"The unification of forces, even if it were perfected, would leave us with two great kingdoms of particles, still not unified. Technically, these are the fermion and boson kingdoms. More poetically, we may call them the kingdoms of substance (fermions) and force (bosons). By postulating that the fundamental equations enjoy the property of supersymmetry, we heal the division of particles into separate kingdoms. Supersymmetry can be approached from several different angles, but perhaps the most appealing is to consider it as an expansion of space-time, to include quantum dimensions. The defining characteristic of quantum dimensions is that they are represented by coordinates that are Grassmann numbers (i.e., anticommuting numbers) rather than real numbers. Supersymmetry posits that the fundamental laws of physics remain invariant transformations that correspond to uniform motion in the quantum dimensions. Thus supersymmetry extends Galileo/Lorentz invariance."