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PCs are Spacefolders: people who can naturally move between the Spheres (planets) through shear force of will (think A Wrinkle in Time).
Additionally, scientists have found a way to boost this ability, making Foldgates (think Fading Suns) which amplify a Spacefolder's power such that she can take large objects (such as spaceships) through space, though there must be a Foldgate at both ends of the journey. Just as in Everway, a gate is required for this. Almost all spheres have at least one naturally-occurring gate somewhere upon them, and these gates connect to one or more specific spheres. Any Spacefolder can use any gate, and automatically knows upon touching it the name(s) of the other sphere(s) it leads to, and can choose which one to go to if there is more than one. Some gates are also one-way, enabling travel to or from spheres, but not the reverse. There are also artificial gates. These are generally called "Foldgates", and no one knows where they came from or who made them. They are only found in space, never within a planet's atmosphere. While the details of design and decoration vary, they all have basic structural similarities: they are all the same size (several kilometers across) and take the form of a set of objects that form a rough circle. Sometimes the circle is complete--a ring--and sometimes it is made up of several pieces not joined. In any case, nothing known can move or damage a Foldgate or any of its pieces--they always remain perfectly fixed in relation to whatever astronomical bodies are nearest. Most often, they are poised at a lagrange point between a planet and its moon, or occasionally between a star and its planet. Foldgates behave noticably differently than natural gates. For starters, they do not identify their destinations, as they have none, or at least no set ones. Instead, a Spacefolder can select any sphere she wishes, and will appear at the nearest Foldgate to that sphere. Spheres are identified by a series of images from the Fortune Deck. The Spacefolder pictures the three cards laid out (in a triangular divinitory pattern) and these serve as the key that activates the Foldgate. During transit via Foldgates, sensitive Spacefolders often catch images of the cards of both the sphere travelled from and the one travelled to. The other major difference between a natural gate and a Foldgate is "luggage". A Spacefolder can take not much more than she can carry through a natural gate, and only a very talented traveller can one or two others with her. But Foldgates somehow magnify the Spacefolders ability many orders of magnitude, enabling her to transit entire starships weighing millions of tons. In practice, anything that can fit through the gate can be taken through it.
In either case, the duration of a journey is a highly variable thing. Generally, a journey between two spheres is the same length in either direction, and is roughly constant from trip to trip. The subjective time felt by those travelling often doesn't match the time as measured by people on either end, but is likewise roughly constant. In both cases, the time taken doesn't appear to have any obvious correspondence to the physical distance crossed.
this journey has minimal objective passage of time, which is roughly constant for any journey. The subjective passage of time is roughly proportional to the distance folded through, and can be as much as a few weeks, for crossing most of the galaxy.
While discovering and using the Foldgates clearly requires sufficient technology for at least simple space travel, the natural gates on nearly every sphere's surface have guaranteed limited contact between sphere's since well before technology. This has assured that no matter what sphere you travel to, there are certain common ideas and traits. The two most important are probably that there are recognizable people almost everywhere--often those spheres that didn't have people to begin with eventually had some settle there, even if there are also aliens native to them--and that nearly everywhere, the Tongue is spoken, if only by some people, as a second language. Once at least one of the spheres that a set of gates connect developed the idea of capitalism, active trade between them often quickly followed. Mostly, this trade took the form of ideas and small, valuable goods (jewelry, portable technologies, etc.), due to the difficulty of transporting bulky objects through the gates. Similarly, wars between spheres are almost unheard of before the discovery of Foldgates, due to the small number of Spacefolders in a given population.
The Fortune Deck is likewise common to all spheres that have discovered Foldgates, and many that have not. While it is sometime brought to a less-advanced sphere by a Spacefolder, it is generally first discovered in the form of hallucinatory/psychic images when using the artificial gates. Eventually, people figured out that the images were a coordinate set, and the right set of coordinates would take you to the gate of your choice. The danger, of course, is in going to an unknown destination, if you don't know the code for the gate you just left from--and there's no way to figure out the code except to try going there. The image(s) you get when transitting are generally a jumble of the place you originated from, and the one you're going to, but often not a complete set of either, so while they give a clue, it still takes some trial and error.
one possibility for the gameworld Fortune Deck:
rather than being found as cards on the various worlds, it is a more recent discovery. when the Foldgates were developed, it was discovered that the intensification/focusing of a Spacefolder's powers that they created had an interesting side effect: whenever one was used, all of the people that passed through it would be left with the distinct impression of an image. this image would always be the same for everyone folding at once, but varied from time to time, even for the same origin, destination, and person or people. however, as Foldgates became more common, and more people used them, it came to light that there appeared to be a finite number of these images, and they always appeared quite stylized and similar, though not always identical--in particular, they sometimes appeared upside-down. nonetheless, the images were consistent enough that people could generally agree on not only what they were, but on meanings/titles for them. and thus the Fortune Deck was born, as people set these images down in permanent form for use as divinatory tools. Some believe that the images revealed when folding have divinatory significance, while others think they are merely a reflection of the folder's mindset--and some believe both. no one really knows where these images come from, or whether the entire set has been discovered yet. while their meanings for divination are not universal, most people come to very close agreement on them, even though Spacefolders and Foldgates have originated independently on hundreds, if not thousands, of worlds.
In addition to low-level interaction between the spheres since time immemorial (or at least since someone (or -thing) figured out how to use the natural gates), the Ancients may have had a direct hand in the similarity of people throughout the universe. Similarly, until capitalism, notions of progress, and eventually industrialization, came to the various worlds, this travel had minimal impact, so mostly the spheres developed in the "usual" way (i.e., mostly like Earth). The biggest difference is the occasional (frequent?) world leaping from the early stages of modern society--say mid-renaissance--straight to (post-)industrial due to the efforts of a spherewalker bringing the right ideas and some technological and/or scientific know-how to worlds that were ready for the concepts but wouldn't have "naturally" developed the ideas for centuries. Likewise, there are some worlds far more advanced than others, because they have chosen not to share their technology. The tension between these two trends (sharing technology with spheres that are not ready for it,and gregariousness in general; and withholding advances in order to maintain superiority, and isolationism in general) has led to a proliferation of worlds with very incongruous tech and social and philosophical levels, such as steampunk, John Carter's Mars, space pirates with slide rules, Barbarella, and Flash Gordon--not to mention Connecticut Yankee in King Arthur's Court, and Once and Future King. And Brazil/Kafka! Spheres are all pretty much in the same habitable range in terms of atmosphere, weather, etc., though some are quite extreme (ice planets, etc.) and a few are really weird (the occasional society floating among the dense clouds of a gas giant or completely aquatic, frex). Some things that are surprisingly not common among spheres are the so-called "natural laws." There are some spheres upon which psychic abilities are common, and others where they are rare, and a few where even those with such abilities lack them. Likewise, there are some technologies that work on some spheres, but not on others.