Foldspace (also known as extrauniversal or extradimensional space, or rarely as hyperspace or hyperdimensional space) is the n-dimensional space in which all universes are contained. It is the medium for FTL travel through a method involving shifting one’s position between 3D planes.
It should be noted that neither the Alliance nor the Terran Empire are aware of the true nature of foldspace, and instead think of it as a singular 4th dimension within the known universe. The EF in the 22nd century however was aware of it and actively studying foldspace with ships such as the EFSV Mandelbrot, albeit to them it was known as hyperdimensional space.
Universes and foldspace have a somewhat special relationship. Universes are, for all intents and purposes, a stable bubble of 3D space on one of the infinite 3-dimensional subsets of foldspace, and have an inherent property of being attracted to their specific 3D plane. Due to this, an universe practically never shifts in any dimension but its specific subset of three. What causes this property is not fully understood.
Due to this, there are two ways to leave or enter an universe: cross its edge in 3D space and exit or enter on the same 3D plane, or move in dimensions other than that universe’s home plane. Foldspace-based FTL drives use the latter method.
Due to the inherent anchoring property of universes however, anything leaving one will also leave that universe’s anchor and drift across infinite dimensions unless a stable miniature universe is created around it. This is also the principle fold drives also utilise for safe travel across foldspace, and why foldspace travel is risky by nature.
Foldspace is the primary means of FTL travel. The simplified procedure consists of a foldship’s drive core tearing a bubble out of the universe itself around the ship, and translating it in n dimensions perpendicular to the source universe’s 3D plane. As that translation places the ship on another 3D plane not aligned with the original, travel in this destination plane maps to a longer distance in the origin plane, allowing for what in the source universe appears to be teleportation.
Foldspace travel is inherently risky. Any misalignment in the actual and calculated destination dimensional plane will result in the return jump to the source plane missing its target, at which point the ship is, for all intents and purposes, lost forever. If the drive core loses power or malfunctions in any way while in warp, its dimensional anchor will fail and cause the ship to drift off course, leading to such a misalignment.
The jump itself cannot be interrupted. If the core loses power during jump initialisation or doesn’t receive enough power, it may malfunction. This can lead to a variety of results, ranging from the jump not reaching its destination plane, to only a part of the ship being jumped to foldspace. For this reason, combined with the core’s intense power draw during the initial jump, foldships tend to have massive capacitor banks to provide the initial surge of energy the core requires. These banks are often slightly oversized to provide more margin for failure.
In addition to physical risks posed by foldspace travel, it may also have negative effects on human mental health. This is due to fold drives’ bubbles often being small enough to see out of and not as strong as natural universes. This exposes human senses to unstable n-dimensional space which they have not adapted to understand. The effects of this are comparable to historical fiction in the cosmic horror genre, and the results of witnessing things simply incomprehensible to the human mind in such fiction.
Human minds, being extremely flexible, can adapt to foldspace though. Many highly experienced foldship crews’ minds have developed means to shut themselves off from the foldspace-induced sensations. Additionally, foldships often feature closeable window shutters, to reduce visual exposure. For less experienced crew members most foldships also have psychologists on board to monitor crew health and provide help where required.
These psychological effects have sparked many urban legends surrounding foldspace, and about eldritch gods which may reside there.
Fold drive cores are a poorly understood device, based on the unknown force anchoring dimensions to a 3D plane. Practically speaking, they have two purposes: tearing a foldship out of realspace in the extremely energy-intensive jump to foldspace, and maintaining, what is essentially, a miniature universe around the ship, to anchor it to the destination plane. They also contain several sensors monitoring the unknown materials and processes inside. These are used to extract necessary values for plotting a course through the destination dimensional plane, arbitrarily chosen by the cores themselves. The cores are also responsible for the specifics of jumps between planes. Their assumptions of each jump are based on a stable alignment with both the source and destination plane, and as such the misalignment mentioned in the Risks section will cause it to miss a jump.
As fold drive cores are very poorly understood after the fall of the EF, Imperial and Alliance cores are slightly modified clones of EF archeotech.
While foldpsace itself is explainable by human theoretical physics, certain properties of it are not. One of these is utilised by both natural universes and fold drive cores; as their dimensional anchoring is based on exotic substances unexplainable by tradtional human science. What is known, is that said substances anchor themselves and a field around them to a given plane, when manipulated in specific ways.
Unrelated to this, foldspace is also home to an extremely high concentration of the fungus-like entity responsible for sentience. It cannot survive in anchored 3D space, such as realspace, outside neural matter like human brains, but thrives in foldspace. This likely plays a role in the previously mentioned psychological effects of fold travel.