Gate of Alchemy
The Gate of Alchemy or Gate of Truth is a fictional metaphysical construct integral to the plot of the anime Fullmetal Alchemist. The Gate is the source of all alchemic energy, often exemplified by the light of an alchemic transmutation. The Gate of Alchemy is depicted differently in anime and manga versions of Fullmetal Alchemist. In the anime, it is depicted as a large, black, two-doored gate that is variable in height and typically suspended in a white void, bears a striking resemblance to Auguste Rodin's "The Gates of Hell". In the manga, it is shown as being a huge clay tablet depicting the Sephiroth.
Nature of the Gate
In the anime
The Gate seems to not be any particular location but rather a dimensional rift to be entered; Dante stated during her confrontation with Hohenheim of Light that an entrance to the Gate exists in every human soul since birth. As people age, the Gate's presence within them is weakened. However, there are certain uses of alchemy that bring a person directly to the "Gate of Alchemy". Those who call upon the Gate, or are sacrificed to reach it, usually lose a portion of their body that is drawn within it by the creatures who inhabit the Gate. These small, purple-eyed, black creatures of unknown origin serve to regulate passage through the Gate, and they can reach outside of it by extending their string-like arms.
The Gate also contains what is referred to as the "Truth", i.e., an inconceivable amount of knowledge about alchemy and even the nature of life itself. It is bestowed upon all those who manage to access the Gate. Not all of this knowledge is always immediately retrievable from memory afterwards, but the knowledge gained includes the methods for binding a soul to an object through a Blood Seal, as well as being able to transmute alchemy without the use of physical transmutation circles. (The alchemist can actually create a circle within themselves by envisioning it in their minds and then activating it with the touching of their limbs; the most common example is a clap of their hands.) The "Truth" even seems to be transferrable to Homunculi, as the Homunculus Wrath was able to perform alchemy without "circles." Most likely due to his experience within the Gate, by using the arm of Edward Elric (that the creatures of the Gate took from him during his attempt at bonding his brother's soul to the armor) along with Edward's leg (which was taken when he attempted to resurrect his dead mother Trisha) and Wrath's unique nature in being a Homunculus that was created with a human body as material.
The Gate of Alchemy can serve as a one-way spiritual or physical door to at least one parallel world (Earth) which has a history almost identical to the world of Amestris up to the point where true alchemy was developed. Interestingly, when a being travels through the gate and appear on earth, they seem to gain knowledge of events that have happened there, it also appears that they gain knowledge of future events. This is shown when Hohenheim of Light explains to Ed how the parallel world(Earth) differs from theirs as a world torn by war, and that worst is yet to come. He then asks Ed if he saw what he was talking about when he came through the gate, and a scene of an mushroom cloud from an Atomic bomb is shown.
In the manga
Like in the anime, the "Truth" exists at the Gate, but rather than being a part of the Gate itself, it originates from an entity residing there (some call it "God" or "the World"). This being, who is featureless save for a frightening grin, meets alchemists who have attempted human transmutation in front of the Gate. For the "toll" of a body part, the entity reveals to the alchemist a portion of the "Truth"; the more that is sacrificed, the more that is revealed. After the transaction, the being keeps the body part(s) that are taken, and is usually seen grinning as the alchemist is flung back into the real world.
The Homunculus Gluttony was designed by Father as an attempt to access the Gate, but instead he is merely an imitation; a "fake" Gate. When Gluttony gets incredibly angry, he will transform into his true form, which consists of his chest opening up into a large, dark void that bears a similar single eye to the one that the true Gate contains. It is this void where everything he consumes remains; the only way to escape it is through the true Gate.
Functions of the Gate
In the anime
The Gate is the basis for alchemy in the world of Fullmetal Alchemist. The energy of alchemy (required to at least decompose and rebuild matter) in the world of Amestris is generated by the souls of deceased humans traveling through the Gate from Earth. If accessed properly, the Gate can also be used for the living to travel from one world to the other. Furthermore, the Gate serves to bestow the "Truth" to those who open it.
In the manga
The functions of the Gate of Alchemy in the manga are similar to that of the anime, but there is no mention of an alternate world existing beyond the Gate that allows for the existence of the souls necessary for the energy of alchemy in the world of Amestris.
Summoning the Gate
In the anime
Alchemists can directly summon the Gate by at least three methods: attempting human transmutation; activating the appropriate transmutation circle on an infant (whose bond between mind, body, and soul is still weak), and sacrificing all or part of a human or Homunculus on the proper transmutation circle.
Once a human attempts to revive another person, he or she literally creates them, "giving birth" as a mother does with her child. That an infant -with a weak bond between body, mind, and soul- is closer to the gate than an adult may suggest that human lives originate from the Gate. This tractability of the mind, body, and soul is what eventually causes alchemists to witness the gate.
In the manga
The only known way for an alchemist to access the Gate of Alchemy in the manga is by means of attempted human transmutation.