Write Back to the Future

So, your friend couldn't get through the one-way time portal before it closed, and as you're standing there wondering what to do, this guy from the Post Office comes up and says that he's got this letter, posted 80 years ago, and addressed to you. Turns out your friend spent ten years building a machine to stop the Big Bad, and he sealed it away right beneath where you're standing now. As you read it, you can't help but marvel that the message actually made it through the postal service.

Write Back To The Future is when a time-traveler stuck in the past sends a message along The Slow Path to their compatriots in the future. It may be a way to get them back; it may be the solution to the problem they're looking for; it may just be an assurance that they had a full and rewarding life in whatever century they got stuck in.

Usually, the message shows up right after the relevant time-travel. In especially clever instances, it may appear before said travel.

The likelihood of this actually working depends on the method used to send the message. If you just drop a letter in a drawer, you're betting that nobody's going to move stuff around for the next sixty years, the desk isn't going to get moved, destroyed, or sold off, et cetera. On the other hand, if you put it in a safe deposit box in a major bank or even behind a loose brick in the masonry, it should be there decades later.

Often, but not always, part of a Stable Time Loop. Inspired by, but not to be confused with what Robert Zemeckis and Bob Gale did.

futureme.org allows you to do this yourself.

Anime & Manga

 * Rave Master. When Haru, Elie, and Sieg Hart travel unintentionally 50 years into the past, Sieg manages to . He then proceeds to write letters to all their friends in the present, informing them of the final battle so they may help the hero. Then he sits down on a stone in order not to change the past and guards the only place where that time travel would be possible,

Card Games

 * The game Chrononauts revolves entirely around time travel, and uses this trope. One of the cards is a "Memo from your Future Self," and basically acts like a counterspell via going back and time and stopping the previous card from being played.

Comic Books

 * In a Big Finish Judge Dredd/Strontium Dogs crossover, Johnny Alpha returns to his own time by leaving notes giving his exact location for his employers to find. Ready to leave, he gives such a note to Dredd, then immediately starts accusing him of betrayal when he fails to dematerialize. It turns out that without his help, Dredd won't survive to deliver the note.
 * Superman Vs. The Terminator: Death To The Future #1 has Cyborg Superman leaving information for the future Skynet in a graveyard sculpture.
 * In the Return of Bruce Wayne story arc, Batman time-travels forward and leaves clues to his whereabouts in and around the past location of Wayne Manor.

Fan Fiction

 * In the Ranma ½ fanfic Exact Change by Ozzallos, Nabiki Tendo does this.

Film

 * Probably the best-known modern example is the letter Dr. Brown writes to Marty at the end of Back to The Future 2. This is an example of "I'm happy in the past", but Marty finds evidence that Dr. Brown is killed a week after writing that letter, thus prompting him to go back and save him.
 * Marty tries to do this in the first film, with a 'Do Not Open Until 1985' letter talking about how Doc gets gunned down. Doc tears it up while proclaiming he can't let it influence the future.
 * Inverted in the Film The Lake House: the letters time-travel; the people take The Slow Path.
 * In the movie 12 Monkeys, as Bruce Willis' character prepares to travel from the future, he is given the telephone number of an answering machine whose tape was found in archaeological research; the whole end-of-the-world problem ensured the tape was not erased for reuse (say that ten times fast).
 * In Frequency, the father sends his wallet (with the killer's fingerprints on it) to his son 30 years later by way of hiding it in a very out-of-the-way spot in the family house (which the adult son still lives in, even though his mother has moved to an apartment).
 * At another point the father communicates with the son by burning a message into the oak table both of them are using.
 * Edward Johnston does this in Timeline, the film of the Michael Chrichton novel. also, one main character muses over what the life story of a knight buried in a recently-discovered grave might be. At the end, it is revealed to be his own grave, with an inscription to those of his friends who returned to the present.

Literature

 * The novel Good Omens has a variation, where the author of the "deliver this message to this specific person at this address on this date" message is not a time traveler, but has the ability to see the future. And includes extra messages, addressed by name, for the three people who try to open the message before it reaches its intended recipient.
 * To elaborate: Agnes Nutter predicted events of future generations, leaving a record for her descendants to consult, one prophecy at a time, in centuries to come. By the time the story is set, intervening generations have annotated many of these documents, including some ribald comments on the page that predicts Agnes's modern-day descendant will have sex with a particular other character.
 * The novel Timeline had several of these. The first clue that time travel is happening is when the head scientist visits the time travel corporation, and suddenly his archaeologists find a thousand-year-old parchment with his handwriting on it, as well as his glasses. At the end of the book, the archeologists find the grave of one of their team who stayed in the past, with a message for them in the epitaph.
 * Lazarus Long uses this trick in Robert A. Heinlein's Time Enough for Love, when he's sent back in time to the early 20th century. It's a complicated set of multiple letters inside letters (with different instructions and dates on which to be opened), since he's writing it to a time two thousand years into the future. The recipients of the letters then use the date they stop being sent to pinpoint when to rescue him.
 * In DJ MacHale's The Pendragon Adventure series, the third book, The Never War, has lead character Bobby Pendragon send his journals detailing what happens during his stay on "First Earth" (Earth, circa 1937) to his friends/acolytes/helpers Mark and Courtney by locking them in a safe deposit box in the town's bank.
 * Also in the eighth book The Pilgrims of Rayne where Aja Killian leaves a message hoping the future will find it.
 * In Isaac Asimov's The End of Eternity, Cooper sends a hidden message through an ad in a magazine, to inform Harlan of where (or to be exact, when) he is.
 * Jack Finney wrote a story in which the main character sent people into the past who wanted to "emigrate" there. They would get themselves in photos—usually around the edges of group photos, for some reason—to let him know they'd arrived safely.
 * Inverted in The Riftwar Cycle book Into a Dark Realm. Pug has a magical box which he uses to send his past self messages.
 * Actually, it is revealed later that
 * In The Hitchhiker's Guide to the Galaxy, Ford and Arthur do this by accident when trapped on prehistoric Earth. Arthur drops his towel during an earthquake, it gets fossilized in a lava flow, then thrown out into space when the planet is blown up by the Vogons two million years later and picked up by the Heart of Gold, where Zaphod interprets it as a message and comes to rescue them. Of course, the Heart of Gold is powered by the Infinite Improbability Drive, so things like this are par for the course.
 * According to the Retcon in The Tertiaty Phase, though, this all happened in the artificial universe in Zarniwoop's office, thereby explaining why the 'real'' Ford and Arthur are still on prehistoric Earth when the Phase begins. (The actual reason is that the Tertiary Phase was based on the third book, while most of the 2nd radio series never happened in the books, and the Retcon was attempting to incorporate the 2nd radio series into the continuity)
 * This trope is mocked in Life, the Universe, and Everything when Arthur and Ford end up on Earth shortly before it's blown up. Arthur gets the idea to warn himself, but Ford points out that it wouldn't work by doing an impression of the hypothetical phone call "'Hello, me? It's me. Please don't hang up.' *shrug* He hung up. This is NOT my first temporal anomaly, you know."
 * Done without the time travel - sort of - in Joe Haldeman's The Forever War, in which Marygay leaves a note for William at the front of his army record, knowing that this will be kept safe to give to him if he survives the war, even though near-lightspeed travel has caused their personal timelines to diverge and therefore he's hundreds of years in her future.
 * The protagonist of The Anubis Gates by Tim Powers sends a message from the past to himself, jotting a note on a book in Pig Latin. This isn't so much an attempt to convey information—he'd already seen the note, and been surprised by it, at a previous point in his time-traveling adventure—so much as a way to self-seal a Stable Time Loop and ensure his earlier self will pay attention to that particular book.
 * Likewise, at the end of Artemis Fowl: The Time Paradox, Artemis
 * The part of the Plot of Robert A. Heinlein's The Doorway to Summer
 * Isaac Asimov's Foundation series features a non-time traveling version of this: Psychohistorian Hari Seldon, having mathematically predicted the future, leaves a set of what amount to video recordings set to automatically play at specific times to reassure the people of the future that they're on track. The fact that one of them turns out to make no sense (well, little sense, one of the characters mentions they were thinking about doing that, but changed their minds because of the current crisis) is the evidence that convinces everyone that the villain called The Mule is a much more serious threat than they had previously thought. (And the fact that after The Mule is defeated the predictions start making sense again is the evidence used by the Foundation to show that a hidden Second Foundation, dedicated to keeping things on track, must exist.)
 * In Connie Willis' time travel books, the historians use so much space in the classifieds trying to communicate with each other and the future that you begin to wonder if any classified ads are "real".
 * There's another short story by Finney where a man in the present day sends out an unaddressed letter complaining of his loneliness, and then a few days later finds a hidden compartment in his antique desk containing a letter from a lonely Victorian woman who received a strange, unmarked letter and wished she knew who to reply to. He writes another unaddressed letter, and again finds a hidden compartment in his desk that contains a letter from the Victorian woman. He realizes that there's one last unopened compartment, and writes to the lady explaining that this will be their last exchange and he wishes for something to remember her by. When he opens the last compartment,.
 * In Diana Gabaldon's Outlander series, Claire and Jamie write to their daughter and son-in-law, storing the letters in a chest addressed to their oldest grandchild.

Live Action TV
"As well as the danger of tangentially altering the timeline in some way, it relies on your messages being undisturbed for decades and subsequently found at the correct time. This is harder than it might seem."
 * Doctor Who:
 * In "Battlefield", the Doctor finds a note in his own handwriting with the body of King Arthur, a message from his future self.
 * "Blink" revolves almost entirely around such messages. First, Sally finds a message addressed to her written behind the wallpaper of a derelict house. When she returns with her friend Kathy, Sally answers the door to someone with a letter for her while Kathy goes upstairs. The letter is from Kathy saying she had a long and happy life, delivered by her grandson; meanwhile Kathy is sent back in time. Another character is asked to keep a message and take it to Sally by The Slow Path, and DVD Easter Eggs are inserted into seventeen unrelated DVDs . At the end,.
 * His conversation with Sally is even more impressive, since the two actually have a dialog using the DVD easter eggs. It turns out that Sally's companion is a conspiracy theorist who has been fascinated by the easter eggs for years. When he hears Sally's responses, he writes her side of the conversation down in shorthand so he can post it to his conspiracy newsgroup. The Doctor later gets a copy of her responses and uses this to record his side of the conversation. Thus the entire conversation is an ontological paradox.
 * River Song uses this tactic all the damn time to get the Doctor's attention, leaving messages in places that are sure to get his attention sooner or later. (It doesn't matter when exactly he finds them, because he can land the TARDIS precisely at the time she asks, even if that was millenia prior.)
 * And in the Big Finish Doctor Who adventure "The Kingmaker", the Doctor and his companions, separated by two years when the TARDIS is stolen (by ) exchange notes by leaving them in the care of the owner of the eponymous tavern. The letters left by Peri and Erimem travel forward to the Doctor in the usual way, while the Doctor leaves his own notes on the assumption that one of his future selves can someday collect them and deliver them to the past.
 * Data's head was used for this, in the Star Trek: The Next Generation two-parter "Time's Arrow".
 * In the episode of Torchwood where Jack and Tosh fall through a crack into the 40s, Tosh leaves notes for the others to find that contain the data they need to work out how to get them back. Requires extensive Willing Suspension of Disbelief, as the message is in two parts: one in an electrical box and one in a tin can in a basement. The chances of them going undisturbed for about sixty years, and Gwen managing to find them both in one day, in the right order, boggles the mind.
 * Similarly, in the series 2 episode "To the Last Man", 1918 Torchwood leaves "time-locked" instructions for 2008 Torchwood to follow regarding the 1918 soldier who's been cryo-frozen in the vault for all this time (except 1 day a year).
 * The Torchwood website advises agents not to try this:

"Sheldon: And initial here, stating that if we ever discover the secret to time travel our first trip will take us to a point five seconds in the future from now. [Leonard initials. They sit for a moment, waiting.] Sheldon: Well, that's disappointing."
 * One episode of Quantum Leap had Sam and Al switching places, and Sam was locked in the imaging chamber. Sam had Al write a letter to a lawyer with the code to unlock the door, which opened immediately when Al put the letter in a mailbox.
 * This was done in reverse in the Stargate SG-1 episode "1969". General Hammond gives a note to Carter as they set out on a mission that ends up sending them to the aforementioned year. It ends up being read by his younger self, who takes the message to heart and helps the team out. In other episodes, the team uses this trope to send a warning to the past not to travel to a certain planet, and on another occasion, they record a video message of themselves to explain their situation to the alternate timeline that resulted from their meddling with past events.
 * This also comes up in an episode of Stargate Atlantis. Due to a freak cosmic accident, Sheppard is transported 48,000 years into the future. In order to help him come back to the present, McKay programs a sophisticated, artificially intelligent simulacrum of himself and leaves it in Atlantis where it waits until Sheppard arrives.
 * In one episode of Friday The 13th The Series, a character tries, and fails, to do this, hiding a note in a desk destined to become an antique. It isn't discovered until after she returns to the present.
 * Worse,
 * In Babylon 5, after  writes a letter to , who delivers it to   just before the beginning of the sequence of events that end in   traveling back in time.
 * also wrote a letter to
 * Heroes: Hiro, in the past, writes an account of his adventures on a series of scrolls and hides them in the hilt of a samurai sword. Conveniently, even though Hiro has been carrying the sword for several episodes, Ando only finds the scrolls after Hiro travels to 1671. (And somehow he gets them in the right order, too.)
 * And considering it's the sword of a famous samurai that has been passed around to many owners over hundreds of years, it's a miracle that not one person has ever noticed that the bottom says "Ando, open" or that there are scrolls falling around inside.
 * Not so unlikely if  had kept it all along, and Linderman only got ahold of it when.
 * Reversal: In Early Edition, the main character mysteriously receives newspapers a day ahead of time. He then spends the rest of the episode trying to avert the tragedies the newspaper describes before they happen.
 * Used pretty well in the sci-fi action series Time Trax—the main character was able to arrange for someone to be taken to the future by dosing the person with the needed time travel drug, then leaving a coded classified ad in a paper that his team in the future was monitoring.
 * In the sixth season of Charmed, when Chris gets dragged back into the future by the sisters write a spell to give him back his powers and stick it under a loose floorboard mentioned previously in the episode. Twenty years in the future, Chris pries up the old loose floorboard, finds the note, and returns to the past with his newly-recovered powers. The sequence is also an example of San Dimas Time, as the sisters 'race' to write and place the spell 'before' Chris opens the floorboard, and were surprised when he re-emerged just a few moments after they planted the note.
 * Though this is justified when you consider that the girls don't really understand time travel, despite attempted explanations by Leo.
 * At the end of Goodnight Sweetheart, when Gary finds himself trapped in 1945, he writes a message of explanation and apology to Yvonne on the wall of his flat and then wallpapers over it - knowing that they're just about to have that wallpaper stripped in the future (where his friend Ron lives in the same flat fifty years later, since Gary still owns it).
 * Inverted in Five Days to Midnight. A case is sent back in time in an attempt to Set Right What Once Went Wrong.
 * The Eureka 4th mid-season finale
 * On the flashback episode of Big Bang Theory Sheldon attempts to do this with the rental agreement he has Leonard sign:

Newspaper Comics

 * Calvin writes a letter to his future self, expressing envy at having experienced life Twenty Minutes Into the Future. When he receives the letter, some days later, he feels sorry for his past self.

Tabletop Games

 * In Continuum, this is a common solution to being "stuck" in the past while your Span recovers; the Scribes specialize in this. Also, taken to extremes, a way of "instant messaging" with fellow spanners. Have an example of this in action.
 * This is also a recommended tactic in GURPS Time Travel and GURPS Infinite Worlds, where the authors suggest giving the message to a law firm to hold until the proper time. Other possible channels exist, they note, but a good lawyer won't even blink at the request.

Video Games

 * In Dark Cloud 2, the diary and letters that Max is writing to his mother....
 * Chrono Trigger uses this with the Rainbow Shell. Leene leaves a message with it meant for Marle, which she reads 400 years after it's written.
 * In Dark Fall 2: Lights Out,

Web Original

 * In Darwin's Soldiers: Pavlov's Checkmate, Shelton's antimatter copy (long story)) uses this to inform modern-day Shelton that he's trapped in 1990.

Webcomics

 * Casey and Andy: President Cleveland sends a message forward a century, to deliver a crotch-punch to his archenemy.

Western Animation

 * In Danny Phantom episode, "The Ultimate Enemy" Jazz (in the present) sends a message to Danny, who's . She used a particularly foolproof method, too:
 * Dexter's Laboratory used a short-term version of this, as Dexter (after opening a space gate to an alien world and being eaten by an alien blob) sends Dee Dee back several hours with a message... but past-Dexter doesn't get it until it's too late (which is to say, just after writing the letter in the first place), due to Dee Dee's ditziness and his own irritability.
 * David Xanatos, from Gargoyles, makes his fortune via this trope. After he's zapped back 1020 years in time by the Phoenix Gate, he leaves a package in the care of the Illuminati, to be delivered to his younger self 1000 years in the future (1975). The package contains three coins (worth thousands of dollars by the time the younger Xanatos receives them and uses them for his first investments, but near worthless for their era) and a lengthy letter to be delivered twenty years after that (1995, one week before the beginning of the episode) that details what he will have to do to ensure that he is sent to the past to send the coins and the letter in the first place. And he even makes a crack about being a "Self-Made Man". Magnificent.
 * His father didn't think very highly of it though. As a wedding present, he gives David a penny, saying it's not worth much now, but in a thousand years it might be worth more.
 * Used in a multi-part Time Travel story arc in Adventures of Sonic the Hedgehog. A couple times, Sonic and Tails are stranded in an impossible situation. Sonic's solution: write a note to their contact in the future, bury/hide it right where they stand, and the solution will be beamed to them the exact second they finish.
 * In the Phineas and Ferb episode "It's About Time!", Phineas and Ferb get trapped in the prehistoric past and send messages back to the present by scratching them into the mud next to a dinosaur footprint they know is going to become fossilized and end up in the local museum.
 * An episode of Teenage Mutant Ninja Turtles: Fast Forward focussed on this. The Turtles received a letter, and later on get separated in groups of two across time and space. It's after Raphael and Michaelangelo from the future tell the same two from the past (in the present) to read the letter that they realize it had been sent by Donatello from 200something.
 * An interesting variation occurs in one episode of the original Ninja Turtles. Carter needs to contact time-travelling allies Landor and Merrik, but they're twenty years in the future. Carter sends a radio message into space, with the intent of bouncing it back to Earth via a star ten light years away. It works.
 * Though not quite the same thing, an episode of the Superfriends has some of the super-heroes trapped in the prehistoric past. To get a signal to the heroes in the present, the trapped heroes bury their communicator in the ground at the precise location where the Hall of Justice will be built thousands of years from now. At that time, the heroes in the present begin to hear a signal coming from the ground. Somehow, this tells them that their friends are trapped in the past. Even as a kid watching this, none of it made any sense.
 * At one point The Tick (animation) and Arthur are stranded in the prehistoric past. Arthur takes months, years carving a message into the hardest stone he can find in the hopes that the message would survive millions of years of erosion to be discovered in the future For those who're curious, some hard granite erodes at a rate of approximately one inch every million years; since they were cohabiting with Australopithecus, letters would probably have to be carved at least a foot deep and wide spaced to still be legible today, assuming they were left exposed to the elements.
 * In the Futurama episode The Late Philip J. Fry, Leela discovers that Fry's been sent into the future, unable to return. She goes into a cave and blasts a pattern of holes in the ceiling. The resulting stalagmite formations over millions of years spell out a touching message for him.

Real Life

 * Stephen Hawking tries this in his "Time Traveller's Party" experiment. He holds a reception for future time travellers, but doesn't publish the time (12:00 on 6/28/2009, for any future people reading this) until after the party, so the only people who could show up would be time travelers. No one did (or he's not telling).