Faster than Light travel.

Faster than Light travel is a little bit of a misnomer. Relativity completely bars a mass accelerating to light speed, let alone beyond it. Of course, Humanity- and indeed every intelligent species- has the ingenuity to get around that, for there is more to physical law than a simple speed limit.

Alcubierre Drive
Technically, it should be called "warp drive", but the term hyperdrive stuck instead. The simplest way to put it is that it squeezes spacetime, fishtailing it out behind the ship while contracting in front.

While the power requirements are significant-- and indeed, engines in general are typically a very large power allocation-- refinements have made it a much less power intensive endeavor to use hyperdrive than to use a Shift drive. A vessel can mount a more compact hyperdrive as well, and because it is both smaller and requires less power overall, a vessel can contain more components aside from the drive itself; this makes it popular for trading vessels especially, as while a warship cannot always efficiently route power and controls to systems with a wealth of space, for a hauler space is money.

The downside, of course, is that it still takes time to transit to the target coordinates. Depending on the refinement of the drive, this can mean days to months to years across the Near-Arm, and this has led those with better drives to dominate politics and military engagements; as the old saying goes, "speed kills". While the deployment rate of even the best hyperdrives cannot match a Shift drive, the power requirement allows for every vessel to mount such a drive, and increases total mobility.

It's notable that hyperdrive is the standard for most states. It's much simpler to maintain than the alternative, it has a lower power requirement and it has a lower total volume, and so in terms of cost, it's the superior alternative for most polities. Indeed, a reasonably quick hyperdrive can still compete in an overall sense, as it even requires less support infrastructure. Ships must be marshaled for counterattack or response to border incursions, and it is for this reason that many hyper-using states keep their fleets at a partial state of readiness all of the time.

Shift Drive
When humanity's attempts to engineer a working hyperdrive failed, the hearts of Man sank. The U.S.S. Columbus, at the time a mighty vessel with all the best technology available to the brief Americo-European collaboration, disappeared with all hands. Historians of later ages would sometimes argue this was a turning point in Earth's internecine wars; without a pressure valve for rising populations, war seemed iminent as even space colonies failed to adequately remove a large number of the humans on the planet.

The solution came from a New York technology firm which would in time come to dominate the entire planet. The attempt to create a working Alcubierre drive was a failure, so it was thought, but what if one attempted to create a wormhole link? After all, if a wormhole could only exist for a bare second without exotic matter, what if one were to make it encompass an entire vessel within that second?

Stahltech's idea worked brilliantly. It required but a precious drop of the exotic matter predicted, just enough to force the wormhole link open...but it also required a massive amount of power. It required two fusion reactors to demonstrate a link to Alpha Centauri in 2045, and this was but only a few minutes before the hole itself began to destabilize. Needless to say, world governments were not overly impressed; the wars that came five years later proved a lack of engagement with the concept, which seemed still theoretical despite its applicability.

After the unification of Earth, state resources were comparatively vast and fairly free to spend; constructing a fleet of Shift capable vessels, and funneling resources into research on the drive and its power requirements, was time consuming but extremely rewarding. To date, only humanity fields Shift Drives for several reasons, as even those with stable antimatter production industries and matter/antimatter reaction plants-- far and away more than is strictly required for a single ship Shift-- can't quite crack what is required to make the device work.

In effect, the Shift Drive creates a wormhole link between two points. the aperture is kept open by the generating ship until all vessels have passed through; the process is extremely fast, taking place in under a second once underway. The power flashes out before an incoming fleet or ship as bursts of gamma or x rays, and also at the origin point. A sure sign that a large fleet has jumped into a system is the flash of high frequency radiation from it. The power requirements- and even with exotic matter the requirement for a wormhole is considerable- is divided into activation energy and mass; the distance traveled is almost irrelevant. This makes it an extremely useful strategic tool of mobility, but lacks the power to make repeated tactical jumps; a jump from low orbit to lunar orbit takes roughly as much power as a jump from Sol to Mintaka.