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This is a setting creation exercise meant to create a setting useful for a space travel RPG.
Earth exists in the setting, but it is burnt out, having been “used up” ages ago. Few planets have the sorts of resources that Earth once had.
Earth has no gate or wormhole, the first being discovered by colonists arriving on Alpha Centauri Prime.
There are remains of non-human sapients, but no living examples have been found. Most never made it off their homeworlds, with the exception of one group, the one that built the gates (see below).
No homeworld has ever been found for these beings. Instead, all that remains of their culture are the gates. What happened to this culture is a mystery to be discovered in play.
Below are some ideas the GM can use, but clever GMs will come up with a solution to the mystery that is both consistent with the facts, and yet fits their campaign well thematically.
The gates failed for some galactic-level reason, stranding all of their cultures, which were largely unable to survive the isolation. In this case it's possible that some few have survived on particularly habitable systems, remaining to be discovered.
In this case, the beings in question evolved to a higher level of existence, becoming energy or something else.
There are several species of Terran fauna that humans have uplifted to full sapience. Some of these have inhabited systems all on their own. Their origins, however, are clear.
Uplifted Terran species include dolphins, octopuses, chimpanzees (bonobos), dogs, and crows which are only sapient in hive-mind flocks.
There have also been a couple of uplifted species found on other worlds.
There are multiple types of FTL drives, each serving various purposes.
There are two main methods of instantaneous transit over large distances.
Normal FTL drives can be modified to ensure survival of transit of wormholes. This is an expensive addition, and few craft have such modifications. More often smaller craft have to be carried by ferries specifically designed for such transit. But there are exceptions, typically military ships.
Worm holes are fairly common; most inhabited systems have one, if only because systems without them tend to be relatively inaccessible.
Gates have been found created by the beings referred to as the Builders for having built them. Humans have not been able to replicate this technology (they have only just begun to understand how to activate these gates). Any craft (even those without any other FTL drives) can transit such a gate.
Even small craft may have FTL drives, but any FTL drives that are not Point-to-point are only up to a few multiples of C (the very fastest small craft move at about 20C, but have very limited ranges), rendering them highly inconvenient for transit between systems, or even to very distant companions. Most use of such FTL is for going from place to place inside of systems where there are no gates built.
These drives vary in some specifics, but all have the following in common:
All such drives create a hyperspace bubble around the craft. This bubble places it out of contact with all normal space-time and matter, but energy can pass in and out of the bubble. Generally the larger the bubble, the more space-time distortion it causes, and the more energy it takes to fight that distortion. So smaller craft with smaller bubbles are faster than those with larger bubbles. In all cases, however, the fields are calibrated so that time inside and outside pass at the same rate relatively, avoiding the issues with moving at (faster than) relativistic speeds. Any miscalibration causes cataclysmic failure that destroys everthing in the Hyperspace Bubble.
As a side-effect of the hyperspace bubble, a shield can be created that blocks a substantial amount of energy from entering the hyperspace bubble (in addition to the hyperspace bubble itself preventing interaction with all matter). The most common use of such shields are simply as protection from space radiation, but the military application allows ships to defend themselves against energy weapons, nuclear detonations, etc.
A craft with no active hyperspace bubble cannot be shielded in this way.