Sphere Packings and Kissing Numbers
The E8 lattice is remarkable in that it gives solutions to the lattice packing problem and the kissing number problem in 8 dimensions.
The general sphere packing problem asks what is the densest way to pack n-dimensional (solid) spheres in Rn so that no two spheres overlap. Lattice packings are special types of sphere packings where the spheres are centered at the points of a lattice. Placing spheres of radius 1/√2 at the points of the E8 lattice gives a lattice packing in R8 with a density of
It is known that this is the maximum density that can be achieved by a lattice packing in 8 dimensions. Furthermore, the E8 lattice is the unique lattice (up to isometries and rescalings) with this density. It is conjectured this density is, in fact, optimal (even among irregular packings). Researchers have recently shown that no irregular packing density can exceed that of the E8 lattice by a factor of more than 1 + 10−14.
The kissing number problem asks what is the maximum number of spheres of a fixed radius that can touch (or "kiss") a central sphere of the same radius. In the E8 lattice packing mentioned above any given sphere touches 240 neighboring spheres. This is because there are 240 lattice vectors of minimum nonzero norm (the roots of the E8 lattice). It was shown in 1979 that this is the maximum possible number in 8-dimensions.
The kissing number problem is remarkably difficult and solutions are only known in 1, 2, 3, 4, 8, and 24 dimensions. Perhaps surprisingly, it is easier to find the solution in 8 (and 24) dimensions than in 3 or 4. This follows from the special properties of the E8 lattice (and its 24-dimensional cousin, the Leech lattice).
Read more about this topic: E8 Lattice
Famous quotes containing the words sphere, kissing and/or numbers:
“Men are rewarded for learning the practice of violence in virtually any sphere of activity by money, admiration, recognition, respect, and the genuflection of others honoring their sacred and proven masculinity. In male culture, police are heroic and so are outlaws; males who enforce standards are heroic and so are those who violate them.”
—Andrea Dworkin (b. 1946)
“What of soul was left, I wonder, when the kissing had to stop?”
—Robert Browning (1812–1889)
“I had a feeling that out there, there were very poor people who didn’t have enough to eat. But they wore wonderfully colored rags and did musical numbers up and down the streets together.”
—Jill Robinson (b. 1936)