We've all heard the familiar rubber sheet analogy for gravity, right? You condense 3-D space into 2-dimensions, and imagine it as a rubber sheet. Masses placed on it produce "gravity wells" in the up-down direction, corresponding to real gravity in a fourth, "gravitational" dimension. Yada, yada, yada... There's just one problem with it -- There is nothing to create the dimples. When you place a mass on the rubber sheet, it creates a dimple (analogous to gravity) because the earth's real gravity pulls it down. But if this dimmple represent's gravity, what creates the dimple in real life? There is nothing. Imagine you perform the rubber sheet experiment in space. There is nothing (no real gravity in 3-D space) to produce a dimple, and hence, no gravity in the 2-D rubber universe. But, now imagine that the entire sheet is being accelerated perpendicular to it's surface (in the extra dimension). Einstein's equivalency principle says that the resulting acceleration will produce a pseudo-force (opposite to the direction of acceleration) that is indistinguishable from gravity. This accelerative force will cause the sheet to dimple, since the masses on the sheet have inertia, and will resist the sheet's acceleration. Now imagine that the entire universe is this sheet, and is accelerating "upwards" in a fourth, "gravitational" dimension (which, by the way, is not time). Since all the mass in the universe has inertia, what is to say it doesn't also have inertia in this fourth dimension. Hence mass will resist the universe's acceleration, and dimple it in the fourth dimension, creating gravity wells. The reason that matter is attracted to a gravity well, then is because it is seeking the lowest point in the gravitational dimension, just like water is attracted towards the lowest spot. Since water, say in a drain, has to move horizontally along the bottom of the sink to reach the lowest point in the vertical direction, similarly, mass has to move in the 3 dimensions of space time (along a hyper-surface) in order to reach the lowest point in the fourth dimension. Therefore, all gravity is created by inertia because matter resists acceleration, and the universe is accelerating in the fourth dimension. ~Caleb P.S. - Anyone think they can poke some holes (or rather, dimples) in this theory? I'd love to know if there's something wrong with it.