Intangible Time Travel

The argument goes that if Time Travel were possible, we'd know about it, because we'd always be running into people asking "What Year Is This?". But who says time travel has to be a physical thing? An infrequent but recurring trope is that of Intangible Time Travel, in which people travel through time only as observers—as spirits or projections, but not as physical beings.

The best part about this is that it can't cause any paradoxes. There's no Grandfather Paradox if you can't kill your own grandfather. You cannot give Shakespeare a copy of the First Folio if you can't even talk to the guy. You can't accidentally kill yourself if you can never influence the events of the past in any way. Thus, it's a great tool for writers to use if they wish to avoid any Logic Bombs or Temporal Paradoxs. It's also a great vessel for a Pensieve Flashback, if the characters in question choose to comment on their past actions.

Because of the nature of the transport, this tends to happen more through magical means than scientific ones, although both have been used over the years.

Sometimes this isn't time travel at all; the "traveler" is having a vision or visiting someone's memory. Other times, the line gets blurred.

Anime and Manga

 * In Princess Tutu Rue is able to relive her early memories this way when It's as weird as it sounds.
 * The manga version of Chrono Crusade has Rosette taking a Journey to the Center of the Mind in Chrono's soul, allowing her to walk through his memories and see them first-hand.
 * In Princess Resurrection traveled back in time as a spirit
 * Negi uses this in Mahou Sensei Negima to show Asuna, and accidentally all his other students (Nadoka used her book to see what they were talking about and ended up showing everyone else the visions), in order to show her what happened to his village and the reason he became a mage so that he could look for the Thousand Master. It was meant to show how them the kind of world they would end up in if they continued to help him as his contractors, and the very real possibility that any or all of them could end up killed (or some equal fate) by demons, or other mages, or what the hell ever else you could think of, the idea being that if he could scare them away they would be safe. And it does scare them... for all of about six seconds, until the realization of the things hes been through (his village being destroyed and the path hes been pushed into as such a young child, having to basically give up his childhood to find out the truth about himself and his family, and find a way to save the villagers who got turned to stone) makes them all so FURIOUS that they basically vow to be his personal army to help him.

Comic Books

 * In 52, Ralph Dibny is shown his wife's murder through magical means, but is unable to stop it from happening.
 * In The Invisibles, the team use The Hand of Glory, a mystical artifact, to fold time down to a point. At that moment, they receive flashbacks and flashforwards of events in their lives and the lives of others. In each instance, they are literally invisible to the people around them.
 * Time travel through psychic projection is pretty commonplace in the series. Compare the "retrieval" of Marquis De Sade and King Mob's adventures in The Roaring Twenties.
 * The Golden Age and Silver Age Superman comics had instances where Superman went intangible whenever he time-traveled under his own power. Of course, this was only in some issues; the writers changed the rules whenever it was convenient. This eventually calcified into a hard rule of time travel in The Silver Age of Comic Books—travelers heading to a time they already existed in would be reduced to intangibility, but would remain solid and able to interact with others if there was no other copy of themselves around (the idea being that by making it impossible to physically be in two places at once, the universe was preventing paradoxes).
 * Used to heartbreaking effect in "Whatever Happened to the Man of Tomorrow?" by Alan Moore, the last Silver Age Superman story—the Legion of Superheroes comes to bid Superman farewell, and Supergirl, being a member, comes with them. The story takes place after Supergirl's death, and Supergirl wonders why she didn't turn intangible and asks Superman what she's like when she grows up. Superman tells her that "Supergirl is in the past", assures her that she grows up to be beautiful... and breaks down sobbing when they leave.
 * In the first collection of The Books of Magic by Neil Gaiman, young Tim Hunter, a potentially very powerful human magician, gets taken back in time in this manner by The Phantom Stranger as part of his education on magic. Later, unusually, he physically travels into one possible future (which people generally tend to see as spirits rather than physical beings) with Mister E.

Fan Works

 * Played with in A Time Apart, a Sailor Moon/Ranma ½ Crossover Fic by Ozzallos. Thrown into a timeline where the Sailor Senshi weren't reincarnated, Sailor Pluto sends Ranma Saotome to watch the fall of the Silver Millennium to explain why she's recruiting him for a replacement band of Senshi.  Ranma's supposed to be only able to watch, but the events of the Fall upset him so much that he manages to subvert the mechanisms that are intended to keep him intangible and apart from the past -- and then goes to town on Beryl's forces at the side of the Silver Millennium's Sailor Senshi.

Film

 * In the movie Constantine, John Constantine goes back in time and sees several scenes from earlier on in the movie, but remains intangible and invisible.
 * In the movie Click, the rewind button on the main character's remote allows him to intangibly review earlier parts of his life.
 * Roughly the first half of Deja Vu has the characters observing the events of several days earlier through a giant monitor in real-time.
 * In "Scrooged" (as a take off on A Christmas Carol) the characters sees his past and is told it's "like a rerun" and they can't be seen or heard. Too bad because he tries to tell his past self to pick up on the fact that the office slut wants him and to not make the mistakes that caused his girlfriend to leave him.

Literature

 * In A Christmas Carol and its derivatives, the protagonist is usually taken backwards and forwards in time to see the errors he has committed how they will leave him unmourned. He is completely invisible, inaudible and intangible through these scenes.
 * Jacen Solo has this ability, called "flow-walking", in the Star Wars Expanded Universe books, in a universe which only rarely features "regular" time travel.
 * At first, in Pastwatch: the Redemption of Christopher Columbus, a machine called the Tempoview allows researchers to view and record history, supposedly unnoticed. (Turns out this isn't the case, and the technology is developed to allow standard Time Travel.)
 * The concept of the technology was used for the Deja Vu film.
 * The short story "The Men Who Murdered Muhammed" by Alfred Bester has a very confusing variant on this.
 * In Kit Pearson's children's book, A Handful of Time, an old pocketwatch allows Patricia Potter to observe her mother's unhappy past - from when her mother was her age and lived in the same house Patricia is staying.
 * In the Odd Thomas series, the titular character has come to the hypothesis that the insubstantial wraiths that gather when some really bad shit is going to go down are actually dicks from the future who enjoy watching human suffering. They aren't totally intangible, though, as upon knowing your knowing about them, they will rectify the situation by making sure you are no longer around to know.
 * Peter F. Hamilton's short story If At First... takes Einstein's theory of not moving one single atom back. Somebody moves his mind back to his childhood years, overwriting the previous pernality, and using his knowledge of future events, alters time. One guy uses it to advance the technological level of society, and the other guy uses it to become a rock star, plagiarizing songs not written yet.
 * In the short Philip K. Dick story Paycheck a "timescope" exists which gives people the ability to see into the future.
 * The novel The Light of Other Days, by Arthur C. Clarke and Stephen Baxter, is about the invention of a kind of wormhole technology that allows it's user to view any location, in real-time. Due to it being relatively easy to replicate, the technology soon gets out of the hands of the government and is available to the general public, which immediately has drastic social consequences (since, essentially, the concept of "privacy" no longer has any meaning). Then, it's discovered that the wormholes can also be projected into the past...
 * Harry Potter has both "real" time travel, in which Never the Selves Shall Meet (Usually because one self will freak out and kill the other), but it also has Pensieves, which allow characters to jump into others' memories and remain intangible.
 * The basis of the short story "Nostolgianauts", in which people start freaking out at the "ghosts" appearing everywhere till they realize that they're tourists from the future. People start wondering when this technology will actually be invented, and the main character gets to witness her nerdy best friend's wealthy, future self coming back to gloat at the bullies who were picking on him in school.
 * Used in Going Postal when Moist sees the past of the Post Office. However, he's still moving around in the present. The place had fallen apart quite a bit between the two time periods, as two of his deceased predecessors found out when one stepped onto a balcony that wasn't there anymore, and one fell down a flight of stairs that wasn't there in the past.
 * Used as the "inspiration for the story" in Last and First Men, as shown by the introduction to the book.
 * Isaac Asimov wrote a number of short stories that involved this concept, most notably "The Dead Past", where it turns out that.
 * Another one features a company that traces genealogy using their device, usually telling customers that they're related to someone famous. The inventor goes crazy when he realizes that he is the first person in his family, back to the beginning of the human race, who has ever done anything of note.
 * In the book JumpMan, the titular time-travel devices can only travel to certain points in history and leave their users a tiny bit out of sync with their timelines, leaving them invisible and (from what I recall) intangible, in accordance with the First Rule of Time Travel ("Don't Touch Anything"). The plot arises when a contest winner is accidentally given an illegal prototype JumpMan which DOESN'T do this...
 * of Warrior Cats can travel through time like this. And he can also interact with time travelers.
 * In Incarnations of Immortality, this happens if one time travels to a time period where they never existed.
 * Dinoverse seems to set this up, as traveling characters become invisible beings of 'pure thought-energy brainwaves', but they're never like this for long - said characters end up possessing the bodies of dinosaurs, acting through them.
 * Daphne du Maurier's novel House on the Strand, which involves several travels from 20th century Cornwall to its counterpart version during The High Middle Ages.
 * In How to Live Safely In A Science Fictional Universe, the physics of the universe prevents you from going back to an event you weren't at and changing that event by throwing you into another timeline. A heartbreaking example is of a young woman who uses time travel to get back to the moment when her grandmother died, because she wasn't there the first time. The grandmother can neither see nor hear her; even though they're in the same room, they're actually separated by a universe.
 * Fans of sci-fi and/or The Inklings have been looking for a long time for the Time Travel story CS Lewis alluded to in the prologue to The Great Divorce, wherein a guy goes into the past, but its immutability means he cannot even eat a sandwich, or keep a raindrop from piercing him clear through. Since the story is as yet unidentified, we do not know if he makes it back to his own time or not.

Live-Action TV

 * In Quantum Leap, Al is able to project himself into the past, but he is visible only to Sam and a selected few other characters (e.g. young children). Sam has to do the hard physical work of putting right what once went wrong, and has to partially take the bodyspace of someone already in that time, he can't just appear via a portal.
 * In Torchwood a device exists that allows someone to be intangibly transported into scenes of strong emotion from the past. When put together with another identical device, it allows the same thing to happen with the future.
 * In Heroes Peter sort of does this, but not intentionally as he couldn't control his abilities. He travels back in time but turns invisible, only one character knows he?s there and his past self isn't aware of him.

Newspaper Comics

 * The dreamscape in Candorville connects to all places and times, so those who are asleep can view the past and the future (though they forget it when they wake.) They can interact with other dreamers in other times and places, but attempts to change the past never seem to work.

Video Games

 * The time travel based RTS game Achron has this in the gameplay. The player is able to freely look at any point on the timeline, but if the time you are looking at is too far in the past you will be unable to make any canges to it.
 * In Tomb Raider: Anniversary, a few cutscenes show Lara doing this through a magical artifact. Since TRA was a remake of TR 1, it was probably in there, too.
 * This is essentially what Ellone does to Squall and company in Final Fantasy VIII, although they're dropped within other people's bodies (which they can't control, but can get the attention of). A powerful enough sorceress can apparently override the "can't control the person you're inhabiting" restriction.
 * The final dungeon of Devil Summoner: Raidou Kuzunoha vs. the Soulless Army has the eponymous protagonist literally traveling through time, climbing his way through a dungeon where every floor represents a later decade. Many of the NPCs in this location are ghosts from their respective time periods, but a few are Intangible Time Travelers, and they comment on how dangerous it is to wander through time in corporeal form like Raidou is doing.
 * Max gains the "future vision" ability in Sam & Max Season Three.

Web Comics

 * Used once in Bob and George. Dr. Light makes a time travelling suit of armor that can make the wearer intangible to prevent unnecessary changes to the timeline. It works great, right up until the armor breaks.

Web Original

 * Spes Phthisica uses the 'true-visions of the future' version.

Real Life

 * According to Einstein, this is how real time travel would have to work. He theorized that it could be possible to travel through time, as long as you didn't disturb even a single atom on the way, meaning you would have to be intangible and invisible.
 * But that means you wouldn't be able to see anything, because light wouldn't interact with your retinas. How pointless.
 * Or hear anything, because the sound waves wouldn't be able to vibrate against your inner ear...
 * But you could still play a mean pinball.
 * Only if you're dumb, too.
 * Another case in which observation changes the outcome.
 * However consider a Flatland scenario where the present is a sheet of paper, the future another sheet laid on top and the past one laid beneath. The entirety of Flatland time would resemble a flick-book. A 3D person could observe any point at any time and would be, from the point of the inhabitants, both invisible and intangible.
 * Come to think of it, shouldn't Hiro from Heroes be unable to see or hear anything when he stops time?
 * He slows it down to a crawl; people with super-speed are still capable of moving when Hiro uses his power.