Got a really really long way to go, and not much time to get there? Superluminal travel might be right for you!

NOTE: The term "superluminal" is one of the many terms I learned from reading the immortal Isaac Asimov, and it means exactly the same thing as the commonly-used "faster than light" (or FTL).

The vast majority of superluminal travel is accomplished by complicated mucking about with hyperspace. Hyperspace is a patch of space-time real estate that need not have a continuous connection with the rest of space-time, thus avoiding all sorts of relativistic limitations (most notably the speed of light).

In the early days of superluminal travel, according to the standard AIF timeline, massive and stationary hyperspace inducers were used to let vessels skate along on their very own patch of hyperspace. This had the very significant ramification that wherever they dropped back to subluminal speed there had better be another hyperspace inducer array, or they were stuck within relativity.

Speaking of relativity, relativistic speeds are not too difficult to achieve with contemporary vessels. A modern high acceleration craft could attain a relativistically-significant fraction of the speed of light in just a few minutes. At such velocities, a crew could cross interstellar distances with very little subjective time. However, for the timeframe of the rest of the mostly-flat space-time galaxy, it still means that they're taking many years to accomplish such voyages. And it should be noted that at such speeds, even tiny particles become dangerous.

Modern ships (and this applies to most ships built within the past millennia) have their hyperspace inducers built into their drive structure, which allows them to jump to hyperspace any time they want. Which is not to say that they would want to, for a great many settings and reasons. This is because being in hyperspace can even more dangerous and vulnerable than travelling in normal space at relativistic velocities. So why do it? Simple: Because you can go really really fast. Most superluminal traffic travels somewhere between 0.1 and 2 parsecs per hour - that's about 30,000 times faster than the speed of light.

NOTE: Yes, we realize that all sorts of 21st century Earth physicists will be spinning in their graves. Let them. We've wound them with wire and surrounded them with magnets, and are using the resulting output to power our homes.

A ship travelling superluminally via hyperspace is still connected to regular space, but that connection appears extremely distorted and incongruous. Like shifting the beam of a flashlight to move along a distant object, where the apparent speed of the spot of light is proportional to how stretched it will seem, so too a superluminal ship will appear massively elongated to observers in regular space-time. A ship moving superluminally may well appear to have a longitudinal dimension greater than the diameter of a star system. You might think that this would make a ship travelling superluminally an easy target - and you'd be right, assuming that you can see it. This is because hyperspace generally conforms to a type of Poisson's ratio, meaning that the lateral dimensions of a ship travelling superluminally are often extremely tiny. It's like the ship is a an incredibly long and and incredibly thin thread. If you didn't have etheric sensors to detect the massive energies of a trip travelling superluminally, you would have a hard time seeing it with regular electromagnetic sensors.

With etheric sensors, though, anything travelling superluminally is quite visible, even with just passive sensors. In fact, not only is the vessel visible, so too is its probable path. Add to this the fact that the damage suffered by something in hyperspace is 100 times greater than in normal space, and it's easy to see how vulnerable a superluminal ship can be.

Due to this vulnerability, ships tend to travel superluminally in distinctive patterns: space lanes and paranoia margins.

Space lanes are not often straight lines, but instead ducking and weaving streams where ships all travel in the same direction in close proximity to each other. This is because the space lanes afford a sort of cumulative safety. While ships that are superluminal are very easily detected etherically, they are extremely difficult to distinguish.  Even more telling, though, is the tendency of space lanes to flinch further than anything can compensate for by travelling at sublight.  All ships travelling superluminally employ etheric sensors to avoid mass, which is increasingly detectable the closer it is to the path of a superluminal vessel.  So, while contact with particles of interstellar dust is unavoidable, anything larger than the size of pea is going to cause ship to try to swerve.  When one ship swerves, the rest of the ships using the space lane are going to tend to follow suit, not wanting to find out the hard way why the other ship swerved.  Travelling 5 orders of magnitude faster than the speed of light means that even the slightest twitch of the space lane will cause it to shift staggering distances laterally in space.  This means that sneaking up on a space lane is pretty nearly impossible.

Similarly, it is pretty common for ships approaching their destination to drop out of hyperspace a safe distance away, and to travel the rest of the last leg in normal space.  This is because most places that are worth stopping at have the inherent risk of others also wanting to be there, and standard paranoia insists on maintaining a suitable margin.  The same thing goes for leaving a point of interest and venturing into hyperspace for a bit of superluminal travel - it's generally left until sufficiently far away from anyone likely to try charging up and taking a potshot at your extending hyperspatial wake.

To cut down on my rambling, here's some obvious questions and answers:

For most AIF games, all this description is actually pretty moot.  Unless the technicalities of interstellar space travel have some significant effect on the game or story, referees would probably do just as well to skip it and pretend that it's qualitatively just like driving on the freeway.