
A doppelganger is the ghostly or in some cases, a physical double of a living person. The word doppelganger is a loanword from German, written there (as any noun) with an initial capital letter Doppelganger, composed from doppel, meaning "double", and ganger, as "walker". In English, the word is conventionally not capitalized, and it is also common to drop the German diacritic umlaut on the letter "a" and write "doppelganger", although the correct spelling without umlaut would be "doppelgaenger".
The term has, in the vernacular, come to refer to any double or look-alike of a person, most commonly in reference to a so-called evil twin, or to bilocation. Alternatively, the word is used to describe a phenomenon where you catch your own image out of the corner of your eye. In some traditions, seeing one's own doppelganger is an omen of death. A doppelganger seen by friends or relatives of a person may sometimes bring bad luck, or indicate an approaching illness or health problem.
The doppelgangers of folklore cast no shadow, and have no reflection in a mirror or in water. They are supposed to provide advice to the person they shadow, but this advice can be misleading or malicious. They can also, in rare instances, plant ideas in their victim's mind or appear before friends and relatives, causing confusion. In many cases once someone has viewed his own doppelganger he is doomed to be haunted by images of his ghostly counter-part. Other folklore says that when a person's doppleganger is seen, the person him/herself will die shortly. It is considered unlucky to try to communicate with such a doppleganger.
Famous Reports of the Doppelganger Phenomenon
Doppelgangers appear in a variety of science fiction and fantasy works, in which they are a type of shapeshifter that mimics a particular person or species for some typically nefarious reason.
A temporal doppelganger is any version of oneself one may meet during time travel. It is an exact likeness of one at a specific time in one's history (or future). Meetings with oneself may occur when one version of oneself travels backwards through the timestream and encounters a younger version of oneself, or when two or more of the same person from different timestreams travel to the same moment in their futures.
Doppelganger Wikipedia