Immaterium

"The Warp is a strange and terrible place. You might as well throw a traveller into a sea of sharks and tell him to swim home as send him through the Warp unprotected. Better it is not to let common man travel through the stars. Better still, let him not know such a thing is feasible."
—Fra Safrane, 5th Aide to Navigator De'el

Overview

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. The Warp is the source of all psychic powers and known instances of so-called "sorcery" or "magic" as well as the home dimension of the powerful entities known as the Chaos Gods and their myriad legions of Daemonic servants. In fact, the terms "Chaos" and the "Warp" are often used interchangeably by those aware of their existence within the Imperium of Man.
Superficially, the Immaterium is Mankind's solution to the problem of faster-than-light travel. The Warp functions as the medium for FTL interstellar travel, with voidcraft entering it through the use of a Warp-Drive. They then use their own mundane reaction drive systems and the aid of a psychic Navigator to navigate between its flowing currents of psychic energy, as if they were moving through an ocean.

Warp Travel

"The Warp is our greatest gift, and also our greatest threat. It is curse and boon, hope and terror, a raging inferno through which we must plunge, or else be lost."
—Navigator D'Halnari

A voidcraft enters the Warp by activating its Warp-Drive. As a starship leaves the material universe it enters a corresponding point in Warpspace. The ship is then carried along by the tides and currents of the Warp, like a pebble in a raging river. After travelling in this fashion for an appropriate time, the voidship uses its Warp engines to drop back into realspace. Because the material universe and the Warp move relative to one another, the voidship reappears in a new position several light years from the starting point. This process is called a Warp jump or a hop, and the process of entering or leaving the Immaterium is known as a drop, shift, or translation.
Journeys through the Warp are usually undertaken in short jumps of up to four or five light years. Longer jumps are unpredictable and dangerous. The tides of Warpspace move in complex and inconsistent patterns, and voidships attempting longer hops often end up widely off-course. Were this limitation to apply to all Warp travel, then Humanity would not have spread throughout the galaxy as it has.
It is possible to make long jumps of many light years by steering a voidship in the Warp itself -- sensing, responding to, and exploiting its currents and thereby directing the craft towards a corresponding point in the material universe. Only the strain of Human mutants known as Navigators can pilot a craft through the Warp in this way.
Some individuals are sensitive to the movements of Warpspace. They can, for example, sometimes tell that a voidcraft is approaching even before it drops back into the material universe. Human sensitivity to the Warp is not generally well-developed.
However, in a minority of people this sensitivity is far more finely-tuned. These people are known as psykers, and they are able to consciously control and use the energy of the Warp to affect the material universe. Navigators possess psychic gifts of a more specialised kind that only allows them to use their powers to actually look into the Immaterium without going insane and then use this knowledge to steer their spacecraft through its flowing currents.

The Astronomican and the Warp

The Astronomican is a psychic homing signal centred upon Terra and directed through the Warp by the mind of the Emperor of Mankind. It is powered by the continuous mental concentration of a thousand Human psykers, whose life essences are slowly drained until they die and must be replaced. The Astronomican cannot be detected in the real universe -- only in the Warp.
It is by means of this signal that the Navigators can steer their voidships through the Warp over long distances. The Astronomican's signal is strongest close to Terra and the Emperor of Mankind who is its source and gets increasingly weaker further away. It extends over a spherical area with a diameter of about 50,000 light years.
The Astronomican does not extend to the extreme fringes of the galaxy, and because Terra is situated in the galactic west, its signal does not reach a massive swathe of the eastern part of the galaxy at all. Nor is the extent or strength of the signal constant -- it can at times be blocked by localised disturbances of the energy within the Warp itself.
Such activity may be compared to the hurricanes or storms of a terrestrial weather system and is known as a Warp Storm. Warp Storms may be so bad, and so long-lasting, that entire star systems are isolated for hundreds of standard years at a time.
A Warp Storm not only obscures the signal of the Astronomican, it is also dangerous for spacecraft travelling nearby. No spacecraft can venture within a Warp Storm and expect to survive, although there are tales of miraculous escapes and of starships being thrown tens of thousands of light years off course.
Warp Storms are not the only dangers within the Warp. There are sentient energies and other immaterial life-forms that inhabit it, creatures formed from, and part of, the shifting stuff of the Warp. They are without fail hostile to all live within the material universe. They are known to Mankind as "Daemons."

Time Displacement

The time differences between realspace and Warpspace are quite drastic. Not only does time pass at different rates in both kinds of space, but it also passes at very variable rates.
Until a starship finishes its jump, it is impossible for a vessel's crew to know exactly how long their journey has taken in realspace. Time passing in realspace is referred to as "real time." Time passing on board a spacecraft in the Immaterium is referred to as "Warp time."
The relationship between real time and Warp time is variable. So, for example, a 100 light year jump will seem to take from 2.5 to 9.5 solar hours to the crew of a vessel in the Warp, but between 3 solar days and 3 solar weeks will have actually passed in realspace.
These times do not include the journey times out to and from the Mandeville Points within each star system before a Warp translation can begin or end.
It can take solar days to weeks of travel at sub-light speed to reach a Mandeville Point, and a similar time to re-enter a new star system from the destination Mandeville Point.
The Imperium is approximately 75,000 light years from edge to edge. A journey of this length could take between 75 and 300 solar days in subjective Warp time, and between 6 and 40 Terran years in real time.

Warp Navigation

Before a starship can enter the Warp, it must first travel at sublight speed to a star system's so-called "Mandeville Point." This is the closest distance that a voidship can safely enter or exit the Immaterium from its origin or destination system, respectively.
Once a spacecraft activates its Warp-Drive, it is plunged into a dimension very different from the material universe. It is convenient to imagine Warpspace as consisting of a relatively dense, almost liquid, energy, devoid of stars, light and life as it is commonly known. Once within Warpspace a voidship may move by means of its main Newtonian reaction drive (such as a standard Imperial Plasma Drive), following powerful eddies and currents in the Warp, eventually reaching a point in the Warp corresponding to a destination in realspace.
The most difficult aspect of Warp travel is that it is impossible to detect the spatial movement of Warpspace once a voidship is in the Warp. The ship can only blindly carry on, its crew trusting that it is going in the right direction. The longer a ship remains in the Immaterium, the greater the chances of encountering some unexpected current that can turn it unknowingly off-course.
Navigation of Warpspace can be achieved in two ways: the calculated jump and the piloted jump.
All Warp-Drives incorporate navigational mechanisms. When the voidship is in realspace, these monitor the ever-shifting movements of that part of the Warp corresponding to the voidship's current position. It is a "window" into Warpspace. By means of observing these movements in the Warp it is possible to calculate a course, corrective manoeuvres, and approximate journey time to a proposed destination.
Calculation relies on the assumption that the Warp-currents observed from realspace do not change significantly during flight. This method is known as a "calculated jump." It is not safe to make a calculated jump of more than four light years at one go. The longer the jump, the greater the chances of a significant change in Warp current movement.
The second, and more efficient, form of Warp-navigation is the piloted jump. This method relies upon two factors: the Navigators and the psychic beacon of the Astronomican. The Astronomican is centred on Terra and is not only controlled by, but is directed by, the psychic power of the Emperor.
The Astronomican is a psychic beacon that penetrates into Warpspace. A Navigator on board a voidship in the Warp is able to pick up these signals and can steer a spacecraft through Warpspace, compensating for current changes as he does so. A piloted jump can cover a far longer distance than a calculated jump. Most piloted jumps are no more than 5,000 light years at a time, but longer jumps have been made.
A typical interstellar voyage might begin with a cargo ship lying in orbit around an Imperial world. Tiny shuttlecraft busily transfer precious minerals, foodstuffs, crew, and manufactured items from the world below. The loading procedure may take solar days or weeks, as the shuttles return time and time again to the huge voidship.
Once loading is complete, the colossal spacecraft slowly accelerates out of orbit under the power of its main reaction drive. The voidship heads outwards towards the rim of the solar system, carefully increasing speed by tiny increments as it does so.
Although the vessel's engines are capable of terrific acceleration, the risk of collision with inter-planetary debris is high if the voidship accelerates too quickly or too much. As the sun shrinks in the ship's wake, the density of debris lessens and the ship's speed reaches approximately one percent of light speed in a vacuum.
After several solar weeks of travel, the voidship arrives at its point of departure into the Warp. This is the Mandeville "jump-point" lying around the star system like the circumference of a circle. This delineates the point at which inter-planetary debris falls below maximum Warp density and gravitic stresses cannot affect the transition from realspace.
Once this invisible line has been crossed, it is safe to activate the Warp engines to enter or exit the Immaterium. A crew careless or foolhardy enough to prematurely activate Warp-Drive would be lucky to find their ship hurled thousands of light years off course. More likely, the voidship would be torn apart and destroyed, never to be heard of again.
With the safe activation of its Warp-Drive, the ship is plucked out of the real universe and enters the dimension of the Immaterium. Its true interstellar journey has begun. Ships travelling in the Immaterium with the aid of a Navigator do so by means of jumps varying in length up to 5,000 light years. Only a long journey would involve more than a single jump. Even so, almost two solar weeks may pass onboard ship before the craft is ready to end its jump. Meanwhile, because of time shifts in the Warp, it is possible that over a standard year has passed in the real universe.
The ship re-enters realspace just beyond the Mandeville Point of its destination solar system. If it is lucky, the ship will come out close to the jump-point, otherwise it may take many extra solar weeks to reach the inner planets. It is always wise to allow a safe margin when jumping towards a star. The results of reentering space within the radius of the Mandeville Point would be the same as prematurely activating the Warp-Drive on the outward journey, and would almost certainly end in disaster.
The ship is now ready for its final haul, beginning by broadcasting to its destination and establishing a new time coordinate. Time in Warpspace is so different from time in normal space that the crew has no idea whether their journey has taken a few solar months or standard years. Initially, the voidship travels at approximately one percent of light speed, decelerating gradually through the denser inner regions. Eventually, the ship reaches its destination, where swarms of tiny shuttles once more make themselves busy loading and unloading cargo and passengers in preparation for the voidship's next journey.

Warpspace Channels

In theory it is possible to travel anywhere in the universe through the Warp. However, the shifting tides of the Warp make it easier to travel from some star systems to others, and short Warp jumps are always more accurate than longer ones.
This is particularly true when moving a large fleet, which may become spread out across several light years of space over an extended journey.
Long established and well-charted Warp channels connect star systems and entire regions of the galaxy, providing relatively predictable conduits through which the majority of Imperial shipping passes.

Warp Gates

A Warp Gate is a point in realspace which is linked to another point in realspace by a tunnel through the Warp. The tunnel somehow avoids the normal disturbances of the Immaterium, allowing a journey to be made within a fixed time and in perfect safety.
The existence of Warp Gates represents something of a mystery, and much debate rages over whether they are natural or artificial. If artificial, then none can say who made them or for what purpose.
Furthermore, some Warp Gates have been artificially enhanced, their entrances delineated by mechanical constructions whose exact function can only be guessed at. Other Warp Gates are mere black holes in space.
Warp Gates occur in the depths of space, at the boundaries of star systems, within star systems, and even on planets.
The largest are easily big enough to permit the passage of spacecraft and are usually situated at the edge of a star system or amongst its outer planets.
Other gates are only large enough to permit the passage of small vehicles, or perhaps Human-sized creatures.
These occur mostly on planet surfaces and lead directly to other gates on the surfaces of other planets. All gates are rare, the smaller types extremely so.
Planetary gates are often disguised, or respond only to electrical, psychic, or other signals, which would seem to indicate a certain amount of intended secrecy on the part of their builders.
All space-going species are prepared to utilise Warp Gates when they find them, although discovering where they lead can often be hazardous.
Many gates appear to be defective, and can dump a ship randomly into the Warp. Others can lead to places which may once have been stars or planets, but which are now no more than empty space.
The possibility of emerging into a distant, hostile alien star empire also has to be considered. It may be that spacecraft vanishing into Warpspace have been transported beyond the galaxy itself.
Warp Gates often become extremely important to whoever controls them, and many are of vital strategic value to the Imperium.
Imperial Commanders and other interests vie for control of a Warp Gate in their sphere of influence, and these relics are often heavily defended against outside attack.

Warp Portals

A Warp portal is a point in space where the Warp and realspace interface, forming a stable entrance into, and exit from, the Warp.
It is not necessary to activate a Warp-Drive to enter the Warp in this manner, although any vessel not equipped with a Gellar Field would be unlikely to survive for long within the Empyrean.
It does not lead to a tunnel, and a voidcraft entering a portal is directly cast into the chance currents of Warpspace.
With careful manoeuvring it may be possible to re-enter normal space using the same portal from the other side. Again, the exact nature of the portals is not understood, and no one knows whether they are mere accidents of nature and non-Euclidian geometries or have some secret purpose.
Some aliens use Warp portals to travel between the Warp and realspace, specifically the creatures known as Enslavers, who live within the Warp itself. Like Warp Gates, portals occur in all places in the universe and may appear on a planet's surface.
Some have a definite physical constituent, whilst others are invisible or take the form of a hole in the ground or a dark cave mouth.
Warp portals do have their uses, for there are recorded instances of spacecraft with damaged Warp-Drives, trapped within the Warp and doomed to destruction, suddenly locating a Warp portal enabling them to return to realspace.

A Thin Veil

The barrier between reality and the Warp is not equally strong across the galaxy. In some places, the veil is thin, allowing the influence of the Warp to leech through. Such areas might be permanent or temporary, and could be of nearly any size, the result of random chance or the intentional acts of malefic Chaos Cults.
Some are the result of experiments performed during the Dark Age of Technology, while others are a side effect of psychic activity or a great effusion of powerful emotion, many deaths or other extreme psychic trauma within a small area.
In the Askellon Sector, for example, the unpredictable currents of the Pandaemonium batter the shores of reality, weakening the barrier between dimensions.
Various disturbing phenomena can manifest in areas where the veil is thin, for the Warp and its influence are never predictable or consistent.
Such places are often considered haunted or ill-omened, for people might hear voices, glimpse movement out of the corners of their eyes, or experience nightmares or even waking visions.
Chronos might run backwards, temperatures plummet or fluctuate, and auspexes and scanners provide inconsistent or contradictory readings.
Individuals might feel a sense of wrongness that they cannot describe, or strange and disturbing sensations, from tingling on the skin to the smell of burning flesh.