Immaterium

The Immaterium (also referred to as the Empyrean, the Aether, the Sea of Souls, the Realm of Chaos, Warpspace or most commonly, the Warp) is an alternate dimension of purely psychic energy that echoes and underlies the familiar four dimensions of the material universe. It is the source of all psychic powers and known instances of so-called "sorcery" and the home dimension of the Chaos Gods and their myriad daemonic servants. Superficially, it is the Warhammer 40,000 solution to the problem of faster than light travel, an equivalent to the Star Wars universe's dimension of hyperspace. This function as a faster-than-light medium for space travel is achieved by the Immaterium being a domain of pure psychic energy, with spacecraft navigating between currents, as in an ocean. The psychic energy that makes up the Immaterium is believed to be the direct result of the existence of sentience in the universe, in particular the Milky Way Galaxy. Considered to be a dark reflection of the material universe, the Warp is an ocean of chaos, raw emotion given energetic form. Stirred by emotion and action, the Immaterium is the true realm of Chaos, home to the dark gods who comprise the Ruinous Powers and their daemonic followers. It also is rumoured to house the spirits of the dead, and therefore can be considered the "Underworld" of the Warhammer 40,000 universe.

Warp Travel
The realm of the Immaterium, while not ideal for beings of the material universe, can be used for faster-than-light communication and travel. Vast amounts of energy are required for a vessel to enter the Immaterium from the phsyical world, and while within it, they are often at the mercy of the currents and eddies that make up the chaotic dimension. Warp travel is not to be taken lightly, and the dangers range from hull damage to daemonic possession. Entire fleets have entered the Warp and never emerged, lost to the infamous Chaos spawn and the corruption and insanity that follows. Time also proceeds a great deal slower in the Warp than in the material universe, creating a form of stasis. In real time, journeys can take months or even years, though to the crew and passengers only weeks seem to pass in the Warp. The timing itself is unpredictable due to the very nature of the Immaterium, and occasionally a fleet of reinforcements will emerge from the Immaterium to find that the war they were sent to fight in has long been lost, or not yet even started!

Necessities of Warp Travel
Several things are necessary for travel within the Immaterium. Primary among these is the ancient device capable of producing what is known as a Gellar Field, a technologically-generated psychic ward that protects the starship by generating a bubble of realspace around the vessel. Without this arcane techno-magic, the souls of those travelling within the Warp would be constantly exposed to the many daemons and psychic creatures that inhabit the twisted realm. For starship jumps any longer than a handful of light-years, a member of the mutant psyker breed known as Navigators must be aboard. These mutant humans are able to 'see' the currents of the Immaterium with their psychic "third eye", and are able to guide a starship to its intended destination with much greater accuracy than normal humans or even computerized aid. However, for a Navigator to fulfil this role, he needs an unchanging reference point, which is provided by the Astronomican.

Dangers of Warp Travel
The Immaterium is far from a safe place to travel through. Great currents and storms can blow starships into uncharted areas of the galaxy and make travel through certain regions impossible. Warp storms that last for centuries can isolate sectors and leave fleets becalmed within the material universe. Even worse, starships could find themselves becalmed within the Immaterium itself, a terrible fate for its passengers as they become playthings for the dark creatures that inhabit that diabolical realm. Sometimes a ship will emerge from Warp space centuries after it left yet its passengers have experienced only a few days onboard, while many more vanish without trace.

The Fast-flowing Stream Analogy
A common explanation of the Warp's mechanics for faster-than-light travel likens it to a fast-flowing stream, where the material world is the banks - by dropping a stone in, it can travel faster than someone walking along the banks, but it is at the mercy of rapids (Warp storms), rocky areas (daemons, Enslavers, etc.) and flooding.

Realm of Chaos
The Ruinous Powers of Chaos and their servants are known to inhabit the portion of the Immaterium often called the Realm of Chaos. Daemons are only able to exist within the Immaterium, unless certain conditions are met within the material realm. The Immaterium is where the servants of the four Dark Gods (Khorne: the Blood God, Slaanesh: the Dark Prince of Pleasure, Tzeentch: the Architect of Fate, and Nurgle: the Lord of Decay) gain their power from, and their followers will undertake any action to increase this power.

Along with the powers of Chaos, the Immaterium is inhabited by many strange creatures formed from psychic energy, foremost among these being the species known as the Enslavers. They are spontaneously created by the shifting energies of the Warp from the nightmares of sentient species, and prowl the Immaterium like sharks. The Enslavers are able to psychically control other species, forcing them to their will, hence their name. Enslavers are able to control almost any species, even creatures such as Tyranids and daemons! However it is thought that the only race immune to the Enslavers are the Necrons, who have no souls nor psychic presence in the Warp. Enslavers exist primarily in the Warp, but can enter the physical universe like a daemon through the unprotected mind of a human psyker. They are able to do this when three Enslavers band together to form a 'trinity', whereupon they form a psychic bond with their psyker victim. Over the course of the next 2-3 days after contact is made, the psyker's organic chemistry changes significantly, the end result being a living, pulsing Warp gate, through which the Enslavers travel into the material universe. Enslavers are rather like an octopus in shape, their main body being made up of two roughly circular parts, one on top of the other. They have 8-12 small tentacles protruding from their bottom half, and 2 larger tentacles ending in suckers. On the smaller, upper half of their body, they have a sensory organ sometimes referred to as an 'eye'; this circular organ is normally reddish in colour. Their leathery skin is able to change colour, but is normally a light brown with pale tentacles.

The Psychic Realm
The Immaterium is a dimension of the spiritual, created, sustained and influenced by intelligent mortal emotions and actions in the physical universe. The mind of every sentient being in the material universe save the Necrons and the C'tan leaves a psychic imprint within the Immaterium, and although the signature of one mind is almost insignificant in the energy or influence it generates, when the imprints of an entire intelligent race of billions upon billions of individuals are combined they have a huge impact on the very nature and shape of the Warp (this seems to imply that the Immaterium is not chaotic by nature, but has been made so by the chaos and war that defines so much of the material realm). When an emotion or faith in an entity or concept grows strong enough, it actually becomes one of the denizens of the Warp. Many of these beings vanish when the emotion or beliefs that create them pass, but the strongest and most archetypal can become self-perpetuating, spreading the thoughts and events that grant them form and power and even interacting with the material world in a way that further strengthens the beleifs and attitudes that give them existence. The strongest of these beings become the gods and daemons associated with a specific group or culture, and almost every single intelligent race or force of will has its universal concepts or religious figures personified in some way by the Warp, be they Chaos, Eldar, Ork, or human.

Psykers
The extrasensory, seemingly magical abilities of the psychically-gifted individuals known in the Imperium of Man as psykers are powered by the energies of the Warp, and when such power is used, the daemons and creatures of the Immaterium are drawn to the psyker's mind like moths to a flame. Those trained in the use of the psychic arts are able to resist such incursion to a degree, but the greatest fear of the Inquisition is the fact that an untrained psyker of sufficient power can become the target of daemonic possession--and a gateway for the intrusion of the daemonic servants of the Ruinous Powers into the physical world.

Warp Rifts
On rare occasions, the realm of the Immaterium is able to break out into the physical universe. These nightmare realms, known as Warp rifts, are usually home to the once-mortal followers of the Ruinous Powers of Chaos, and are often the site of titanic atrocities.

The most well-known Warp rifts in the Milky Way Galaxy include:
 * The Eye of Terror, created when the Eldar inadvertently caused the creation of Slaanesh, and is now home to the traitorous Chaos Space Marine Legions and their other human followers like Chaos Cultists, Traitor Imperial Guardsmen and the tech-priests and Traitor Titan Legions of the Dark Mechanicum.
 * The Maelstrom
 * The Storm of the Emperor's Wrath
 * Van Grothe's Rapidity

Warp Storms
The Warp is an extremely volatile medium. Regions of material space can be affected by the constant ebb and flow of the Immaterium. These "Warp Storms" can isolate star systems and entire sectors from each other, by making the Imperium's normal methods of travel and communication impossible as the Warp surrounding those regions becomes too chaotic for a starship to safely travel through or for an astropath to send or receive a telepathic message through the psychic "interference." These storms can last for any amount of time, ranging from weeks to centuries, and can occasionally break out into the material universe, creating a temporary Warp rift that can cause a Chaos incursion.

Controlling the Warp
There are conflicting accounts as to whether it is Tzeentch, the Emperor of Mankind, or a creature known as the "numan" (from the Inquisition War Trilogy) that prevents the Immaterium from spilling out and completely engulfing the material universe. However, recent canon seems to imply that the anti-psychic technology of the Necrons contributes, too. It is also possible that the laws of physics act as a barrier. It should also be noted that the Immaterium seems to only be relevant to the Milky Way Galaxy (although the Tyranids' case poses a question), implying that it is either a unique occurrence, that there may be other "Warp shadows" of other galaxies where sentient life exists or that the Warp continues in other areas, but uninhabited by daemons due to the lack of sentience. A paradox of thoughts is apparent: the extra-dimensional entities of the Warp are sustained by the minds and souls of living creatures within the physical world. So what would occur to the Warp if all life was obliterated through the fall of the physical reality? It would probably cease to exist.