Immaterium

The Immaterium (also referred to as the Empyrean, Aether or Warp) is an alternate dimension.

Superficially, it is the Warhammer 40,000 solution to the problem of faster than light travel, an equivalent to the Star Wars Universe's hyperspace. This is achieved by the Immaterium being a domain of pure energy, with spacecraft navigating between currents, as in an ocean.

The energy that makes up the Immaterium is believed to be the direct result of sentience. Considered to be a dark reflection of the material universe, it is an ocean of chaos, raw emotion given form. Stirred by emotion and action, the Immaterium is the realm of Chaos, home to the dark gods and their 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, can be used for faster-than-light communication and travel. Vast amounts of energy are required for a vessel to enter the Immaterium, and while within it, they are often at the mercy of the currents and eddies that make up the dimension.

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. 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 started.

Necessities of Warp Travel
Several things are necessary for travel within the Immaterium. Primary among these is the ancient device known as a Gellar field, a psychic ward that protects the starship. Without this arcane techno-magic, the souls of those travelling within the Warp would be exposed to the many daemons and creatures that inhabit the twisted realm. For 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, and are able to guide a ship to its intended destination with much greater accuracy than normal humans. However, for a Navigator to fulfill this role, he needs a 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, ships 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 they left yet only have experienced a few days onboard, while many more vanish without trace.

The Fast-flowing Stream Analogy
A common explanation of the warp likens it to a fast-flowing stream, where the material world is the banks - by dropping a corpse in, it can travel faster than someone walking along the banks, but is at the mercy of rapids (warp storms), Rocky areas (daemons, Etc) and flooding. This analogy is explained further in the Fall of Medusa V booklet, available with White Dwarf 317 (UK) and online at the Fall of medusa V website.

The Realm of Chaos
The powers of Chaos and their servants are known to inhabit the Immaterium. 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, foremost among these being the species known as Enslavers. They are formed 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 there will, hence their name. Enslavers are able to control almost any species, even creatures such as tyranids, necrons and deamons! Enslavers exist primarily in the warp, but enter the physical universe through the unprotected mind of a human psyker. They do this by three enslavers banding together to form a 'trinity', whereupon they form a psychic bond with the psyker. Over the course of the next 2-3 days, the psykers organic chemistry changes significantly, the end result being a living, pulsing warp gate, through which the enslavers travel. 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 there bottom half, and 2 larger tentacles ending in suckers. On the smaller, upper half of there body, they have a sensory organ sometimes referred to as an 'eye'; this circular organ is normally reddish in colour. There 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 and influenced by mortal emotion and action. The mind of every sentient creature leaves an imprint within the Immaterium, and although the signature of one mind is almost insignificant, when the imprints of an entire race are combined they have a huge impact on the very nature 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 in the material realm) When an emotion or faith grows strong enough, it becomes one of the denizens of the Warp. Many of these beings vanish when the emotion that creates them passes, but the strongest become self-perpetuating, spreading the thoughts and events that grant them form and power. The strongest of these beings become the gods associated with a group, and almost every single race or force of will is represented, be they Chaos, Eldar, Ork, or Human.

Psykers
The abilities of psychically gifted individuals known as psykers are powered by the energies of the Warp, and when such action is undertaken, 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.

Warp/Realspace Overlays
On rare occasion, the realm of the Immaterium is able to break over into the physical universe. These nightmare realms are often home to the once-mortal followers of the Powers of Chaos, and are often the site of titanic atrocities.

The most well known overlays are:
 * The Eye of Terror, created when the Eldar inadvertently caused the creation of Slaanesh, and now home to the traitorous Chaos Space Marine Legions and their followers.
 * The Maelstrom
 * The Storm of the Emperor's Wrath
 * Van Grothe's Rapidity

Warp Storms
Regions of material space can be affected by the ebb and flow of the Immaterium. These 'warp storms' can isolate star systems and sectors from each other, by making the Imperium's normal methods of travel and communication impossible. 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/Realspace overlay.

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 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 or that the Warp continues in other areas, but uninhabited by Demons due to lack of sentinence.

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?

This question was answered though in the second edition Codex Tyranids, which included an Inquisitor, musing on the monolithic nature of the Tyranid threat, in which he mentioned even the forces of Chaos would not be so enduring, for the day when mankind ceased to exist, the warp would as well.