Immaterium

The Immaterium (also referred to as the Empyrean, Aether or Warp) is an alternate dimension in the fictional Warhammer 40,000 universe.

Superficially, it is the Warhammer 40,000 solution to the problem of faster-than-light travel, an equivalent to the hyperspace concept popular in science fiction. As the Immaterium is a domain of pure energy unaffected by physical laws, faster-than-light feats are possible within it.

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.

Requirements
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. The extreme outer sections of the in-setting galaxy however are too remote for the Astronomican to reach, leaving them largely if not totally untouched by humans.

Dangers
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. Vessels travelling through the Warp are constantly under attack by the malign lifeforms that inhabit that dimension. If a ships' Gellar Field fails, the vessel and inhabitants are torn apart in ways some consider physically impossible, by daemonic entities and the pure ferocious energy of the Warp.

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), dangerous creatures (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.

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. They come in many forms, but all are mindless, and all are predatory, seeking out unprotected souls to feed on.

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 such as thrak'kar are powered by or draw power from 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. Daemonic entities can use an unshielded psyker's mind like a gateway into the material realm. Once this has occurred often a chain reaction starts, to the point where common 'ungifted' minds can be possessed and used as gateways. 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 can become the target of daemonic possession, or if sufficiently powerful, control the daemonic entities of the Warp. Due to the influence of Chaos within the warp, such individuals are almost always megalomaniacal, seeking to use the daemons and power of the Warp for personal gain.

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
 * The "Perdus Rift"

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 what prevents the Immaterium from spilling out and completely engulfing the material universe. Possible causes (potentially all of them) are:


 * Tzeentch, the Emperor of Mankind, or a creature known as the "Numan" (from the Inquisition War Trilogy) that keeps it at bay.


 * The anti-psychic technology of the Necrons, as implied in recent canon.


 * The laws of physics.

It should also be noted that Warp travel and Psykers accessing the Immaterium weakens the Warp/Real Space barrier by tiny increments (as psychic activity is often noted as a 'weak spot' between the Warp and our dimension), which suggests that if the Warp had never been accessed by lifeforms of this dimension the two would remain totally separate.

Apparently, 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 is uninhabited by Demons due to 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 is noted in the Necron Codex that the Warp was once a calm realm of pure energy, but uncontrolled psychic activity and emotion has 'whipped up' the Warp into a realm of insanity and danger. The irony of the situation is that the beings that seek to consume the galaxy in the Warp would perish if such an event occurred.

Still, as warp creatures are the result of a particular emotion reflected in the warp, it is possible they are too singular in existence and purpose to fully appreciate this paradox.