To honor another great soul...
F.D. 'Tony' Smith passed away last December, and he had much to say that inspired me to investigate various areas of Math I would otherwise overlook or fail to understand. Like Steve Dufourny; I came to realize that the properties of spheres in various dimensions have a relation to Physics that is profound. People expect them to be simple but they are not. One might think adding dimensions would allow you to increase the volume or surface area of a sphere unendingly, but this is not what is real. Instead a sphere has maximal (hyper-) volume in 5-d and maximal (hyper-) surface in 8-d. Tony Smith had this to say:
Sphere, torus, Klein bottle, Möbius strip, etc are all basic geometric concepts.
The simplest of these is the sphere.
When people tried to use math to classify spheres of various dimensions, they found out that classification was not at all simple, but had lots of subtleties. For example, a thing that looks like a sphere from combinatorical/piecewise linear point of view can (in some dimensions) have many different smooth/differential structures:
sphere - number of possible smooth/differential structures
S1 - 1
S2 - 1
S3 - 1 = 2x1 / 2 (but S3 is a subset of any exotic R4# and there are uncountably many exotic R4 spaces)
S4 - 1
S5 - 1
S6 - 1
S7 - 28 = 8x7 / 2 = 23 x (23 - 1) / 2
S8 - 2
S9 - 8
S10 - 6
S11 - 992 = 32x31 = 25 x (25 - 1)
S12 - 1
S13 - 3
S14 - 2
S15 - 16,256 = 128x127 = 27 x (27 - 1)
S16 - 2
S17 - 16
S18 - 16
As John Baez has noted,
there are various distinct questions floating around, including:
A) how many topological manifolds are homotopy-equivalent to the sphere?
B) how many PL ( = piecewise-linear = combinatorical) manifolds are homeomorphic to the sphere?
C) how many smooth manifolds are PL equivalent to the sphere?
For dimension 3, question A is the Poincare conjecture.
It was proven by Grisha (Grigori) Perelman.
For dimension 3, questions B and C are solved and the answer is 1.
For dimension 4, question A is solved (in the 1980s, by Freedman) and the answer is 1.
For dimension 4, question C is solved and the answer is 1.
For dimension 4, question B is open (the smooth Poincare conjecture in dimension 4).
To try to make sense of this look at spheres by their Homotopy Groups PI(k)(Sn),
which is roughly the number of ways you can wrap a k-sphere around an n-sphere.
For example, PI(n)(Sn) is the infinite cyclic group Z, and each element of Z corresponds to a winding number of a wrapping of Sn around Sn.
If you want to look at homotopy groups from the point of view of all spheres of all dimensions, and take the orthogonal group O(n) as the group of rotations/reflections of Sn, then you can say that O(infinity) is the orthogonal group for infinite-dimensional real space which contains as subgroups all orthogonal groups O(n) for all finite n and is effectively the symmetry of all spheres of whatever dimension.
Then you find that the homotopy relation is periodic with period 8:
Bott periodicity PI(n+8)(O(infinity)) = PI(n)(O(infinity))
The orthogonal structure is directly related to Clifford algebra and Clifford algebra also has the periodicity structure Cl(8N) = Cl(8) x ...(N times tensor product)... x Cl(8)
So, in some sense the geometry of spheres is described by Clifford algebra which is why I use Clifford algebra as the basis for my physics model.
Once you describe spheres, you can use that to describe
torus - sphere with a hole
Klein bottle - sphere with a twist
... etc ...
so
I think that Clifford algebras are a nice Math way to describe spheres which are the basic structures of the universe.
Tony