## Solving Hilbert’s sixth problem

### Commutative vs. non-commutative geometry

Last time I introduced the fundamental relationship of (Hamiltonian) mechanics:

ρ12= ρθ +    θρ
θ12 = θθ + x ρρ

This looks very abstract, and nothing like the usual formulation but we will show that this leads to classical and quantum mechanics.

However, we want to take a closer look first at the relationship and notice a very interesting analogy. Suppose x = -1.Where have we seen a relation like this before? How about complex number multiplication?

z = a+ib, w = p+iq
z*w = ap - bq + i (aq + bz)

Im(zw) = Im(z) Re(w) + Re(z) Im(w)
Re(zw) = Re(z) Re(w)  - Im(z) Im(w)

Since x=-1 corresponds to quantum mechanics, it is no wonder that quantum mechanics is expressed best over complex numbers!

In general the products ρ and θ can be proven to be skew-symmetric and symmetric but I am going to skip the proof. What I want to concentrate today is the concrete realizations of the two products which is a very interesting story with unexpected mathematical links.

It turns out that there are two such realizations, one based on state space, and one based on Hilbert space. However, those realizations tell a much larger story, that of commutative and non-commutative geometry.

In mathematics there is a nice duality between geometry and algebra. The easiest way to see this is to consider the ordinary 2D plane and old fashion Euclidean geometry in a plane. Then add a coordinate system and express all lines and circles as algebraic relationships. For example a circle with center at position (a, b) and radius r obeys (x-a)^2+ (y-b)^2 = r^2. And then all usual geometric theorems can be expressed algebraically.

This duality cuts very deep across many mathematical concepts and structures, and in particular generalizes to non-commutative geometries where the notions of point and lines are not well defined. In non-commutative geometry at core, the very notion of distance is modified from an infimum (the shortest distance between two points is a line which minimizes the distance) to a supremum.

There is a dictionary of correspondence of mathematical structures between commutative and non-commutative realm (http://arxiv.org/pdf/math/0408416v1.pdf):

 Commutative Noncommutative measure space von Neumann algebra locally compact space C∗- algebra vector bundle ﬁnite projective module complex variable operator on a Hilbert space real variable sefadjoint operator inﬁnitesimal compact operator range of a function spectrum of an operator K-theory K-theory vector ﬁeld derivation integral trace closed de Rham current cyclic cocycle de Rham complex Hochschild homology de Rham cohomology cyclic homology Chern character Chern-Connes character Chern-Weil theory noncommutative Chern-Weil theory elliptic operator K-cycle spin Riemannian manifold spectral triple index theorem local index formula group, Lie algebra Hopf algebra, quantum group Symmetry action of Hopf algebra

To this list on the commutative side I will add: phase space and to the noncommutative side I will add Hilbert space. I will also discuss only quantum mechanics (meaning the elliptic composability case of x=-1)

Here are the realizations of the two products

 Commutative Noncommutative Product ρ (skew symmetric) A ρ B=A 2/ ħ(sin (ħ/2)∇) B Moyal bracket A ρ B = i/ħ (AB-BA)   commutator Product θ (symmetric) A θ B=A (cos (ħ/2)∇) B A θ B = 1/2(AB+BA) Jordan product

Where  is the Poisson bracket (where the first partial differential acts on the left and the second partial differential acts on the right).

The particular choice of realization depends on the problem under consideration and the ease of the formalism to solve it, but both realizations give the same final answer. Let us play a bit with the noncommutative side products to derive an (trivial in this formalism) identity (but which is absolutely essential).

We will use [,] as a notation for the commutator, and {,} for the anti-commutator (Jordan product). Then consider this:

[A,[B,C]] – [[A,B],C] and {A,{B,C}} – {{A,B},C}

those are called associators because they measure the lack of associativity in the products [], {}.

[A,[B,C]] – [[A,B],C] = [A, BC-CB] – [AB-BA, C]=
ABC-ACB-BCA+CBA   - ABC+BAC+CAB-CBA = BAC+CBA – ACB-BCA

{A,{B,C}} – {{A,B},C} = {A, BC+CB} – {AB+BA, C}=
ABC+ACB+BCA+CBA   - ABC-BAC-CAB-CBA = -(BAC+CBA – ACB-BCA)

In fact the two associators are proportional, and this is the consistency relationship between dynamic and ontology!!! It is this relationship that transforms the two products into a mechanic (quantum or classical)! What this shows is that the two products can be combined into an associative product. Why is this important? Because it allows for the introduction of probabilities into the formalism and the notion of a state/phase space which is needed if we have to make experimental predictions, doing physics and not pure math. In fact, the table above should be extended as follows:

 Commutative Noncommutative Product ρ (skew symmetric) A ρ B=A 2/ ħ(sin (ħ/2)∇) B Moyal bracket A ρ B = i/ħ (AB-BA)   commutator Product θ (symmetric) A θ B=A (cos (ħ/2)∇) B A θ B = 1/2(AB+BA) Jordan product Associative product f⋆g = f θ g + i ħ/2 f ρ g the start product AB = AB (matrix multiplication) Ordinary complex number multiplication