Hyperspace, also known by other names such as subspace, is a physical region of the universe which has unusual properties that are accessible to technologically advanced societies. It is contrasted with realspace, the lower-dimensional region of the universe where most structure lies.
While hyperspace may be thought of as being "outside" the universe, both hyperspace and realspace are two contrasting regions of the same structure, and hyperspace must be understood in reference to this; ultimately, it is best described as the part of four-dimensional space that is not realspace.
In contrast to its cosmic overall size, the 3D realspace brane is folded on a microscopic scale within the fourth spatial dimension. The average distance between folds is close to the Planck length, ~10-35 metres, a few hundred times smaller than the total thickness of the fourth dimension, although the size of each fold is around 1016 times larger, corresponding to the scale of the electroweak interaction.
Hyperspace is the higher-dimensional space between the folds, and it is possible for objects to escape the realspace brane and travel in hyperspace. This requires the use of non-orientable wormholes, which in turn can only be produced by interacting with hyperspace, and this necessitates the use of energy densities close to the Planck energy density (~10113 joules per cubic metre). However, it allows the objects to cover a shorter distance between any two points than would be possible if they were forced to travel along the folds, allowing for what is functionally faster-than-light (FTL) travel.
Due to the instability of bound states in greater than three spatial dimensions, matter cannot travel through hyperspace unaided; instead, it will disintegrate immediately and dissolve upon contact with normal-fluid hypermatter. Instead, faster-than-light drives must be used to restrict their particles to a layer of hyperspace, a three-dimensional volume derived from the realspace brane. This volume will inevitably inherit some of the microscopic curvature of the realspace brane, and this limits their effective velocity; the maximum possible speed is 1016 c, corresponding to a particle moving locally at the speed of light c in a volume that is completely flat. There have been claimed observations of one or more precursor races travelling in excess of 1060 c, but these are rare and are usually put down to measurement errors. Why these errors only seem to occur for precursor vessels is as yet unexplained.
The other main feature of hyperspace is hypermatter, an Essence-enhanced superfluid that maintains this convoluted geometry against both gravitational collapse and quantum tunnelling, and likewise prevents particles on the realspace brane from simply tunnelling through hyperspace unaided. Uneven distributions of hypermatter are responsible for the existence of hyperspatial anomalies, from subspace storms to twisted "Escher space".
Hyperparticles, the quanta of this superfluid, are colloquially known as "tachyons" after imaginary-mass particles that would, were it not for tachyon condensation, intrinsically travel faster than light. Hyperparticles can make use of hyperspace to move with apprent superluminal speed, but this does not involve them having imaginary mass.
Since gravity has the ability to extend into the extra dimensions, hyperparticles often coalesce into larger structures. These structures are limited by the fact that, as previously noted, bound states cannot exist in four-dimensional space. The most common hyperspace structure is therefore nebulous hyperspace fog. Also sometimes encountered, but exceedingly rare, are hypermatter stars, large four-dimensional vortices that are only visible by their ellipsoidal cross-sections within a hyperspace layer.
A small fraction of hyperparticles, specifically those that are travelling more slowly than c to a realspace observer - less than 10-16 c, or a mere 30 nanometres per second, in hyperspace - also have a gravitational influence on realspace structures such as galaxies and galaxy clusters (the "dark matter" effect).
As most hypermatter is at a temperature of close to absolute zero, it can be used as a nearly inexhaustable heat sink; it is this feature of hyperspace that makes a great deal of modern technology possible, from populous ecumenopoleis to starships without external radiators. Much like other superfluids, hypermatter has both a frictionless superfluid component and a viscous "normal" component, and the amount of the normal component increases with temperature. This normal fluid can impair long-ranged sensor navigation and physically hampers travel within hyperspace, hence regions of comparatively dense or high-temperature hypermatter are most affected by this and are thus known as hyperspace fog. Hyperlanes are fogless strips of hyperspace, and are very valuable to the nations that control them. Meanwhile, in deep gravity wells such as in the immediate vicinity of planets and stars, hypermatter is so dense that fog renders most kinds of FTL impossible.
The microscopic extra dimensions in which realspace is folded provide additional degrees of freedom for hyperparticles. The kinetic energy resulting from momentum along these dimensions is trapped as mass, which can be released by bringing hyperparticles into realspace. This is often known as "zero-point energy extraction", as most hyperparticles used for this are in their minimum-energy state. Furthermore, careful refolding of realspace can alter the effective physics experienced in certain hyperspace layers, changing the values of physical constants or (by allowing lower amounts of extra-dimensional kinetic energy to be stored as mass) adding extra generations of quarks and leptons.
Although often suspected to not have a natural origin, the lack of known boundaries to hyperspace suggests that it has existed since the beginning of the universe. When it was first exploited is not known, although the Kormacvar were able to engineer a layer of hyperspace, the Borealis Grid, over five billion years ago. In the similarly distant past, the Zhulultu are believed to have warped Tuurosian hyperspace into "hellspace" with so-called schismatic energy, rendering its local hypermatter more violent and chaotic. More recently, Oikoumene artefacts over seventy million years old possess hyperspatial technologies; it has been suggested that Amemoriam uses hyperspatial wormholes to create basement universes in which realspace is folded in different manners to produce altered physics.
Records of such ancient technologies are sparse, however, and more relevant to modern hyperspatial technologies is the history of contemporary civilisations. The earliest known utilisation of hyperspace by such a power is the Draconid invention of the warp drive in the 251st millennium BC, but it was not until the late 28th century AD that the most recent revolution in hyperspatial technology occured, sparked by the discovery of precursor relics and the Technoospheric singularity. This led to the development of more complicated hyperspatial devices, each using numerous basic elements arranged to give complicated effects, many of which are capable of manipulating Essence. The hyperspatial revolution has had an important effect on developing the technological synchronicity of the entire Gigaquadrant, although it should be noted that hyperspace is not the only way to develop into higher Tiers; stellar energy and antimatter storage can be used to summon huge amounts of power without hypermatter, for instance.
The typical path of utilising hyperspace is that a civilisation begins to prototype hyperspatial technology when it is able to produce particles with energies approaching the Planck energy, which is no easy task. A late Tier 4 or early Tier 3 might be able to construct primitive FTL drives or very simple hypermatter power sources by using the decay of microscopic black holes, which will emit Planck-energy photons. A civilisation approaching the end of Tier 3 can sustain these power levels by focusing the radiation of entire stars, and might gain the ability to develop their first advanced hyperdrives and other hyperspatial technologies. Tier 2 and 1 civilisations will eventually be able to distribute such energies (corresponding with attaining Type 2.5 on the Kardashev scale) and use hyperspace on a larger scale.
Most hyperspace technology falls under four categories, in order of prevalence: negative mass, hypermatter utilisation, FTL travel and communications, and time manipulation.
Matter held within hyperspace but fixed to multiple folds acts as negative mass from the perspective of observers on the folds, which has many technological applications. While negative masses cannot, contrary to popular belief, be used directly for anti-gravity, they can be used to manufacture gravitational dipoles which act as "Faraday cages" for gravitational fields, expelling the effect of gravity from anything inside the cage. This makes them an important component of artificial gravity devices, as they minimise the leakage of gravitational flux. Furthermore, its ability to gravitationally repel positive mass adds a propulsive effect which not only can be used as anti-gravity and reactionless drives, but is also important for artificial gravity as it somewhat cancels out the negative impacts of having a planet's worth of gravitatational mass inside a structure.
The metric shield is essentially a gravitational Faraday cage which functions as an impenetrable barrier against the worldlines of all particles, to prevent them from travelling via a certain path. This allows the shields to be simultaneously used as both a direct defence and as a cloaking device. Metric shields can either allow passage through the shield in one direction but not in the other (like a one-way mirror, in which case they are also known as "nightshades", because light emitted from the structure cloaked by the shield can be seen, but light from outside cannot be reflected back), or they can simply prevent any travel through the shield whatsoever. However, metric shields are not truly impenetrable: they do not always reach far into subspace, and they can be destroyed by black holes and hyperspace weapons.
Pocket universes (or "basement universes") are perhaps the most advanced form of hyerspace engineering, comparable to time travel in terms of the technological capabilities required. They can be used for high-energy computation, engineering, or experiments that involve carefully controlled conditions without outside interference, or simply increasing the amount of space available for a particular purpose by making a structure that is bigger on the inside than on the outside. They can also be used destructively by encasing a target within one and then destroying the universe itself, taking the target with it.
Once hypermatter power becomes a mature technology, it is often weaponised.
Superlasers are particle beams that are powered by hypermatter to incredibly high energies, where each particle carries around a gigajoule of kinetic energy each. This is often distributed between the net velocity of the beam and the thermal energy of the plasma. Handheld superlaser weapons (often known as "Death Rays" and fired from "rayguns") are arguably the most powerful infantry weapon. At the other end of the scale, "hyperlasers" are an alternative to antimatter weapons as a means of destroying various large objects such as planets and stars.
Hypermatter radiation weapons affect hyperspatial technology itself (for that reason, they are also known as "hyperspace weapons"), making propulsion that relies on them difficult or even impossible. Some of the most powerful ones, often known by names such as "mass shadow generators", can destroy any high-mass objects such as planets, stars, and even black holes. The most powerful hyperspace weapons, "subspace compression waves" or "Gridfire", can destroy entire sectors of space at (much like all hypermatter propagation) superluminal speeds. These effects are most easily achieved by modifying high-speed travel such as the "jump drive", but more advanced mechanisms work by detonating a specialised warhead during a faster-than-light jump, pushing ambient hypermatter into realspace and thereby causing a far larger and more expansive release of radiation than would otherwise be possible. A few subspace compression events are known to have been caused by sending "thunderbolts", gravitational waves with singularity-like gravity, through hyperspace.
Interdiction fields are accumulations of hypermatter drawn towards an object by non-gravitational means such as hyperspace tractor beams, producing a large but unstable artificial gravity well that can be removed (potentially destructively) at will. Such gravity wells are able to divert the course of any object with mass, prevent nearby FTL travel with dense hyperspace fog, and in battle they can be used to pull unguided (and some guided) projectile weapons away from vulnerable areas of a starship and towards places where they can be destroyed by point-defence weapons or more powerful armour or shielding. The large inertial mass surrounding the interdiction field prevents the ship generating it from accelerating for the duration that it is activated.
- Credits go the The Culture series for the name "Gridfire" and the concept, but it is a good way to explain the so-called "galaxy busters". It is possible the Forerunner weapon from the Haloverse (and fanfic) works like this.
Once emitted, long-wavelength uncharged radiation (radio waves, low-energy neutrinos or gravitons) can propagate through hyperspace easily, allowing them to be used for FTL communications. Although most readily blocked by hyperspace fog, radio waves are the most commonly used due to the ease with which electromagnetic radiation can be manipulated; antennae connected to hyperspace for this purpose are sometimes known as ansibles.
Matter, as previously described, requires more sophisticated techniques to keep it intact. There are six main types of FTL, three of which are known as hyperdrives and must be built into whatever is using them:
- Warp drives involve surrounding starships with negative-mass "warp bubbles" and lowering them into hyperspace, which allows them to bypass the more convoluted parts of the folding of the realspace brane. They are the slowest form of hyperdrive, with a maximum speed of around 550 kc.
- Transwarp drives can go up to approximately 100 Mc. The term "transwarp drive" is most often used by civilisations that have traditionally used regular warp drives, but it is also vague in that it can be used to refer to any form of propulsion that is faster than conventional warp travel. Many civilisations use more specific names, such as "quantum slipstream drives", for their form of transwarp.
- Jump drives work by "locking on" to a comet, planet, star, or other body with sufficient mass. This gives different types of jump drive different ranges and different speeds depending on what mass is sufficient; for intergalactic drives with a speed of 1016 c, this limit is greater than the diameter of the known universe. Most species with jump drives discover hyperluminal communications and scanning technology from the same relics that they use to develop the drives, while other species have to rely on transwarp-speed communications.
It is important to note that these limits are relative to the local rest frame of hypermatter, and are not absolute velocities. The speed u' of an object in a frame travelling at speed v relative to one in which it is travelling at speed u is given by the relativistic velocity-addition formula, u' = (u-v)/(1-uv/c2), from which one can see that a speed of v = c2/u gives infinite u'. For example, a speed of only 545 m/s relative to the local hypermatter rest frame is required to have a 550 kc warp drive in the hypermatter frame travel at infinite speed. Any faster, and the drive will be travelling backwards in time.
The other three FTL methods typically require structures that are independent of the user:
- Krasnikov tubes are permanent artificial tunnels in which spacetime is warped in a similar manner to in a warp bubble, so that the distance inside is shorter than the distance outside. A network of Krasnikov tubes constitutes a megastructure. Occasionally, Krasnikov tubes can arise naturally between hypermatter stars ejecting materials at superluminal speeds (an "Alderson path").
- Wormholes are superficially similar to Krasnikov tubes; however, there are differences: krasnikov tubes require much more matter to build in order to warp spacetime in the intended way, and while Krasnikov tubes can be accessed at any point, wormholes must be entered or exited at the ends of the tunnel. Both Krasnikov tubes and wormholes can be used to attain speeds equal to that of intergalactic jump drives. Permanent wormholes can be made by means such as opening Kerr loops in black holes. A wormhole drive allows a starship to temporarily create a wormhole for its own transit.
- Transfer beams are not directly linked to any speed, and are often treated as a form of "teleportation". Despite the name, information is not literally carried via beam; instead; the teleported object is instead surrounded by a form of weak deflector shield that protects it from the dangers of hyperspace travel. During slower-than-light teleports, these shields leak hypermatter, resulting in the formation of plasma vortices around them when they pass through an atmosphere. At faster-than-light speeds, no leakage occurs, and the transfer beam can bypass most defences, allowing it to be weaponised by carrying items such as explosives.
A related technology to hyperdrives and basement universes is "ghost phasing", in which objects are displaced away from realspace, preventing sensors from detecting them (hence the name) or weapons (besides those that are similarly phased) from interacting with them.
As a result of relativity of simultaneity, any faster-than-light jump has reference frames in which it is backwards time travel, a fact that is also demonstrated by the velocity-addition formula mentioned in the previous section. However, "true" time travel is inhibited as superluminal journeys between two locations affect the direction of four-dimensional hyperparticle flux between them, directing starships along routes that limit their pastwards time travel thus causing their journeys to be affected so as to maintain causality. These currents can be disrupted by perturbations of precisely-controlled magnitude and distribution, destroying their enforcement of causality, but this requires highly advanced hyperspatial technology and is computationally and energetically intensive even for civilisations capable of such a task. If done incorrectly, the process will either have no effect or it will render any hyperspatial travel impossible in the affected region. Still, travelling through time to the past is certainly possible.
Besides time travel, time manipulation includes changing the flow of time, such as making time loops, time locks, and inducing time dilation. This is often incorporated into projectile weapons, creating devices such as chronon or chroniton missiles (named after quantum-scale timelike wormholes), which have effects such as penetrating phasic deflector shields by being in a state of "temporal flux" or a certain period of time "out of sync" with the shields.
More complex time-altering devices are capable of editing history and events. These include weapons such as missiles that erase the target from time, also known as "chronon torpedoes", which are much more sophisticated devices, using technology such as Chronoscopic.