Bifrost

In Norse mythology, Bifröst (/ˈbɪvrɒst/ (listen)) or Bilröst is a burning rainbow bridge that runs between Midgard (Earth) and Asgard, the realm of the gods. The bridge is referred to as Bilröst in the Poetic Edda, which was compiled in the 13th century from earlier sources, and as Bifröst in the Prose Edda, written by Snorri Sturluson in the 13th century, as well as in the poetry of the Skalds. In both the Poetic Edda and the Prose Edda, the bridge is alternately called Ásbrú (Old Norse "Bridge of the Æsir").

According to the Prose Edda, the bridge ends in the heavens at Himinbjörg, the seat of the god Heimdallr, who guards it against the Jötnar. The destruction of the bridge during Ragnarök by the forces of Muspell is foretold. Scholars have suggested that the bridge may have originally represented the Milky Way, and they have drawn parallels between the bridge and another bridge in Norse mythology, the Gjallarbrú.

Etymology
Scholar Andy Orchard suggests that Bifröst could mean "shimmering path." He notes that the first element of Bilröst – Bil (meaning "a moment") – "suggests the fleeting nature of the rainbow," which he associates with the first element of Bifröst – the Old Norse verb bifa (meaning "to shimmer" or "to shake") – and observes that the element evokes notions of the "shimmering brilliance" of the bridge. Austrian philologist Rudolf Simek says that Bifröst either means "the swaying path to the heavens" (also drawing from bifa) or, if Bilröst is the original form of both (which Simek considers likely), "the fleetingly glimpsed rainbow" (possibly associated with bil, meaning "moment, weak point").

References
Two poems in the Poetic Edda and two books in the Prose Edda contain information about the bridge:

Poetic Edda
In the Poetic Edda, the bridge is mentioned in the poems Grímnismál and Fáfnismál, where it is called Bilröst. In one of the stanzas of Grímnismál, where the bridge is mentioned, Grímnir (the god Odin in disguise) imparts cosmological knowledge to the young Agnarr, including that Bilröst is the best of all bridges. Later in Grímnismál, Grímnir states that Ásbrú "burns everything with flames" and that the god Thor wades through the waters of Körmt and Örmt and the two Kerlaugar every day:

Benjamin Thorpe translation:
Körmt and Örmt, and the two Kerlaugar:
Thor must wade through these
every day when he goes to the council
at Yggdrasil's ash;
for as the Ás-bridge burns entirely with flame,
the holy waters boil.

Henry Adams Bellows translation:
Kormt and Ormt and the Kerlaugs together
Must Thor wade through every day,
(When he is doomed to give, he moves on
To the ash tree Yggdrasil;)
For the sky’s bridge burns completely with flame,
And the holy waters bubble.

In Fáfnismál, the dying dragon Fafnir tells the hero Sigurd that the gods will meet with their spears at Óskópnir during the events of Ragnarök. From there, the gods will cross Bilröst, which will break apart as they cross, forcing their horses to swim through a vast river.

Prose Edda
The bridge is mentioned in the books Gylfaginning and Skáldskaparmál of the Prose Edda, where it is called Bifröst. In chapter 13 of Gylfaginning, Gangleri (King Gylfi in disguise) asks the throne figure of High what path exists between heaven and earth. Laughing, High replies that the question is not wise and explains that the gods built a bridge between heaven and earth. Incredulously, Gangleri asks if he has not heard the story before. High says that Gangleri must have seen it and notes that Gangleri may call it a rainbow. High states that the bridge is made of three colours, is very strong, and "was built with more art and skill than any other construction."

High remarks that although the bridge is strong, it will break if the "sons of Muspell" try to cross it, and their horses will have to swim across "great rivers." Gangleri comments that the gods did not "build the bridge in good faith if it can break, for they can do whatever they want." High responds that the gods are not to blame for the bridge breaking, as "nothing in this world is certain when the sons of Muspell attack."

In chapter 15 of Gylfaginning, Just-As-High says that Bifröst is also called Ásbrú, and that the gods ride across it every day with their horses (except Thor, who wades through the boiling waters of the rivers Körmt and Örmt) to reach Urðarbrunnr, a sacred well where the gods hold their court. To reference this, Just-As-High quotes the second of the two stanzas in Grímnismál where the bridge is mentioned (see above). Gangleri asks if fire burns on Bifröst. High says that the red in the bridge is burning fire, and without it, the frost giants and mountain giants would "rise into the heavens" if anyone could cross Bifröst. High adds that there are "many beautiful places in heaven" and that "everywhere there is divine protection."

In chapter 17, High tells Gangleri that the place Himinbjörg "stands at the edge of the sky, where Bifröst reaches the heavens." Describing the god Heimdallr in chapter 27, High says that Heimdallr lives in Himinbjörg by Bifröst and guards the bridge from the mountain giants while sitting at the edge of the heavens. In chapter 34, High quotes the first of the two stanzas in Grímnismál where the bridge is mentioned. In chapter 51, High foretells the events of Ragnarök. High says that during Ragnarök, the sky will split, and from the split, the sons of Muspell will ride out. When the "sons of Muspell" ride over Bifröst, it will break, "as was said above."

In the Prose Edda, in the book Skáldskaparmál, the bridge is mentioned only once. In chapter 16, there is a work by the 10th-century skald Úlfr Uggason, who refers to Bifröst as "the way of the powers."

Theories
In his translation of the Prose Edda, Henry Adams Bellows notes that the stanza in Grímnismál where Thor is mentioned and the stanza about the bridge might mean that "Thor in the final days of destruction, when the bridge burns, must go on foot." However, another interpretation is that the rainbow bridge becomes hot in the sun when Thor leaves the sky (i.e., when a thunderstorm passes).

John Lindow points out a parallel between Bifröst, which he believes is "a bridge between earth and heaven or earth and the world of the gods," and the bridge Gjallarbrú, "a bridge between earth and the underworld or earth and the world of the dead." Several scholars have suggested that Bifröst could represent the Milky Way.

Adaptations
In the final scene of Richard Wagner’s 1869 opera Das Rheingold, the god Froh conjures a rainbow bridge over which the gods travel to Valhalla.

Bifröst appears in the comics of the Marvel Comics character Thor and in later film adaptations of these comics. In the 2011 film Thor from the Marvel Cinematic Universe, Jane Foster describes Bifröst as an Einstein-Rosen bridge, a transport medium used to traverse space in a short time.

In the episode "The Bellero Shield" from the original television series The Outer Limits, an alien known as the "Bifröst Alien" appears, as it slides down a beam of light to Earth. In the dialogues of the series, some characters view this beam as similar to Bifröst in Norse mythology.