Discworld (world)
The Discworld is the fictional world where English writer Sir Terry Pratchett's Discworld fantasy novels take place. It consists of an interstellar planet-sized disc, which sits on the backs of four huge elephants, themselves standing on the back of a world turtle, named Great A'Tuin, as it slowly swims through space.
The Disc is the setting for all forty-one Discworld novels; it was influenced by world religions which feature human worlds resting on turtles, as a setting to reflect situations on Earth, in a humorous way. The Discworld is peopled mostly by three main races: men, dwarfs, and trolls. As the novels progress, other lesser known races are included, such as dragons, elves, goblins, and pixies.
Pratchett first explored the idea of a disc-shaped world in the novel Strata.
Great A'Tuin
Great A'Tuin is the Giant Star Turtle who travels through the Discworld universe's space, carrying four giant elephants who in turn carry the Discworld. The narration has described A'Tuin as "the only turtle ever to feature on the Hertzsprung–Russell diagram".Great A'Tuin's sex is unknown to the inhabitants of Discworld, but the subject of much speculation by some of the Disc's finest scientific minds. The sex of the World Turtle is pivotal in proving or disproving a number of conflicting theories about the destination of Great A'Tuin's journey through the cosmos. If, as the Discworld version of the popular "Big Bang theory" states, Great A'Tuin is moving from the Birthplace to the Time of Mating, then at the point of mating the civilisations of the Disc might be crushed, simply slide off, or else the entire world will end. The hypothesis is that all stars in the sky are also worlds carried by giant turtles, and that when all the turtles meet they will mate passionately, for the first and only time; from that mating, it is hypothesised that new turtles would be born to carry a new pattern of worlds. Attempts by telepaths to learn more about Great A'Tuin's intents have not met with much success, mainly because they did not realise that its brain functions are on such a slow timescale. All they have been able to discern is that the Great A'Tuin is looking forward to something.
The other theory, described as being popular among the Discworld's academics as the Discworld version of the steady state theory, which in-universe is known as the "steady gait" theory, is that Great A'Tuin came from nowhere and is going to keep swimming through space to nowhere forever. Eric shows Great A'Tuin being made instantly from nothing, seemingly in support of the "steady gait" theory; however, the events in The Light Fantastic, in which the Great A'Tuin attended the hatching of eight baby turtles, each with four baby elephants and a tiny discworld of their own, would seem to support the Big Bang hypothesis.
The little turtles have since gone off on their own journeys. Whether this was the event the Great A'Tuin was looking forward to or merely one step towards its ultimate goal is not mentioned.
Great A'Tuin has been mentioned to frequently roll on its belly to avoid asteroid and comet collisions, or even to snatch these projectiles out of the sky which might otherwise destroy the Disc. These stunts do not affect the Disc's population, other than to induce severe seasickness on anyone who happens to be looking at the night sky at that time. A'Tuin has been known to do more complex rolls and corkscrews, but these are rarer. This is similar to real-world sea turtles' habit of rolling over with their shell down to protect themselves from sharks.
Due to the Great A'Tuin's travelling through the universe, the night sky of the Discworld changes markedly over the course of decades, as the turtle departs older constellations and enters new ones. This means that astrologers must constantly update and alter their horoscopes to incorporate all-new zodiacs.
A tiny sun and moon orbit the Great A'Tuin, both about in diameter when described at the start of the series, but the description of their diameter is increased to at least later in the chronicles. The moon is slightly closer to the Disc than the sun. One half is covered with silvery glowing plants, which feed the lunar dragons. The other half is burnt black by the sun. The moon rotates and completes a full revolution in about a month; the full moon occurs when the luminescent side is completely visible from the Disc; the new moon when the dark side is shown. The sun's orbit is so complex that one of the elephants has to cock its leg to allow the sun to continue on its orbit.
According to the wizards of Unseen University, Chelys galactica, and thus Great A'Tuin, are composed largely of the fictional element chelonium, the properties of which are apparently known to them, but not to readers.
Magic
Magic is the principal force in the Discworld and operates in a similar vein to real-world elemental forces such as gravity and electromagnetism. The Disc's "standing magical field" is essentially the local breakdown of reality, which allows a flat planet on the back of a turtle to even exist. The force called "magic" is really just a function of the relative absence of reality in the local area, much in the same way that the absence of heat is described as "coldness". Magic warps reality in much the same way as the real universe's gravity warps its space-time. The act of performing magic is, essentially, telling the universe what form it must take, in terms that the universe cannot ignore. This is very draining to magic users, due to Discworld science's Law of Conservation of Reality, which states that magic and mundane methods require the same energy expenditure. This is why most Discworld wizards store magic in a staff, which is a sort of capacitor for magical energy.On the Discworld, where magic has more in common with particle physics than Houdini, high-level background magic occurs when a very powerful spell hits, creating myriad sub-astral particles that severely distort local reality. Building a house in a region where this has happened is extremely dangerous, as it is mentioned that an individual may not remain the same species, shape or level of sanity as they were when they entered. Medium levels cause odd effects, such as coins landing on their edges and turning into caterpillars. Areas with larger than normal quantities of background magic tend to display unusual qualities, even for the Disc. Very high quantities of magic can knock a hole in Discworld reality, leading to an invasion by Lovecraftian monstrosities from the Dungeon Dimensions, or, almost as bad, the world of the Elves.
In the Discworld universe, magic is broken into elementary particulate fragments in much the same way that energy and other forces are in real-world quantum physics. The basic unit of Discworld magic is the thaum, equal to the amount of mystical energy required to conjure up one small white pigeon, or three normal-sized billiard balls. Several SI-modifiers have been applied to it in the books. Magic can be measured with a thaumometer, which is described as a black cube with a dial on one side. A standard thaumometer is good for up to a million thaums, beyond that level, Discworld reality starts to break down. Parodying the introduction of the metric system, later Discworld novels refer to the introduction of the newer unit, the Prime, to avoid arguments over the standard sizing of pigeons. It is defined as the magical energy required to move one pound of lead one foot, referencing the joule.
The thaum also appears to be a particle, the Discworld physics equivalent of the atom. "Splitting the thaum" revealed that it was in fact composed of numerous sub-particles, called resons which in turn are created from a combination of up to five "flavours": up, down, sideways, sex appeal, and peppermint.
In the opening books, the number eight is significant and has magical properties on the Disc, and should never be spoken by wizards, especially in certain places. Doing so may allow the ancient dungeon dimension creature "Bel-Shamharoth the sender of eight" to break through. On the other hand, eight turns up in many places one would expect the number seven in the real world. After The Colour of Magic, both the colour and the number eight no longer appear as dangerous.
The Disc's magical field is centred on Cori Celesti. Everyday natural forces, such as light and magnetism, are muffled by the power of the Disc's magical field, and rather than a magnetised needle, navigators on the Disc use a compass with a needle of the magical metal octiron, which will always point towards Cori Celesti. Light is so oddly affected by magic that, as it passes into the Disc's atmosphere, it actually slows down from millions to hundreds of miles an hour. One odd effect of this is that the Disc has time zones, however, as a flat world, it should not. Another effect is that, as reported in Thud!, the red- and blue-shifting of light becomes noticeable when travelling at speeds of merely.
The power of belief
Reality is spread thinly on the Disc, so events may be affected by expectations, especially those of 'intelligent' species such as humans, dwarfs, etc. As such, the Discworld is not governed by real-world physics or logic but by belief and narrative resolution. Essentially, if something is believed strongly enough, or by enough people, it may become true. Jokes such as treacle mines and drop bears are real on the Disc. In reality lemmings do not actually rush en masse off cliffs; on the Disc, they do, because that is what people believe. These concepts of Discworld physics are also exploited in both wizard and witch magic. For example, when a character wishes to turn a cat into a human, the easiest way is to convince the cat, on a deep level, that he is a human. In fact, the main reason the Auditors of Reality dislike sentient beings in the Discworld universe is that the Auditors are the personifications of the real-world laws of physics, but the Discworld physics' power of belief and the humans' ignorance constantly remake the world, making their work fruitless.More significantly, it is also belief that gives Discworld's gods their powers. Discworld gods start off as tiny spirits, and gain power as they gain believers; this is explored most thoroughly in Small Gods. A similar effect has led to the personalisation and "reification" in the Discworld universe of mythological beings symbolising abstract concepts, such as Death, the Hogfather and other anthropomorphic personifications. In Hogfather, the assassin Mr. Teatime tries to kill the patron of Hogswatch by using an old magic that involves controlling a person with a part of their body, in order to stop children from believing in him, and almost succeeds. It is also possible for humans to gain godly powers through the power of belief, typically venerated rulers such as the Kings of Djelibeybi as seen in Pyramids, and the Duchess Annagovia of Borogravia in Monstrous Regiment. It is possible for gods to derive power from misotheists, as demonstrated in Small Gods when Om takes a liking to the ardently anti-Omnianist Sergeant Simony. As demonstrated in
Such is the nature of belief on the Disc that temporary new gods come into being on a regular basis, and often for such mundane things as stuck drawers; Anoia, Goddess of Things That Get Stuck in Drawers, came into her powers in such a manner, though she was previously a volcano goddess.