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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. Each in turn then 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 “pulls the piste” and 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. Early experimentation proved that you can't pull too close to a planet, star or other massive body, the accepted theory is that the addition of the gravity well distorts the wedge and breaks the slide, causing the travelling body to re-enter normal space along the entire length of its journey, its molecules evenly distributed over a massive area. It's as good a theory as any, as nothing that has tried to pull a close planetary piste has ever come out the other side and while there is an equation to predict minimum safe distance based on the mass of travelling ship and mass of destination all but the most daring and desperate military actions like to aim for a good couple of days distance just to be on the safe side.

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 a combat vessel intended to leave its home system is astronomically expensive to build and maintain while very easy to cripple.

It also places limits on ship design - because of cost you are only coating one edge in ardanium, but you need to make sure that will be the only edge ever in contact with the gradient, so it is often the long side and ships tend to be long and low rather than short and high. If the bottom of your tall ship clips a clump of dark matter on the gradient, theorised as possible as it only cracks the dark matter not removes it, there is a risk that ship would “topple” and one of the non ardanium surfaces would touch the gradient and be destroyed. At least that is what is theorised because any ship or test bed that did happen to vanished into whatever collection of obscure quantom waveforms you become when the gradient goes wrong. No one is entirely happy with the word “topple” either to describe a process of rotational geometry that is happening at a point where normal physics and geometry don't apply, but once again nobodies managed to come up with a better term that can be understood by people without 3 PhD's in theoretical physics.

While ships can traverse a gradient, it has proven impossible to transmit a signal that can maintain coherence during the slide. As a result, all interstellar communication needs to be delivered via ships, leading to lucrative mail services and and a thriving courier culture. Cost for your postal system is often more proportional to volume than mass - the bigger your object to be shipped, the bigger the courier it needs, so the fewer ships that can carrier it and the more ardanium they needed to build.