A social group related to, but distinct from, the Hiders, the Leavers are entities who have for whatever reason decided to simply leave Terragen society behind forever and have nothing more to do with it. This departure may or may not be secret. Leavers are sometimes, and more derogatorily, known as Bailers (as they are seen to be 'bailing out' of Terragen society). Although some Terragen starships are capable of velocities of 0.99c and above, and explore outwards at that speed, the overall expansion rate of the border of Terragen space and the wormhole Nexus is significantly slower than this, though still a significant fraction of the speed of light. This provides enough of a margin for Leavers to be able to outrun Terragen civilisation. Leaver motivations vary a great deal. Some are one-way explorers, or self-appointed emissaries. Other simply dislike Terragen society and wish to leave it entirely. Others still are criminals of various kinds. Some Leavers do apparently plan to return to Terragen space, but only after Terragen civilisation has gone wherever it is the other ancient galactic civilisations have gone.
The first known Leavers were a group of AIs who, during the most dangerous period of the Technocalypse era, simply left without warning, much to the displeasure of many of those who were trying to fight the ongoing catastrophe. Others also left during this period. They were never heard from again, although the Argus Array has tracked a fast-moving object that might possibly be the original AI Leavers. Another early group was the Semperist Survival Fleet, which founded the extensive Refugium empire beyond the sphere of Terragen expansion; eventually (of course) that sphere caught up with the Refugium and engulfed it.
During times of trouble the numbers of Leavers tend to increase, as was particularly the case during the Version War, but even in times of peace there is a steady trickle of them heading off for the far reaches of the universe... One group of Leavers since then was a large percentage of Clade Tavi, after the Kedric Incident. Another more exploration-orientated group of Leavers is the Magellan series of Scions. Some also include Wintermute's Far Edge as a type of Leaver. Some historians also include the Diamond Belters, the Pansolipsists and other similar groups as technically being Leavers who have found a way to defend and separate themselves from Terragens society as a whole. Most consider them to at the very far end of the Leaver bell curve, if truly Leavers at all.
The main problem faced by any potential Leavers is the fact that the Terragens sphere of influence is expanding at a significant fraction of the speed of light. To stay beyond Terragens society Leavers must thus move faster than this, and so use relativistic ships of one kind or another, normally launched from as far away from civilisation as possible. Once launched they race off across the universe, not responding to any signals beamed after them. Most Leaver vessels seem to accelerate to a high percentage of the speed of light, but no faster, apparently cruising at a velocity that is selected as a compromise between their Leaving as fast as possible and their local time flow rate being slowed to a crawl by time dilation. Some Leavers, apparently more solipsist in their outlook, heading for a particular goal, or fleeing something (perhaps the 'cancer of Terragen civilisation') simply accelerate constantly. The former type of Leaver vessels do not carry a powerful on-board drive, but instead use a separate (and sometimes disposable) booster drive to reach cruising speed, then a much smaller sustainer drive to maintain it against friction with the interstellar medium and other losses. All Leaver ships are built using at least a nanotech level of technology, and are designed to be as durable and as long-duration as possible given that technology. However, they all have to be able to maintain their drives while in flight, otherwise they will eventually fail, with bad consequences for all aboard. They do this by a variety of means. If the drive is being used as a sustainer motor to keep the Leaver ship at a constant speed rather than accelerate it constantly, then it is off most of the time. Some Leavers simply switch off the drive periodically to allow maintenance to occur. Others use drives designed to be maintainable while running, some by cycling between multiple redundant systems, others by more ingenious means.
Because they intend to stop as little as possible, Leaver vessels are equipped to extract everything they can from the interstellar environment through which they pass (generally by means of ramscoops). Most Leaver ships are highly streamlined against the interstellar medium, not just to maintain their speed with the minimum of effort, but also to reduce their cross-section, and thus the amount of shielding they carry and the number of impacts they suffer as they travel. Because of this Leaver ships are often small and needle-like in configuration. Most Leaver vessels thus hold the Leaving entities in a virch environment to keep the vessel itself as small as possible. This does not mean that there are not embodied Leavers, but they are much rarer, and their vessels are usually much more Spartan environments than those of disembodied Leavers. All Leaver systems, and especially computer systems, are constructed to be as durable as possible in the harsh environment of interstellar space, using enhanced checking and repair of program and data integrity. One major problem that Leavers suffer more than other interstellar travellers is that of availability of raw materials for repairs, shielding and other uses. Although using the interstellar medium for raw materials is possible, the low average density of the interstellar medium (approximately one atom per cubic centimetre) makes this fairly unproductive. For this reason some Leavers travel through interstellar gas and dust clouds as much as possible (as they can be up to a hundred times more dense than the average interstellar medium).
Others carry large reserves of raw materials, and stop every now and again to take on supplies in some suitably resource-rich region. In this case their ship design is optimised so that stopping and starting still allows their average speed to be faster than the expansion of Terragens space. It is speculated that not adopting a strategy like this may have doomed some early Leavers. The most basic Leaver ships are those running virch inhabitants. These are often very small, using an ion drive for sustaining thrust (once their boost motor is done), and collecting power from starlight. Their inhabitants often run rather slowly. Fuel for the ion drive and sufficient of other elements to keep the vessel shielded and maintained are picked up from the interstellar medium and occasional stops and passes through dense interstellar clouds. To help them travel without stopping some Leavers use ships designed and built using only the elemental mix available from the interstellar medium; the original designs allowing this to be done were derived from the first games of Elemental Taboo.
Some Leaver groups and other commentators have pointed out that improvements in drive technology will eventually cause Terragen civilisation to overtake them regardless of how far and how fast they go, as there is no way for them to match the level of innovation that will occur in Terragens space. Even making for other galaxies will not work in the long term. Because of this, some Leaver groups have adopted a different strategy to the majority of Leavers. From their point of view to remain out of the mainstream of Terragen society on a permanent basis heading for any place inside the galaxy, or any other galaxy, is probably useless. This leaves only intergalactic space as a possible destination. The Leavers who subscribe to this approach set course for the emptiest part of the sky they can find and travel there at as high a speed as possible. When they arrive they intend to slow down to a low coasting speed before drifting on through eternity using slowed virtual subjective time to stretch out their existence. Vast arrays of mirrors to collect the intergalactic starlight and/or thermoelectric generators based on very long-lived radioisotopes are used to provide power in addition to more conventional means. Between massive amounts of preparation and running themselves very slowly, Leavers who choose this course should theoretically be able to last a very long time, employing survival strategies originally invented for use in deep time, but adapted for use in the present-day universe, perhaps interspersed with stops at galactic halo stars when new resources are truly necessary.
Rejoiners are offshoots of the various groups of Leavers who have later rejoined Terragen society. Some of these re-joinings have been voluntary as the politics or views of the Leaver society have changed with time. Others are involuntary with the Leavers, usually relying on old technologies, being overtaken by ships using more advanced drive technologies, or by entities taking advantage of things such as the Carina Rush wormhole which allow them to 'jump ahead' faster than lightspeed would normally allow.