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My original thought was to have different sorts of gravity due to magical spells, but looking at the 7th level spell reverse gravity made me pause - if this simple spell only lasts a short time and has a very specific effect, spells that were long-lasting and general would have to be 9th level or higher! Thus the use of such magics by the Scro for their ships would be almost impossible.
Trying to circumvent this problem, I noticed something. All forms of gravity that we know of (in real space or arcane space) are attractive. The reverse gravity spell makes gravity act in a repulsive fashion. This must be contrary to the basic laws of the universe, thus the high level of the spell. Spells which alter the direction of gravity but still allow it to remain an attractive force should be much simpler.
I therefore came up with the following theory and a new set of
spells affecting gravity. All three spells can be made permanent with the
permanency spell (XP cost = 500 x spell level).
Theory of Nonbinary Gravity
There is a very brief state in which the gravity of objects is neither binary (with down on both sides of an object) nor spherical (like a planet): a disclike state that begins to bulge out into a double-saucerlike state, and finally to spherical. However, the difference between a binary-gravity state and a spherical-gravity state is probably only on the order of a few hundred pounds of material (i.e., very minor, cosmically speaking). Essentially gravity is an either-or situation (like quantum mechanics' wave-or-particle theory) and natural situations that fit neither state are going to be so rare as to be practically nonexistant. Magic can alter the effects of gravity, of course.
Greater Gravity Alteration
Level: Sor/Wiz 5
As lesser gravity alteration, except as noted above and that it can even alter gravity greater than 90 degrees from normal, allowing walking on ceilings.
Level: Sor/Wiz 7
Components: V, S, M
Casting Time: 1 action
Range: Medium (100 ft. + 10 ft./level)
Area: One 10 ft. cube per level (S)
Duration: 1 minute/level
Saving Throw: None
Spell Resistance: No
You cause the fundamental gravitational force of matter to align itself
against itself, resulting in an area of no gravity (in other words, every
other square inch of the area of effect aligns its gravity one way, while
the remaining spaces align their gravity in the opposite direction, canceling
each other out). This effect is similar to being next to a crystal shell
or a starbeast -- there is no pull towards that object. Without gravity,
creatures can easily push themselves off of the surface, although without
the ability to fly moving in the air is difficult (as a full-round action,
a character can "swim" in the air, moving a number of feet equal to a Strength
check minus 10, or half that with a move-equivalent action*).
Material Component: A paste made of iron filings, diamond dust, and grav blood worth 50 gp.
*If Andy Collins already covered moving in zero-gravity in his Shadows of the Spider Moon spelljammer campaign setting he wrote for Polyhedron, ignore this improvised rule
and use that instead.