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Once ardanium was discovered, and its properties were understood, it turned out that friction was everything and there was no such thing, really, as the speed of light.

There is an upper limit to how fast things can travel under circumstances even vaguely resembling normal, and light has a tendency to travel at or near that upper limit, but this arbitrary velocity at which acceleration becomes impossible and funny things happen to time actually comes from another factor entirely. Dark matter.

Dark matter is everywhere, literally, it actually occupies all of space, including the tiny proportion also taken up by other things such as nebulae, solar systems, dust clouds and ambient vacuflora drifts. Dark matter is the substance which prevents anything from exceeding 299,792,458 metres per second. Dark matter is the term physicists have historically used to describe anything which causes universal effects that they cannot otherwise explain.

It's as good a term as any, though, for the crust on spacetime that locked the Human race and the other two local spacefaring biologicals into tiny spheres around their home systems for the first few centuries of their respective exploratory eras, until each in turn found a small deposit of ardanium and discovered that mineral's interesting attitude to gravity under different radiation exposures.

Existing somewhere below the early 21st Century periodic table (and on a slightly different layer), ardanium still has a relational link to the alkaline metals. However, the electron shells are directly interfaced with a side on lattice (theoretically impossible and nobody is particularly happy with any model used to explain it) of the Habgood boson, also known as the pseudo-graviton or - due to an unfortunately timed joke by a top particle physicist in hearing of members of the press - popularly and erroneously, the Quirky-Quark.

This unusual interaction of particles allows this extremely irregular element to act in a manner almost indistinguishable from stable matter, despite the fact that it should have a half life measure in fractions of one percent of a nanosecond. It is literally held together by fake gravity from the Habgood boson and it is this effect which also gives it its unique capacity to manipulate gravitational effects when exposed to streams of composite Higgs quasiwavelets and depressed semistrange fermions, shunting the electrons around in their alignment and causing them to 'fall'.

For Humans the luxury of small scale, plane applied artificial gravity came first, so that when people finally slid down their first gradient to a distant world, they did so with a comfortable one Gee on the deck and with the option of the same on the ceiling or any other available surface if they really wanted.

Pick a direction to be 'forwards', pick another to be 'right' and you've by default defined 'up'. Expose the ardanium core of your gradient drive to the correct mix of exotic quanta and unlikely sounding wave forms and the arbitrary two dimensional plane you defined with your two decisions becomes stretched into a three dimensional concave sided wedge, although not in any way that makes sense to the perceptions of any entity yet encountered. On the surface of that wedge, the dark matter becomes stretched thin and cracked, so that as your ship slides down the slope back towards Normalised space, it can exceed the speed limit normally imposed. Get it right - and most people do most of the time - and when friction reasserts itself and the gradient under your ardanium hull layer flattens out you will have traversed many times the distance light normally travels in a year in a matter of days or weeks and be close enough to your target to complete your journey on chemical or ionic engines in a similarly reasonable period. Get it wrong and you'll be in the middle of nowhere and have to start again, assuming you can.

The ardanium layer must be maintained across the whole contact surface of the ship, any non ardanium parts will lose physical coherence due to the distorted spacetime of the gradient, destroying the ship without a trace. Commercial and personal ships can be constructed with the bottom of the ship coated in a thin layer of ardanium, also used to provide gravity on-board (this configuration has the advantage of making a lot more sense to the crew than having a surface above them or off to the sides on contact with the gradient), this really only has to be thick enough to survive micrometeorite and intra atmospheric particle damage. Warships, on the other hand, must maintain a layer thick enough to sustain weapon damage, which means that and combat vessel intended to leave it's home system is astronomically expensive to build and maintain.