted to use Slipstreams for FTL radio, but that it didn't work very well, so they went back to couriers.
 * This is because Slipstream is highly unstable. Traveling through it requires one not only to be a good pilot but to be lucky (something about quantum physics). It is stated several times that a machine, even an AI, is unable to successfully navigate in Slipstream, as machines can't guess. An attempt was made to map out Slipstream, but it failed.
 * In one episode, Hunt laments the need to wait for a reply when the Andromeda is more than a few light-minutes away.


 * The 2000s Battlestar Galactica apparently relies solely on radio (referred to in-story simply as "wireless") for ship-to-ship communication, and faster-than-light comm does not exist; ships on recon missions must be provided with rendezvous points before departure lest they never be able to find the fleet again.
 * However, the presence of some kind of FTL transmissions is implied with the Cylons. They have possess tracking beacons mentioned early on that can be traced after an FTL jump (see the Olympic Carrier in the first regular episode), plus the very nature of Resurrection technology requires FTL transmissions, as the Resurrection ship/hub is often shown as being in different star systems from the point of the Cylon's death and yet works relatively fast.

Tabletop Games

 * Traveller, where all interstellar messages have to be delivered by courier (although those can still jump). This has a natural impact on the way the universe works.
 * Deliberately invoked in The Fifth Frontier War, a tabletop board game based on the Traveller universe. Players were required to plot moves multiple turns in advance. The presence of Admirals, intelligence gathering, or other factors could reduce the preplanning by one or two turns.
 * Note that this is almost exactly the same problem faced in H. Beam Piper's universe.

Video Games

 * Halo. All messages are carried on ships that travel between planets. Described as the futurisitc Pony Express.
 * This is more a UNSC problem though as the Forerunner definitely had FTL communications (it's unclear if the Covenant have this technology or not). Some scenes also suggest that the UNSC has at least "short" range FTL sensors as for instance they seem able to detect ships incoming at FTL speed somehow. It's also notable that their never seems to be any delay in communications even over large distances inside systems.
 * In the books it's shown that they'll send probes into slipstream, which will then scan for anything traveling FTL before returning to regular space, and then transmit the data back.
 * The book Contact: Harvest has a Kig-Yar (Jackal) Privateer ship find the human colony of Harvest. All Covenant ships are equipped with a Forerunner device known as a "luminary", which scans for Forerunner energy signatures (and other factors) and relays the data back to High Charity. Tampering with luminaries is forbidden under the pain of death (not the least reason being that they're Forerunner relics and are, thus, holy). The Kig-Yar attempt to do just that in order to prevent their luminary from letting the Covenant know about the discovery. They fail. The rest is history.
 * The Liir (dolphin-like aliens) from Sword of the Stars use communication drones equipped with FTL engines. For gameplay reasons, there is no delay even at very long distances that require many years for the ships themselves to cross.
 * The drones have a much smaller mass, meaning teleportation calculations would be much quicker (i.e. faster "movement").
 * Technically, the Liir don't have FTL. Their "stutter-drives" are teleporters, moving a ship a tiny distance millions of times per second, imitating physical movement. It's difficult to imagine a computer able to do this faster than the speed of light. It's likely they move at very high STL speeds and are not experiencing Time Dilation effects due to the ship actually standing still at any given moment.

Web Comics

 * In Freefall, it's specifically said that the only way to communicate between star systems is by sending messages via ship. As most interstellar travel is via sublight vessels (FTL Travel being quite expensive, and possibly dangerous going by the drive's name: Dangerous And Very Expensive ] drive), this means that a message and its response could take up to many months, as the ship carrying it travels on its normal route.

Anime and Manga

 * Planetes features a phone on a lunar colony with a Windows-esque progress bar on a video screen that gives the delay between messages between the Moon and Earth (about a second and a half)
 * Stellvia of the Universe has this as a plot point. Videomail between Foundations travels slower than light, so it takes hours for the messages from Shima and Rinna to reach each other.

Literature

 * 2001: A Space Odyssey, in which an interview with astronauts on their way to Jupiter has to be edited together to remove the long delay times in the conversation.
 * Greg Egan's Schilds Ladder features a universe with neither ftl communication nor travel. Being written by Greg Egan, most interstellar travel is done by transmitting your mind, with only a few "anachronauts" crawling around in starships to investigate the future they find along the way; either way all travellers are doomed to miss decades of time. One planet solves the problem by putting the entire population into what amounts to slow-motion until the traveler returns.
 * One of the entries in the X Wing Series has the squadron tapping in on a pirate conversation. They figure out how far apart the two conversing parties are by the length of the pauses, and by doing so, pinpoint their locations.

Video Games

 * Usually, the Metroid games avoid this by having no form of communication through space, but in Metroid Prime: Hunters, it's stated that a signal was sent to various bounty hunters via a telepathic frequency.

Real Life

 * Real Life, so far. This clip shows how tricky surface-to-orbit conversations can be without an ansible to hand.
 * Even Digital Satellite TV (e.g. DirecTV, Dish Network) is delayed about 4 seconds relative to the cable feed the shows come from. This delay is mostly due to the processing time necessary to compress the data on-the-fly, however; the actual signal-propagation delay to and from the satellite is only a quarter of a second.
 * Recently, scientists have discovered that Entanglement is possible in macroscopic scale. Quantum entanglement experiments make it seem as if systems are somehow "communicating" faster than light through correlations between their states, although if quantum mechanics is correct it's (provably) impossible to actually use this to send information faster than light.