Time travel is the ability to travel, surprisingly enough, through time. Time travel can be done either to the future or to the past; however, travel to the past carries the risk of causality violation or paradoxes. Travel to the future at the precise rate of one second per second is quite common - in fact, it appears to be a requirement in our universe.
The general method of time travel is by using a time machine. Such devices can take the shape of anything, from a lounge chair to a phone booth to a DeLorean. Improperly calibrated, these devices have been known to not work, or plain and simply blow up.
 Time travel in fiction
Time travel is a popular topic in science fiction, either directly or as a plot device. Writers from H. G. Wells to Gene Roddenberry to Russell T. Davies (and earlier writers of Doctor Who) have all incorporated time travel into their work.
Another variant on typical time travel (while not actually a version of traveling) is the Groundhog Day plot device, where time appears to be stuck in a loop, similar to a broken record, although usually the loop is longer than a record skipping, to make it more interesting. This allows the protagonist(s) to repeat the events of the period numerous times. To prevent the loop from repeating infinitely, the mind of one or more characters is somehow immune. This concept has also been presented in various TV shows, as well as a few movies.
To be honest, time travel has been done to death in fiction. Approximately 45 billion gazillion books, comics, bad movies and episodes of Star Trek have dealt with the topic in one form or another. HG Wells and Mark Twain kicked it off over a century ago, and you'd think having those titans of literature as the competition would discourage people, but no. Occasionally someone comes up with a new twist on the idea (eg. the low-budget 2004 film Primer which focuses on how petty and selfish genuine geeks would become if they actually did build a time machine). The most clever recent use of the device was as a pretext for telling an army of basement-dwelling every-last-detail obsessive-fans to go f*** themselves, as in the recent Star Trek reboot movie.
 Aims of time travel
 Building a utopia
A popular motivation of any would-be time traveler is the desire to go back and "correct" the past, such as the classic example of aborting Mrs. Hitler's pregnancy at the end of 1888. However, one of the criticisms of this is that had little Adolf never been born, there would be no way of ascertaining if someone even worse would have taken his "place" in history. Another humorous example is this first-person account of such an attempt:
- "I went back in time and killed Mal Stacey as a child. As everybody knows, Mal Stacey is the guy who flew the Enola Gay and dropped the "Little Boy" nuke on to Tokyo, causing Japan's immediate surrender in World War II. Thus, my actions saved the lives of everyone in Tokyo. What I didn't foresee, however, is that by removing Mal Stacey from the timeline, it would lead to the events of both Hiroshima and Nagasaki being bombed instead." - the unsung hero
Changing history in this way would prove the possibility of parallel universes (possibly an infinite number of them). We might prevent Hitler from coming to power, avoiding the Second World War and the Holocaust, but we also know (from our knowledge of history before we changed it) that these things had already happened. We cannot deny the existence of the universe where these events happened, since we have traveled from that point, but we have created another universe in which history takes a very different course. Another problem, called the "Grandfather Paradox" could arise out of this scenario. Say our time traveler aborted Mrs. Hitler's pregnancy then there would be no reason in the future for a time traveler to go back and do the deed, hence, Mrs. Hitler would carry the pregnancy to term and little Adolf would still be born.
 Putting yet more lies in history books
Another, less-often stated aim of time travel would be to go back in time and accurately report history.
Professors and students who have unraveled the mysteries of time travel have used it to go back in time and gain first hand understanding of historical events. A classic example would be Bill and Ted's Excellent Adventure, in which the protagonists go back in time and meet historical figures in order to complete a history report, although this plot was centered around a larger one in which the protagonists' music forms the basis of a utopian society - another aim of time travel mentioned above.
 Capitalist wet dream
Any sensible person with a working time machine would, of course, use it to get rich on the stock markets or lotteries, by using information from the present in the past, or from the future in the present.
 Just to make things clear
Interestingly, the events that one had changed would already have happened, so in reality, one wouldn't actually be changing anything, unless we do not correctly understand time (we don't, but not 'incorrectly' enough to make a mistake like this [we think]). So any changes that had been made would really be self-fulfilling prophecies. We think. How are you with advanced theoretical physics?
 Time travel is real
Before the Internet came along, even if a time traveler from the future (or distant past; can't forget that) did arrive in our time, only a very small number of people would necessarily know about it. There are limits to the number of people with whom one can interact meaningfully without muscling one's way onto Oprah or something; a feat a Terminator could perhaps accomplish, but no guarantees. So maybe that's why they go to the past when they want to stay low profile and visit a (very much non-)historic bridge reopening in 1940s Canada, or, say, be a weirdly dressed extra in a silent movie. It's senseless even if it was a time traveler - how would she have gotten reception? To whom was she talking, all the other people with cell phones? And why in a silent movie?</ref> they come to the present day with its series of tubes when they want to disseminate an important message widely.
Enter John Titor.
 John Titor, libertarian from the future
In 2000, someone claiming to be an authentic military chrononaut from the year 2036 appeared on several time travel and conspiracy theory forums and, after filling us in about how his time machine runs on "two top-spin, dual positive singularities," a "standard off-set Tipler sinusoid," and an "electron injection manifold," was good enough to field some questions.
That's immediately puzzling, as if you ask the majority of people arriving at a geographic location in a car how they got there, they wouldn't describe the principles of the internal combustion engine but say something like: you turn a key to get it going, then press foot pedals to control the speed and turn a wheel to control direction. Or more likely just: "I drove".
He gets the Grandfather Paradox thing out of the way immediately, by ascribing the status of fact to the many-worlds theory of quantum physics, which states essentially that every conceivable motion made at the quantum level is made in at least one of an infinitude of parallel dimensions. So he can't mess up his own time by fooling with ours, which he then merrily proceeds to try to do.
He tells a story of the future that reads like a Newt Gingrich sci-fi novel: a civil war divides the United States along urban/rural lines, and just when the virtuous yeomen are about to pummel the corrupt and decadent (read: gay/black/liberal/secular) city dwellers, Russia decides to take advantage of the internal weakness and starts firing nukes in every direction for no discernible reason. This results in the destruction of urban America, China, and Europe. So the capital of America is moved to Omaha (How Omaha survived a nuclear was is never answered), the country is subdivided into five districts, and it settles in for an existence of god-fearing, self-reliant pastoral bliss straight out of Heinlein. Oh, except for the mad cow disease epidemic, which is eating more brains than a stoned Hannibal Lecter. Cool story, right?
Other things that make future America sound keen, just keen:
- Olympics are canceled after 2004 (big loss, and absolutely happened).
- No more centralized banking. Alex Jones will be thrilled.
- Christians worship on Saturday. Seventh Day Adventism goes mainstream, apparently, and all it took was a nuclear exchange.
- The family and religion take center stage in life, unlike in the current era where they mean nothing, NOTHING!
- Global warming didn't cause any lasting problems. We can finally put that hysteria to bed.
- They regularly spit out "microsingularities" and General Electric builds time machines out of them. It's nice to know predatory megacorporations will come through "N Day" intact.
- Everyone homeschools. And they can somehow still build time machines; maybe Andrew Schlafly is really on to something.
- John's time machine is built into his Corvette. And while it's no DeLorean, it seems to be a step up from 1985-era flux capacitor technology.
As for his mission in the past, it seems that UNIX has its own Y2K set to go off in 2038, and to prevent this 2036 scientists need an IBM from 1975 to debug the world's computers (what, no Windows?). Uh, okay, but what is John doing in 2000? Well, fucking around on Art Bell's website, apparently, and gleefully rubbing our noses in our inevitable nuclear doom, alongside getting pictures of loved ones that vanished in the Civil war. Titor on multiple occasions expresses contempt for 2000 CE-era people's weakness, complacency, and stupidity, and all but says we deserve what we're going to get. Then, in 2001, he disappeared from the tubes back to 2036/his parents' basement/a local padded cell.
So is it real? Well, the technology is in no sense real except for the IBM/UNIX thing, Titor was sending cranky faxes to Art Bell as early as 1998 predicting Y2K collapse (though this could easily have been a different Titor with his own GE Time Corvette from a separate "worldline"), and roughly none of Titor's predictions have come true in the last 10 years without some serious reaching.
So yeah, it's totally real, perhaps but not for our timeline.
 See also
- John Titor Archive - At Above Top Secret? Surely not.
- John Titor Times - Maintained by somebody named Oliver Williams.