The lack of anachronisms is often used as evidence against the possibility of time travel.

Anachronism literally means "out of time". It can be any object or event, in fiction or in the real world, that appears out of place because it isn't appropriate or is impossible for the time.

In relation to theories of time travel, the question of anachronisms has been discussed as possible disproof of (or proof for) the possibility of travelling in time. If it is possible to travel in time, why isn't there any historical evidence that people from the future have visited the past? And why are there no obvious anachronisms in our own time?

It can be assumed that if time travel is possible at any future time, that temporal explorers would travel throughout history, and the longer time travel techniques existed, the more people would travel. It might be considered inevitable that these people would leave some anachronistic evidence behind. The time traveller themselves would likely be a notable anachronism, since even something as simple as spoken language changes rapidly over time. The lack now and throughout history of any anachronistic technology, objects, or ideas could be taken as proof that time travel does not happen and therefore is not possible. "There are no anachronisms", this argument goes, "so there never will be any travellers with time machines".

== Breadcrumb trails and hidden clues ==

The corollary of this argument is that if anachronisms exist, then this might be evidence for time travel. Some people look at history and cause and effect, and find things which appear not to have happened in a sensible chronological order, such as unbelieavble accomplishments or sudden advances in technology or thought.

A further theory, explored especially by science fiction and fantasy authors, suggests that time travel may be monitored for the specific purpose of controlling anachronism; the suggestion is that in the future there may be a kind of "time police." This group would control what people were able to do when they travelled in time, and perhaps be responsible for "correcting" errors and eddies in the space time continuum, returning history to its 'pure' path as if it had not been tampered with. This idea is strongly related to a non-deterministic or "chaos" theory of history, suggesting that any anachronistic influences would by nature be destructive, changing everything in the time line to follow. If such a body of "time police" existed, it would be hard to imagine that they would perform this task perfectly leaving no evidence behind, though one may consider that if time travel techniques were perfected, there would be an infinite number of opportunities to correct a single change. The "time police" theory also presents a dramatic example of what is commonly called the grandfather paradox or the time loop paradox: if an anachronism in time was in fact corrected, how would anyone in the future know to go back and correct it? For more exploration of time paradoxes, see the time travel article.

Both progressive and regressive anachronisms could be the result of time travel, but regressive anachronisms tend to be much stronger proof. If, say, dinosaurs are found on a remote pacific island, that could be the result of time travel. But it could equally be that this particular island has somehow been sheltered from whatever wiped out the rest of the dinosaurs. By contrast, if Julius Caesar is caught on film in ancient Rome wearing a pair of Levis, that could only be a result of time travel (on two counts).

== Other Explanations ==

The scarcity of anachronisms may exist because of "time police", or because time travel is impossible. However, there are other (possibly less dramatic) explanations:

Time Travel may be very difficult. This would make anachronisms either very rare or non-existent. It might also limit time-travel to larger organisations who would be more responsible in their uses of the technology. (because governments and large corporations are known for their responsibility and integrity.)

The human species may become extinct shortly after the invention of time travel, so there won't be enough time [sic] to flood the past with tourists. The technology might also be developed on another planet or even by another species. Maybe time travel is itself a dangerous technology?

It may not be possible to travel back in time past the creation of some specific artifact. The Tippler Method is an example of a method with this kind of limitation. In this case we'd expect to see no time travellers now, but at soon as the relevant artifact is constructed we'd expect visitors from the future to be a fact of life.

== Example Anachronisms ==

If the pyramids were not built by aliens, must they have been built with technology from a future civiliation? They're certainly impressive monuments in both scale and design, but the Egyptians were skilled masons with an almost infinite supply of labour.

What about other feats of ancient engineering? From the solar calendar that is Stonehenge to the statues of Easter Island, there are a number of ancient monuments that have raised eyebrows over their feasability given the technology and government of the time. Did their builders get a helping hand?

How could Leonardo da Vinci have imagined the helicopter, centuries before it first took off? But Leonardo's helicopter could not in fact fly, and wasn't a proper helicopter anyway. Leonardo also had some tank designs: another temporal anomaly? And what was it that really sent Leonardo mad?

How come the Greeks had a clocklike computer over 2000 years before clockwork's 'natural' development in the 18th Century, in the shape of the Antikythera mechanism? Perhaps the Time Police ensured that the secret of clockwork was hidden for long enough for technology to catch up. After all, the last mechanism went down in a ship wrecked in the Mediterranean.

The Baghdad Battery has puzzled people for years.

A more fanciful suggestion is that the fabled island of Atlantis was one giant anachronism, and it was drowned in an attempt to hide this "fact".

Some people argue that these examples and others show that there have been careless Time Travellers and show that Temporal Secret Agents have been here on containment and damage limitation missons. These are said to be the smoking guns of the ultimate conspiracy theory.

== History ==
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2002-3: Published by Wikimedia, Edited by Wikipedia contributors, Entitled Anachronism and time travel

2002: Published by h2g2, Written by a girl called Ben, Entitled Hellmans' Anachronism: an hypothesis about historical proof of future time travel