FTL

Millennium Falcon Cockpit, from Star Wars (1977)
Millennium Falcon Cockpit, from Star Wars (1977) (later repackaged as Star Wars: Episode IV A New Hope. ), A classic cinematic representation of FTL travel

FTL or  Faster Than Light travel is a staple of most Space Opera, even the hard SF versions thereof (the exception being the recent trend towards space opera limited only to the solar system and maybe to a single nearby star).

Einstein proved that ftl is impossible in our space-time. Logic and hard science dictates that it is possible to have two but not all of these three things: Relativity, FTL, and Causality (no paradox); we can get around Einstein by dumping causality (quantum entaglement and nonlocality in effect does this to some degree but only on the quantum scale, not on the macroscale of spaceships).

Science fiction uses various plot devices to get around this, e.g. hyperspace, jump-point, warp drive, wormholes, etc.

Hyperspace for example is a purported region of space-time in which the laws of physics differ from our familiar space-time, hence allowing superluminal speed, or alternatively in which remote parts of ordinary spacetime are close together, and in which therefore ftl is possible. In Star Trek this is called Warp Drive.

Jumpspace assume instantaneous ftl transit via handwavium tech (like wormholes but without having an actual navigable wormhole), sometimes this requires certain distance from a planet or star, in other settings there are only certain points where this is possible.

Wormholes and stargates are huge static portals by which ftl transit is possible, but only between two fixed points. Of these, navigable wormholes are the only ones remotely possible given our current understanding of the physical universe, although even they would look nothing like the water-ripple special effects portals of pop sci fi such as Stargate, and require converting a gas giant or an entire star to exotic matter.

A pair of jump-points resembles a wormhole in that it provides a shortcut between two remote points in spacetime.  Unlike wormholes, there is no modification to curved spacetime itself, and hence no need for elaborate wormhole technology such as exotic matter. Rather the ship itself transrotates out of realspace into hyperspace, and then back again into realspace.  With a ship transiting a wormhole, the front half of the ship can have emerged at one end of the universe whilst the rear half is still back at the other end. With jumpspace, the whole ship disappears from the first area of realspace, before appearing or emerging at the other end

There is the tendency among some lovecraftian-influenced horror-fantasy scifi universes, the best known being Warhammer 40K, to replace the Star Trek/Star Wars/SF hyperspace, which is essentially just another form of rational physical reality accessible via ordinary technology, with terrifying alternate dimensions, making ftl a sanity-risking enterprise.

The Alcione-verse uses the premise of Relativity, FTL, and Acausality (macroscale nonlocality) to allows FTL at the cost of a paradoxical universe; every ftl transit adds to the overall paradox debt. To ensure ftl isn’t too easy or comfortable or accessible everywhere, I’ve also brought in paraspace weirdness and lovecraftian elements.