Tree of life

From Wikipedia, the free encyclopedia

An 1847 depiction of the Norse Yggdrasil as described in the Icelandic Prose Edda by Oluf Olufsen Bagge

17th-century depiction of the Tree of Life in Palace of Shaki Khans, Azerbaijan

The concept of a tree of life has been used in biology, religion, philosophy, and mythology. A tree of life is a common motif in various world theologies, mythologies, and philosophies. It alludes to the interconnection of all life on our planet and serves as a metaphor for common descent in the
evolutionary sense. The term tree of life may also be used as a synonym for sacred tree.^[1]

The tree of knowledge, connecting to heaven and the underworld, and the tree of life, connecting all forms of creation, are both forms of the world tree or cosmic tree, according to the Encyclopædia Britannica,^[2] and are portrayed in various religions and philosophies as the same tree.^[3]

Religion and mythology

 

Various trees of life are recounted in folklore, culture and fiction, often relating to immortality or fertility. They had their origin in religious symbolism.

Ancient Persia

In pre-Islamic Persian mythology, the world tree is a large, sacred tree which bears all seeds. Ahriman (Ahreman, Angremainyu) created a frog to invade the tree and destroy it, aiming to prevent all trees from growing on the earth. As a reaction, God (Ahura Mazda) created two fish staring at the frog to guard the tree. The two fishes are always staring at the frog and stay ready to react to it. Because Ahriman is responsible for all evil including death, while Ahura Mazda is responsible for all good (including life) the concept of world tree in Persian Mythology is very closely related to the concept of Tree of Life.

Another related issue in ancient mythology of Iran is the myth of (Mashy and Mashyane) two trees who were the ancestors all of living being. This myth can be considered as a prototype for the creation myth where living beings are created by Gods (who have a human form).

Ancient Egypt

Worshipping Osiris, Isis, and Horus

In Egyptian mythology, in the Ennead system of Heliopolis, the first couple, apart from Shu and Tefnut (moisture and dryness) and Geb and Nuit (earth and sky), are Isis and Osiris. They were said to have emerged from the acacia tree of Iusaaset, which the Egyptians considered the tree of life, referring to it as the "tree in which life and death are enclosed." Acacia trees contain DMT, a psychedelic drug associated with spiritual experiences. A much later myth relates how Set killed Osiris, putting him in a coffin, and throwing it into the Nile, the coffin becoming embedded in the base of a tamarisk tree.^[4]

The Egyptians' Holy Sycamore also stood on the threshold of life and death, connecting the two worlds.

Armenia

In ancient Armenia, the Tree of Life (Կենաց Ծառ) was a religious symbol and was drawn on walls of fortresses and carved on the armor of warriors. The branches of the tree were equally divided on the right and left sides of the stem, with each branch having one leaf, and one leaf on the apex of the tree. Servants stood on each side of the tree with one of their hands up as if they are taking care of the tree.

Assyria

The Assyrian Tree of Life was represented by a series of nodes and criss-crossing lines. It was apparently an important religious symbol, often attended to by eagle-headed gods and priests, or the King. Assyrilogists have not reached consensus as to the meaning of this symbol. It is multi-valent. The name "Tree of Life" has been attributed to it by modern scholarship; it is not used in the Assyrian sources. In fact, no textual evidence pertaining to the symbol is known to exist.

Baha'i Faith

The concept of the tree of life appears in the writings of the Baha'i Faith, where it can refer to the Manifestation of God, a great teacher who appears to humanity from age to age. An example of this can be found in the Hidden Words of Bahá'u'lláh:^[5][6]

"Have ye forgotten that true and radiant morn, when in those hallowed and blessed surroundings ye were all gathered in My presence beneath the shade of the tree of life, which is planted in the all-glorious paradise? Awestruck ye listened as I gave utterance to these three most holy words: O friends! Prefer not your will to Mine, never desire that which I have not desired for you, and approach Me not with lifeless hearts, defiled with worldly desires and cravings. Would ye but sanctify your souls, ye would at this present hour recall that place and those surroundings, and the truth of My utterance should be made evident unto all of you."

The concept can be broken down still further, with the Manifestation as the roots and trunk of the tree and his followers as the branches and leaves. The fruit produced by the tree nourishes an ever-advancing civilization.

A distinction has been made between the tree of life and the tree of the knowledge of good and evil. The latter represents the physical world with its opposites, such as good and evil and light and dark. In a different context from the one above, the tree of life represents the spiritual realm, where this duality does not exist.^[7]

China

In Chinese mythology, a carving of a Tree of Life depicts a phoenix and a dragon; the dragon often represents immortality. A Taoist story tells of a tree that produces a peach every three thousand years. The one who eats the fruit receives immortality.

An archaeological discovery in the 1990s was of a sacrificial pit at Sanxingdui in Sichuan, China. Dating from about 1200 BCE, it contained three bronze trees, one of them 4 meters high. At the base was a dragon, and fruit hanging from the lower branches. At the top is a strange bird-like (phoenix) creature with claws. Also found in Sichuan, from the late Han dynasty (c 25 – 220 CE) is another tree of life. The ceramic base is guarded by a horned beast with wings. The leaves of the tree are coins and people. At the apex is a bird with coins and the Sun.

Christianity

In addition to the Hebrew Bible verses, the tree of life is symbolically described in the Book of Revelation as having healing properties: "[1] And he shewed me a pure river of water of life, clear as crystal, proceeding out of the throne of God and of the Lamb. [2] In the midst of the street of it, and on either side of the river, [was there] the tree of life, which bare twelve [manner of] fruits, [and] yielded her fruit every month: and the leaves of the tree [were] for the healing of the nations." (Revelation 22:1-2)

In Catholic Christianity, the Tree of Life represents the immaculate state of humanity free from corruption and Original Sin before the Fall. Pope Benedict XVI has said that "the Cross is the true tree of life." ^[8] Saint Bonaventure taught that the medicinal fruit of the Tree of Life is Christ himself.^[9] Saint Albert the Great taught that the Eucharist, the Body and Blood of Christ, is the Fruit of the Tree of Life.^[10]

In Eastern Christianity the tree of life is the love of God.^[11]

The Church of Jesus Christ of Latter-day Saints

The tree of life appears in the Book of Mormon in a revelation to Lehi (see 1 Nephi 8:10). It is symbolic of the love of God (see 1 Nephi 11:21-23). Its fruit is described as "most precious and most desirable above all other fruits," which "is the greatest of all the gifts of God" (see 1 Nephi 15:36). In another scriptural book, salvation is called "the greatest of all the gifts of God" (see Doctrine and Covenants 6:13). In the same book eternal life is also called the "greatest of all the gifts of God" (see Doctrine and Covenants 14:7). Because of these references, the tree of life and its fruit is sometimes understood to be symbolic of salvation and post-mortal existence in the presence of God and his love.

Swedenborgianism

According to Swedenborgianism, the first twelve chapters of Genesis are a symbolic retelling of ancient truths. In his Arcana Coelestia,^[12] Emanuel Swedenborg (1688–1772) expounded on the symbolism and underlying spiritual meaning of both Genesis and Exodus, and the symbolism regarding the tree of life.

Europe

11th century Tree of Life sculpture at an ancient Swedish church.

In Dictionaire Mytho-Hermetiqe (Paris, 1737), Antoine-Joseph Pernety, a famous alchemist, identified the Tree of Life with the Elixir of Life and the Philosopher's Stone.

In Eden in the East (1998), Stephen Oppenheimer suggests that a tree-worshipping culture arose in Indonesia and was diffused by the so-called "Younger Dryas" event of c. 8000 BCE, when the sea level rose. This culture reached China (Szechuan), then India and the Middle East. Finally the Finno-
Ugaritic strand of this diffusion spread through Russia to Finland where the Norse myth of Yggdrasil took root.

Georgia

The Borjgali (Georgian: ბორჯღალი) is an ancient Georgian Tree of Life symbol.

Germanic paganism and Norse mythology

In Germanic paganism, trees played (and, in the form of reconstructive Heathenry and Germanic Neopaganism, continue to play) a prominent role, appearing in various aspects of surviving texts and possibly in the name of gods.

The tree of life appears in Norse religion as Yggdrasil, the world tree, a massive tree (sometimes considered a yew or ash tree) with extensive lore surrounding it. Perhaps related to Yggdrasil, accounts have survived of Germanic Tribes' honouring sacred trees within their societies. Examples include Thor's Oak, sacred groves, the Sacred tree at Uppsala, and the wooden Irminsul pillar. In Norse Mythology, the apples from Iðunn's ash box provide immortality for the gods.

Islam

The Tree of Immortality (Arabic: شجرة الخلود) is the tree of life motif as it appears in the Quran. It is also alluded to in hadiths and tafsir. Unlike the biblical account, the Quran mentions only one tree in Eden, also called the tree of immortality, which Allah specifically forbade to Adam and Eve.^[13][14] Satan, disguised as a serpent, repeatedly told Adam to eat from the tree, and eventually both Adam and Eve did so, thus disobeying Allah.^[15] The hadiths also speak about other trees in heaven.^[16]
However, according to the Ahmadiyya movement in Islam, Quranic reference to the tree is symbolic; eating of the forbidden tree signifies that Adam disobeyed God.^[17][18]

Jewish sources

Etz Chaim, Hebrew for "tree of life," is a common term used in Judaism. The expression, found in the Book of Proverbs, is figuratively applied to the Torah itself. Etz Chaim is also a common name for yeshivas and synagogues as well as for works of Rabbinic literature. It is also used to describe each of the wooden poles to which the parchment of a Sefer Torah is attached.

The tree of life is mentioned in the Book of Genesis; it is distinct from the tree of the knowledge of good and evil. After Adam disobeyed God by eating fruit from the tree of the knowledge of good and evil, he was driven out of the garden of Eden. Remaining in the garden, however, was the tree of life. To prevent the man's access to this tree in the future, Cherubims with a flaming sword were placed at the east of the garden. (Genesis 3:22-24)

In the Book of Proverbs, the tree of life is associated with wisdom: "[Wisdom] is a tree of life to them that lay hold upon her, and happy [is every one] that retaineth her." (Proverbs 3:13-18) In 15:4 the tree of life is associated with calmness: "A soothing tongue is a tree of life; but perverseness therein is a wound to the spirit."^[19]

The Book of Enoch, generally considered non-canonical, states that in the time of the great judgment God will give all those whose names are in the Book of Life fruit to eat from the Tree of Life.

Kabbalah

 

Judaic Kabbalah Tree of Life 10 Sephirot, through which the Ein Sof unknowable Divine manifests Creation. The configuration relates to man

Jewish mysticism depicts the Tree of Life in the form of ten interconnected nodes, as the central symbol of the Kabbalah. It comprises the ten Sephirot powers in the Divine realm. The panentheistic and anthropomorphic emphasis of this emanationist theology interpreted the Torah, Jewish observance, and the purpose of Creation as the symbolic esoteric drama of unification in the Sephirot, restoring harmony to Creation. From the time of the Renaissance onwards, Jewish Kabbalah became incorporated as an important tradition in non-Jewish Western culture, first through its adoption by Christian Cabala, and continuing in Western esotericism occult Hermetic Qabalah. These adapted the Judaic Kabbalah Tree of Life syncretically by associating it with other religious traditions, esoteric theologies, and magical practices.

Mesoamerica

The concept of world trees is a prevalent motif in pre-Columbian Mesoamerican cosmologies and iconography. World trees embodied the four cardinal directions, which represented also the fourfold nature of a central world tree, a symbolic axis mundi connecting the planes of the Underworld and the sky with that of the terrestrial world.^[20]

Depictions of world trees, both in their directional and central aspects, are found in the art and mythological traditions of cultures such as the Maya, Aztec, Izapan, Mixtec, Olmec, and others, dating to at least the Mid/Late Formative periods of Mesoamerican chronology. Among the Maya, the central world tree was conceived as or represented by a ceiba tree, and is known variously as a wacah chan or yax imix che, depending on the Mayan language.^[21] The trunk of the tree could also be represented by an upright caiman, whose skin evokes the tree's spiny trunk.^[20]

Directional world trees are also associated with the four Yearbearers in Mesoamerican calendars, and the directional colors and deities. Mesoamerican codices which have this association outlined include the Dresden, Borgia and Fejérváry-Mayer codices.^[20] It is supposed that Mesoamerican sites and ceremonial centers frequently had actual trees planted at each of the four cardinal directions, representing the quadripartite concept.

World trees are frequently depicted with birds in their branches, and their roots extending into earth or water (sometimes atop a "water-monster," symbolic of the underworld). The central world tree has also been interpreted as a representation of the band of the Milky Way.^[22]

Middle East

The Epic of Gilgamesh is a similar quest for immortality. In Mesopotamian mythology, Etana searches for a 'plant of birth' to provide him with a son. This has a solid provenance of antiquity, being found in cylinder seals from Akkad (2390–2249 BCE).

The Book of One Thousand and One Nights has a story, 'The Tale of Buluqiya', in which the hero searches for immortality and finds a paradise with jewel-encrusted trees. Nearby is a Fountain of Youth guarded by Al-Khidr. Unable to defeat the guard, Buluqiya has to return empty-handed.

North America

In a myth passed down among the Iroquois, The World on the Turtle's Back, explains the origin of the land in which a tree of life is described. According to the myth, it is found in the heavens, where the first humans lived, until a pregnant woman fell and landed in an endless sea. Saved by a giant turtle from drowning, she formed the world on its back by planting bark taken from the tree.

The tree of life motif is present in the traditional Ojibway cosmology and traditions. It is sometimes described as Grandmother Cedar, or Nookomis Giizhig in Anishinaabemowin.

Serer religion

In Serer religion, the tree of life as a religious concept forms the basis of Serer cosmogony. Trees were the first things created on Earth by the supreme being Roog (or Koox among the Cangin). In the competing versions of the Serer creation myth, the Somb (Prosopis africana) and the Saas tree (acacia albida) are both viewed as trees of life.^[23] However, the prevailing view is that, the Somb was the first tree on Earth and the progenitor of plant life.^[23][24] The Somb was also used in the Serer tumuli and burial chambers, many of which had survived for more than a thousand years.^[23] Thus, Somb is not only the Tree of Life in Serer society, but the symbol of immortality.^[23]

Turkic world

The Tree of Life, as seen as in flag of Chuvashia, a Turkic state in the Russian Federation

The World Tree or Tree of Life is a central symbol in Turkic mythology. The blue sky around the tree reflects the peaceful nature of the country and the red ring that surrounds all of the elements symbolizes the ancient faith of rebirth, growth and development of the Turkic peoples.

Biology

 

Fig. 1: A speculatively rooted tree for rRNA genes, showing major branches Bacteria, Archaea, and Eukaryota.

The tree of life is a metaphor describing the relationship of all life on Earth in an evolutionary context.^[25] Charles Darwin talked about envisioning evolution as a "tangled bank" in On the Origin of Species; however, the book's sole illustration is of a branched diagram that is very tree-like.

From the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state. As we here and there see a thin, straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus (Platypus) or Lepidosiren (South American lungfish), which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.

—Charles Darwin, On the Origin of Species^[26]

Graphical representation of the modern "Tree of Life" on the Web project

The evolutionary relationships of the tree of life were refined using genetic data by the American microbiologist Carl Woese, the discoverer of the domain Archaea and a pioneer in molecular (genetic) methods in evolutionary biology. In February 2009, BBC One broadcast an animated, interactive tree of life as part of its "Darwin Season."

The Tree of Life Web Project is an ongoing Internet project containing information about phylogeny and biodiversity, produced by biologists from around the world. Each page contains information about one group of organisms and is organized according to a branched tree-like form, thus showing relationships between organisms and groups of organisms.

The neuroanatomical term arbor vitae (tree of life) describes the branching pattern between the cortical grey matter and subcortical white matter of the cerebellum.

Popular culture

Art and architecture

A 2½ story high "Tree of Life" sculpture by Wisconsin artist Nancy Metz White was installed in Mitchell Boulevard Park in Milwaukee in 2002. The tree is made of brightly painted welded steel and forge flashings recycled from Milwaukee heavy industry.

Austrian symbolist artist Gustav Klimt portrayed his version of the tree of life in his painting, The Tree of Life, Stoclet Frieze. This iconic painting later inspired the external facade of the "New Residence Hall" (also called the "Tree House"), a colorful 21-story student residence hall at Massachusetts College of Art and Design in Boston, Massachusetts.^[27]

Contemporary Welsh artist Jen Delyth created a Celtic Tree of Life symbol, in part based on ancient Celtic veneration of trees and traditional Celtic designs.

Music

In their album Emissaries the black metal Melechesh make a reference to the Tree of Life in their song "Touching the Spheres of Sephiroth."

American rock band O.A.R. featured a tree of life both on the cover art and on the actual c.d. for the album In Between Now and Then

Double album "Bath"/"Leaving Your Body Map" by avant-garde metal band maudlin of the Well was constructed based upon a parallel qabalistic Tree of Life structure.

The double album Axis Mutatis by the electronic group The Shamen contains in some limited editions the instrumental album "Arbor Bona Arbor Mala." The title refers to the tree of life, the ancient symbol found in virtually all Shamanic cultures, linking the underworld with the earth and the heavens. Also, on the cover of Axis Mutatis appears a representation of the tree of life by William Latham.

In Korean boy band, EXO's Music Video MAMA, the twelve forces (EXO's members) will be the one to restore the tree of life.

Rapper Ab-Soul uses the tree of life as the cover of his Control System album

Fiction

Literature

• In George Herbert's poem The Sacrifice (part of The Temple, 1633), the Tree of Life is the rood on which Jesus Christ was crucified.
• In C. S. Lewis' Chronicles of Narnia, the Tree of Life plays a role, especially in the sixth published book (the first in the in-world chronology) The Magician's Nephew
• In Robert Jordan' Wheel of Time the Tree of Life – "Avendesora" – as the last of its kind plays a pivotal role. This tree also linked to the Buddhist "Bodhi" tree, beneath which the Buddha attained Nirvana
• "Tree of Life" is the name of a plant in Larry Niven's Known Space novels that, when consumed by Pak Breeders at a certain age, transforms them into Pak Protectors.
• In the Roger Zelazny's 1978 novel The Chronicles of Amber: The Courts of Chaos prince Corwin encounters Ygg (a nick from Yggdrasil), a tree who speaks and is planted on the border between Order and Chaos, between Amber and Courts of Chaos
• In The Sea of Trolls written by Nancy Farmer, the Tree of Life (Yggdrasil) is a place holding magical powers.

Video games

• The Norse Tree of Life, Yggdrasil, is either featured or referenced in many games, including those of the Tales RPG-series, the 2002 video game Wild Arms 3 and the 2008 video game Too Human.
• In the Atari 2600 game Swordquest: Fireworld, the map of the game world is patterned after the Kabbalah Tree of Life.
• In the 1997 video game Breath of Fire III, Yggdrasil, overseer of the world's forests, features a minor role. The mutant plant Peco, a party member, becomes able to channel Yggdrasil after a time skip of several years, during which Peco made extended visits to Yggdrasil.
• In the 2002 video game Warcraft III: Reign of Chaos, a Tree of Life is the central building of the Night Elf race.
• In the Etrian Odyssey series, a great tree named Yggdrasil is present in all four games, serving as a major plot device to the story.
• In the 2007 video game Dragoneer's Aria, The Great Spirit guards a World Tree.
• In the 2008 video game Prince of Persia, a gigantic, ancient tree in the middle of the desert is used to keep the evil deity Ahriman sealed in a temple at its trunk. This game's story heavily borrows from Zoroastrianism.
• In the 2009 video game Uncharted 2: Among Thieves, the Tree of Life grows within the city of Shambala, and the sap or fossilized resin from the tree is seen to be consumed by the inhabitants of the city. This in turn grants the user incredible regenerative abilities, strength, increased height and possible biological immortality. The sap from the tree represents the Cintamani Stone of Buddhist mythology, a giant raw sapphire with supposed wish-fulfilling properties.
• In the 2009 video game Dragon Quest IX, the player must harvest fyggs from Yggdrasil, the World Tree, to attain passage into the realm of the Almighty.
• In the 2010 video game Darksiders, the Tree of Life is located in Eden where War (a Horseman of the Apocalypse) sees the future and a way to defeat Abaddon The Destroyer. The Tree of Life gives War the Armageddon Blade needed to kill him.
• In the 2012 video game Darksiders II, the Tree of Life is a gateway that connects all the realms of existence, allowing The Horseman Death to travel to different maps within the game.

Film

• Darren Aronofsky's 2006 film The Fountain (as well as the 2005 graphic novel based on the screenplay) centers on immortality given by the Tree of Life.
• In the 2008 movie The Librarian, the religious mention of the Tree of Life is clearly seen in parts with a Crusade-era picture of a knight with his shield in that of the Tree of Life. Another part of the movie depicts a fake secret area beneath a New York City museum, where there are historical items such as the Fountain of Youth and Noah's Ark. At the end of the movie the camera angle changes and the ground's walking surface is revealed to be that of the Tree of Life.
• In Dragon Ball Z's third movie Tree of Might, a giant tree named the Tree of Might is represented as an evil version of the Tree of Life. Its roots take so much nutrients from the planet it has been seeded on that it kills the planet to support its fruit and growth. It is also a very massive tree much like the Tree of Life can be represented as.
• In the 2009 film Avatar, the Na'vi live in Hometree, the spiritual and physical home of the tribe; over 300 meters tall, Hometree is connected with all the other plant life of Pandora through a neural-like network. They revere the Tree of Souls, which is also connected with all other living things.
• Alex Proyas' 2009 film Knowing ends with the two young protagonists directed towards the Tree of Life.^[28]
• The Tree of Life is a Terrence Malick film released in May 2011, starring Brad Pitt, Sean Penn and Jessica Chastain.
• The Marvel Cinematic Universe film franchise features a Tree of Life, in a more science-based version of the mythical tree. In the 2011 Marvel Studios superhero film Thor, the Asgardian warrior Thor explains that the Nine Realms of the Asgardian cosmos are linked by Yggdrasil, the Norse mythological Tree of Life, which is here interpreted as a nebula in space connecting the planets in an orbit.

Anime

• In the anime Nadia: The Secret of Blue Water there is a giant tree beneath Antarctica that is identified as the Tree Of Life by Captain Nemo
• In the anime Ah! My Goddess Yggdrasil is a "program" that acts as the operating system for all life/reality.
• In the anime Genesis of Aquarion the Tree of Life is being fed to create a new Genesis.
• In the anime Rin - Daughters of Mnemosyne (Mnemosyne (anime)) The Tree of life is an eternal tree that bears fruit that grants immortal life to woman, while men are turned into short-lived angel-like creatures who have sex with and then kill the immortal woman.
• In the anime Neon Genesis Evangelion the tree of life was the source of all souls. Evangelion Unit-01 became the tree of life that once existed when it possessed the fruit of wisdom and fruit of life during Third Impact.
• In the anime Yu-Gi-Oh! 5D's the final antagonist Z-One, uses a deck of cards that are parts of the tree of life.
• In the anime Fullmetal Alchemist the tree of life is depicted on the alchemist's gates (their ability to perform transmutations)
• In the anime Sword Art Online, there is a large tree named the Yggdrasil, or the Tree of Life, sitting in the center of the VRMMORPG Alfheim Online.
• In the anime "Naruto", the Ten-Tailed Beast is revealed to be the harbinger of all life and mystical powers and its true form is a tree.

Other

• The logo of American health service and insurance company CIGNA makes use of a tree of life motif.

Decorative arts

5-kuruş-coin features the tree of life.

• The grandfather of British studio pottery, Bernard Leach, famously used a 'tree of life' on many of his works. Something which was continued by his son David Leach, among others.
• A motif of the tree of life is featured on Turkish 5 Kuruş coins, circulated since early 2009.

Physical "trees of life"

• The Arborvitae gets its name from the Latin for "tree of life."
• The Tule tree of Aztec mythology is also associated with a real tree. This Tule tree can be found in Oaxaca, Mexico.
• There is a Tree of Life in the island country of Bahrain in the Persian Gulf.
• Metaphor: The Tree of Utah is an 87-foot (27 m) high sculpture in the Utah Bonneville Salt Flats that is also known as the Tree of Life.
• In some parts of the Caribbean and in the Philippines, the coconut is considered the "tree of life" as its parts can easily be used for short/medium term survival such as for food, shelter, and various implements.
• Disney's Animal Kingdom theme park features an artificial tree dubbed "The Tree of Life," which has about 325 carvings of different species of animals. Inside the tree is the It's Tough to be a Bug! attraction.
• The West African Moringa oleifera tree is regarded as a "tree of life" or "miracle tree" by some because it is arguably the most nutritious source of plant-derived food discovered on the planet.^[29] Modern scientists and some missionary groups have considered the plant as a possible solution for the treatment of severe malnutrition^[30] and aid for those with HIV/AIDS.^[31]

Virtual particle

From Wikipedia, the free encyclopedia

In physics, a virtual particle is a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that exists for a limited time. The concept of virtual particles arises in perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines. ^[1][2]

Virtual particles do not necessarily carry the same mass as the corresponding real particle, and they do not always have to conserve energy and momentum, since, being short-lived and transient, their existence is subject to the uncertainty principle. The longer the virtual particle exists, the closer its characteristics come to those of ordinary particles. They are important in the physics of many processes, including particle scattering and Casimir forces. In quantum field theory, even classical forces — such as the electromagnetic repulsion or attraction between two charges — can be thought of as due to the exchange of many virtual photons between the charges.

The term is somewhat loose and vaguely defined, in that it refers to the view that the world is made up of "real particles": it is not; rather, "real particles" are better understood to be excitations of the underlying quantum fields. Virtual particles are also excitations of the underlying fields, but are "temporary" in the sense that they appear in calculations of interactions, but never as asymptotic states or indices to the scattering matrix.

Antiparticles should not be confused with virtual particles or virtual antiparticles.
Note that it is common to find physicists who believe that, because of its intrinsically perturbative character, the concept of virtual particles is a frequently confusing and misleading one, and is thus best to be avoided. ^[3][4]

Properties

The concept of virtual particles arises in the perturbation theory of quantum field theory, an approximation scheme in which interactions (in essence, forces) between actual particles are calculated in terms of exchanges of virtual particles.^{[citation needed]} Such calculations are often performed using schematic representations known as Feynman diagrams, in which virtual particles appear as internal lines.

A virtual particle does not precisely obey the formula m²c⁴ = E² − p²c².^[5] In other words, its kinetic energy may not have the usual relationship to velocity–indeed, it can be negative. The probability amplitude for it to exist tends to be canceled out by destructive interference over longer distances and times. A virtual particle can be considered a manifestation of quantum tunnelling. The range of forces carried by virtual particles is limited by the uncertainty principle, which regards energy and time as conjugate variables; thus, virtual particles of larger mass have more limited range.^{[citation needed]}

Written in the usual mathematical notations, in the equations of physics, there is no mark of the distinction between virtual and actual particles. The amplitude that a virtual particle exists interferes with the amplitude for its non-existence, whereas for an actual particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, actual particles are viewed as being detectable excitations of underlying quantum fields. Virtual particles are also viewed as excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles. Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled.

There are two principal ways in which the notion of virtual particles appears in modern physics. They appear as intermediate terms in Feynman diagrams; that is, as terms in a perturbative calculation. They also appear as an infinite set of states to be summed or integrated over in the calculation of a semi-non-perturbative effect. In the latter case, it is sometimes said that virtual particles contribute to a mechanism that mediates the effect, or that the effect occurs through the virtual particles.^{[citation needed]}

Manifestations

There are many observable physical phenomena that arise in interactions involving virtual particles. For bosonic particles that exhibit rest mass when they are free and actual, virtual interactions are characterized by the relatively short range of the force interaction produced by particle exchange.^{[citation needed]} Examples of such short-range interactions are the strong and weak forces, and their associated field bosons. For the gravitational and electromagnetic forces, the zero rest-mass of the associated boson particle permits long-range forces to be mediated by virtual particles. However, in the case of photons, power and information transfer by virtual particles is a relatively short-range phenomenon (existing only within a few wavelengths of the field-disturbance, which carries information or transferred power), as for example seen in the characteristically short range of inductive and capacitative effects in the near field zone of coils and antennas.^{[citation needed]}

Some field interactions which may be seen in terms of virtual particles are:

• The Coulomb force (static electric force) between electric charges. It is caused by the exchange of virtual photons. In symmetric 3-dimensional space this exchange results in the inverse square law for electric force. Since the photon has no mass, the coulomb potential has an infinite range.
• The magnetic field between magnetic dipoles. It is caused by the exchange of virtual photons. In symmetric 3-dimensional space this exchange results in the inverse cube law for magnetic force. Since the photon has no mass, the magnetic potential has an infinite range.
• Electromagnetic induction. This phenomenon transfers energy to and from a magnetic coil via a changing (electro)magnetic field.
• The strong nuclear force between quarks is the result of interaction of virtual gluons. The residual of this force outside of quark triplets (neutron and proton) holds neutrons and protons together in nuclei, and is due to virtual mesons such as the pi meson and rho meson.
• The weak nuclear force - it is the result of exchange by virtual W and Z bosons.
• The spontaneous emission of a photon during the decay of an excited atom or excited nucleus; such a decay is prohibited by ordinary quantum mechanics and requires the quantization of the electromagnetic field for its explanation.
• The Casimir effect, where the ground state of the quantized electromagnetic field causes attraction between a pair of electrically neutral metal plates.
• The van der Waals force, which is partly due to the Casimir effect between two atoms.
• Vacuum polarization, which involves pair production or the decay of the vacuum, which is the spontaneous production of particle-antiparticle pairs (such as electron-positron).
• Lamb shift of positions of atomic levels.
• Hawking radiation, where the gravitational field is so strong that it causes the spontaneous production of photon pairs (with black body energy distribution) and even of particle pairs.
• Much of the so-called near-field of radio antennas, where the magnetic and electric effects of the changing current in the antenna wire and the charge effects of the wire's capacitive charge may be (and usually are) important contributors to the total EM field close to the source, but both of which effects are dipole effects that decay with increasing distance from the antenna much more quickly than do the influence of "conventional" electromagnetic waves that are "far" from the source. ["Far" in terms of ratio of antenna length or diameter, to wavelength]. These far-field waves, for which E is (in the limit of long distance) equal to cB, are composed of actual photons. It should be noted that actual and virtual photons are mixed near an antenna, with the virtual photons responsible only for the "extra" magnetic-inductive and transient electric-dipole effects, which cause any imbalance between E and cB. As distance from the antenna grows, the near-field effects (as dipole fields) die out more quickly, and only the "radiative" effects that are due to actual photons remain as important effects. Although virtual effects extend to infinity, they drop off in field strength as 1/r² rather than the field of EM waves composed of actual photons, which drop 1/r (the powers, respectively, decrease as 1/r⁴ and 1/r²). See near and far field for a more detailed discussion. See near field communication for practical communications applications of near fields.

Most of these have analogous effects in solid-state physics; indeed, one can often gain a better intuitive understanding by examining these cases. In semiconductors, the roles of electrons, positrons and photons in field theory are replaced by electrons in the conduction band, holes in the valence band, and phonons or vibrations of the crystal lattice. A virtual particle is in a virtual state where the probability amplitude is not conserved. Examples of macroscopic virtual phonons, photons, and electrons in the case of the tunneling process were presented by Günter Nimtz ^[6] and Alfons A. Stahlhofen.^[7]

History

Paul Dirac was the first to propose that empty space (a vacuum) can be visualized as consisting of a sea of electrons with negative energy, known as the Dirac sea. The Dirac sea has a direct analog to the electronic band structure in crystalline solids as described in solid state physics. Here, particles correspond to conduction electrons, and antiparticles to holes. A variety of interesting phenomena can be attributed to this structure. The development of quantum field theory (QFT) in the 1930s made it possible to reformulate the Dirac equation in a way that treats the positron as a "real" particle rather than the absence of a particle, and makes the vacuum the state in which no particles exist instead of an infinite sea of particles.

Feynman diagrams

One particle exchange scattering diagram

The calculation of scattering amplitudes in theoretical particle physics requires the use of some rather large and complicated integrals over a large number of variables. These integrals do, however, have a regular structure, and may be represented as Feynman diagrams. The appeal of the Feynman diagrams is strong, as it allows for a simple visual presentation of what would otherwise be a rather arcane and abstract formula. In particular, part of the appeal is that the outgoing legs of a Feynman diagram can be associated with actual, on-shell particles. Thus, it is natural to associate the other lines in the diagram with particles as well, called the "virtual particles". In mathematical terms, they correspond to the propagators appearing in the diagram.

In the image to the right, the solid lines correspond to actual particles (of momentum p₁ and so on), while the dotted line corresponds to a virtual particle carrying momentum k. For example, if the solid lines were to correspond to electrons interacting by means of the electromagnetic interaction, the dotted line would correspond to the exchange of a virtual photon. In the case of interacting nucleons, the dotted line would be a virtual pion. In the case of quarks interacting by means of the strong force, the dotted line would be a virtual gluon, and so on.

One-loop diagram with fermion propagator

Virtual particles may be mesons or vector bosons, as in the example above; they may also be fermions. However, in order to preserve quantum numbers, most simple diagrams involving fermion exchange are prohibited. The image to the right shows an allowed diagram, a one-loop diagram. The solid lines correspond to a fermion propagator, the wavy lines to bosons.

Vacuums

In formal terms, a particle is considered to be an eigenstate of the particle number operator a^†a, where a is the particle annihilation operator and a^† the particle creation operator (sometimes collectively called ladder operators). In many cases, the particle number operator does not commute with the Hamiltonian for the system. This implies the number of particles in an area of space is not a well-defined quantity but, like other quantum observables, is represented by a probability distribution. Since these particles do not have a permanent existence,^{[clarification needed]} they are called virtual particles or vacuum fluctuations of vacuum energy. In a certain sense, they can be understood to be a manifestation of the time-energy uncertainty principle in a vacuum.^[8][9]

An important example of the "presence" of virtual particles in a vacuum is the Casimir effect.^[10] Here, the explanation of the effect requires that the total energy of all of the virtual particles in a vacuum can be added together. Thus, although the virtual particles themselves are not directly observable in the laboratory, they do leave an observable effect: Their zero-point energy results in forces acting on suitably arranged metal plates or dielectrics. By other hand, Casimir effect can be interpreted as relativistic van der Waals force.

Pair production

In order to conserve the total fermion number of the universe, a fermion cannot be created without also creating its antiparticle; thus, many physical processes lead to pair creation. The need for the normal ordering of particle fields in the vacuum can be interpreted by the idea that a pair of virtual particles may briefly "pop into existence", and then annihilate each other a short while later.

Thus, virtual particles are often popularly described as coming in pairs, a particle and antiparticle, which can be of any kind. These pairs exist for an extremely short time, and mutually annihilate in short order. In some cases, however, it is possible to boost the pair apart using external energy so that they avoid annihilation and become actual particles.

This may occur in one of two ways. In an accelerating frame of reference, the virtual particles may appear to be actual to the accelerating observer; this is known as the Unruh effect. In short, the vacuum of a stationary frame appears, to the accelerated observer, to be a warm gas of actual particles in thermodynamic equilibrium.

Another example is pair production in very strong electric fields, sometimes called vacuum decay. If, for example, a pair of atomic nuclei are merged to very briefly form a nucleus with a charge greater than about 140, (that is, larger than about the inverse of the fine structure constant, which is a dimensionless quantity), the strength of the electric field will be such that it will be energetically favorable to create positron-electron pairs out of the vacuum or Dirac sea, with the electron attracted to the nucleus to annihilate the positive charge. This pair-creation amplitude was first calculated by Julian Schwinger in 1951.

The restriction to particle–antiparticle pairs is actually only necessary if the particles in question carry a conserved quantity, such as electric charge, which is not present in the initial or final state.
Otherwise, other situations can arise. For instance, the beta decay of a neutron can happen through the emission of a single virtual, negatively charged W particle that almost immediately decays into an actual electron and antineutrino; the neutron turns into a proton when it emits the W particle. The evaporation of a black hole is a process dominated by photons, which are their own antiparticles and are uncharged.

Actual and virtual particles compared

As a consequence of quantum mechanical uncertainty, any object or process that exists for a limited time or in a limited volume cannot have a precisely defined energy or momentum. This is the reason that virtual particles — which exist only temporarily as they are exchanged between ordinary particles — do not necessarily obey the mass-shell relation. However, the longer a virtual particle exists, the more closely it adheres to the mass-shell relation. A "virtual" particle that exists for an arbitrarily long time is simply an ordinary particle.

However, all particles have a finite lifetime, as they are created and eventually destroyed by some processes. As such, there is no absolute distinction between "real" and "virtual" particles. In practice, the lifetime of "ordinary" particles is far longer than the lifetime of the virtual particles that contribute to processes in particle physics, and as such the distinction is useful to make.