As promised, here's the first in a series of essays covering the background and technical details of the SF setting I've created for Perilous Waif. This has been an interesting project, because I wanted a relatively hard SF setting that still allowed for the typical space opera sorts of story plots. So somehow I had to get an interstellar civilization without having a singularity, replacing humanity with AIs, or otherwise rendering normal people completely irrelevant.
The FTL method is a key foundation of this kind of setting, but in this case I wanted it to be a fully integrated part of the universe instead of just a magic plot device. The mechanism I eventually settled on is a bit complex, but on the good side it adds a lot of richness to the tactics of space combat in a very natural way.
In this setting advanced physics research has revealed that our universe is simply one of a series of large 4-dimensional spaces embedded in a common higher-dimensional space. These universes are nested like a series of concentric hyperspheres, and it is possible for inhabitants of a given universe to travel to the two neighboring universes in the series. There is also a stable mapping of locations from one universe to another, so if you shift universes in Mars orbit you’ll always emerge near the same corresponding location in the target universe.
The ‘hyperspace’ universes are the ones nested inside our universe. The geometry of the higher-dimensional space means that each nested universe is ‘smaller’ than its container by a factor of pi^3, meaning that if you make a trip in the first hyperspace universe instead of normal space the distance you need to cover will be shorter by that amount. So interstellar travel is accomplished by shifting to a hyperspace universe where the distance is thousands of times smaller than in normal space, and no actual FTL movement is involved. The various universes nested inside normal space are collectively referred to as ‘hyperspace’, while individual universes are ‘layers’ designated by Greek letters (i.e. Alpha Layer, Beta Layer, Gamma Layer, etc.)
The universes ‘outside’ normal space are collectively referred to as ‘subspace’. As you travel into subspace distances increase by a factor of pi^3 in each universe, making it useless for travel. The average mass density also drops quickly, leaving them with very little in the way of interesting features like stars or planets. As a result subspace is normally of interest only to scientists, and is rarely visited.
A starship normally moves between different layers of hyperspace using a device called a hyperspace converter, which is a large piece of complex nanotechnology. While the actual transition between layers happens in microseconds it takes at least a minute for even the fastest hyperspace converter to power up, and on larger ships a cycle time of five to ten minutes would be normal. Civilian ships, especially cargo vessels, often save money by using designs that have a long cycle time but place less stress on the hyperspace converter.
The design of FTL ships is constrained by two important scaling laws. First, the energy needed for a hyperspace transition is relative to the surface area of a sphere enclosing the ship, so larger ships find it easier to fit in enough fusion reactors to run the hyperspace converter. However, transition also subjects the ship to large mechanical stresses that become worse the bigger it is. Both of these factors are easily managed for Alpha transitions, which have relatively low energy costs and transition stress, but get geometrically worse for each layer beyond that.
Making a hyperspace transition near a massive object tends to be dangerous, because a gravity well greatly increases the transition stress. Military ships normally avoid making transitions in a local gravity field stronger than 0.01g, while civilian shipping treats 0.001g as a hard safety line. This constraint applies to both the origin and destination points of a transition, which can make visiting uncharted space rather hazardous for the unwary.
While travelers (and invaders) might like to shift rapidly between different layers of hyperspace, this is easier said than done. Each transition dumps a fantastic amount of waste heat into the hyperspace converter, which is normally buried deep inside a ship to protect it from damage. Hyperspace transitions also produce a temporary disturbance in the dimensional barrier between the layers, which makes further transitions dangerous (much like a gravity well) for a period proportional to the diameter of the ship’s transit bubble. So small ships with superior engineering might be able to zoom around changing layers every few minutes, but capital ships will normally maintain a more stately pace of one or two transitions per hour.
A few major nations have developed the technology to create permanent, stable wormholes between normal space and the Alpha Layer. While this requires a large capital investment, it can be a worthwhile project in systems that have a large volume of civilian interplanetary traffic. Unlike a normal hyperspace transition, using a portal requires no special equipment and imposes minimal transit stress on the ship.
Unfortunately portals between the Alpha and Beta Layers are far more difficult to build. While small systems capable of moving people or sensor drones have been demonstrated, a version sized for ships would be far too expensive to have any real use. Portals to the higher layers are even more difficult, due to the high levels of transit stress that the portal system would have to stabilize.
Several nations have adapted this technology to create a portable system for their larger warships, allowing them to peek into adjacent hyperspace layers using small temporary portals. Sometimes called hyperspace periscopes, these devices have been demonstrated all the way up to the Delta Layer (albeit with very small aperture sizes).
All universes that can actually be visited run on the same laws of quantum mechanics (otherwise ships and people entering them would immediately stop working). But ‘cosmological’ physics (gravity, dark energy and everything else that in RL hasn’t been unified with quantum mechanics) can vary from one universe to another, and universal constants can also have slightly different values. Between these differences and the rapidly increasing mass density of the higher layers hyperspace looks very different from normal space.
Adjacent to normal space, with relatively mild transit stress between the two universes. The Alpha Layer is ~30 times ‘smaller’ than normal space, and is heavily used for local travel within a solar system. At normal long-distance travel speeds of ~1,000 kps a starship in the Alpha Layer would take ten years to traverse a light year of normal space.
With an average mass density almost a thousand times higher than normal space, the Alpha Layer is characterized by large galaxies full of dense star clusters. The region adjacent to human space is on the fringes of one of these galaxies, and contains far more stars than the corresponding region of normal space. But the vast majority of them are giants of 3-10 solar masses, which burn out quickly and produce huge numbers of supernovae. Neutron stars and black holes are also extremely common, and the relative abundance of heavy elements is far higher than normal space.
There is no native life in the Alpha Layer, and permanent colonies are rare. The average planet will be sterilized by a supernova or gamma ray burst about once every thousand years, a fact that has largely discouraged the establishment of permanent human colonies. In civilized areas robotic monitoring systems track such events, and all shipping will know to avoid the Alpha Layer when a blast wave is due to pass through. In less civilized areas monitoring can be incomplete or even completely absent, making travel somewhat dangerous (especially for smaller ships).
Despite the hazards, large-scale mining operations are often set up in the Alpha Layer to take advantage of the high abundance of heavy elements. Heavily populated colonies also put monitoring systems and other static defenses in the Alpha Layer, where they can easily intercept interplanetary traffic.
The next universe up from the Alpha Layer, with higher transit stresses that require more expensive ships. The Beta Layer is ~900 times ‘smaller’ than normal space, and is sometimes used for long interplanetary trips (i.e. visiting the Oort cloud, travel between distant binary stars). At normal long-distance travel speeds of ~1,000 kps a starship in the Beta Layer would take four months to traverse a light year of normal space.
The Beta Layer is a universe where the competition between matter and antimatter never ran to completion. Instead some galaxies are made up of matter while others are antimatter, and the cosmic background radiation is dominated by a harsh glare of matter-antimatter annihilation. The region adjacent to human space is in intergalactic space, but there is a thin sprinkling of antimatter halo stars. These systems are often claimed by nations with active antimatter weapon programs, although even with modern technology mining antimatter and processing it into warheads is a dangerous process.
With substantially higher transit stress than the Beta Layer, this universe didn’t become accessible until the development of compact fusion power plants and diamondoid structural materials. Thanks to the scaling factor of ~27,000, a ship in the Gamma Layer can cross the equivalent of a light year of normal space in only 4 days. The first great wave of interstellar exploration and colonization used the Gamma Layer, and it is still used by the majority of interstellar cargo shipping.
The Gamma Layer is a universe whose initial expansion was slower than in normal space, and as a result virtually all hydrogen was burned into heavier elements before it expanded enough to become transparent. There are very few stars, since there isn’t much for them to burn, and in fact most of the mass in the universe has become sequestered in black holes. The region adjacent to human space is an intergalactic void, making it conveniently lacking in navigational hazards.
Major nations often build large-scale fortifications in the Gamma Layer to protect access to important systems, since the ~1,000,000 km range of heavy energy weapons is enough to interdict access to an entire solar system in normal space.
The transit stress to this layer is so high that only heavily armored vessels can survive entering it, making it uneconomical for most civilian purposes. But being ‘smaller’ than normal space by a factor of 810,000 means that ships that are able to use it can cross a light year of normal space in a bit over 3 hours, making trips of tens or even hundreds of light years relatively quick. Most military vessels use the Delta Layer for its greater mobility, as do courier ships and express transports, and the second great wave of exploration and colonization began with the construction of the first Delta-capable ships
The Delta Layer’s physics is rather bizarre compared to the lower layers, due to the fact that it has a cosmological repulsive force that becomes stronger than gravity over distances greater than 10^8 km. This generally prevents the formation of objects larger than a small moon, leading to a universe filled with diffuse clouds of partially-ionized gas. This medium is actually dense enough to cause thermal damage to relativistic objects, and can give rise to immense storm-like phenomena that block long-range sensors and last for millennia.
With a relative scaling of 24 million, a ship in the Epsilon Layer would be able to cover a light year in only six minutes. Such speeds would open up the entire Local Group to human colonization, so it’s too bad it’s impossible to get there.
The problem is that the energy needed to enter the epsilon layer is so high you’d need a 2 km ship packed full of antimatter reactors to run the hyperspace converter, but the transition stress is so high that even a solid block of diamondoid material would be ripped apart if it’s more than a few hundred meters across. Since both the power output of antimatter reactors and the tensile strength of the best structural materials are currently limited by fundamental physics rather than engineering details, it is generally believed that accessing the Epsilon Layer is impossible.