# What exactly is the relationship between the algebraic formulation of Quantum Mechanics and the geometric formulation of Classical Mechanics?

Okay so if we consider a particular physical system, the classical description of the system starts by first introducing a symplectic manifold, which is the cotangent bundle of a configuration manifold. There is the canonical Poisson Bracket which forms an algebra of functions on the manifold (i.e. the observables) and the time evolution of an observable $f$ is just $\{H,f\}$ where $H$ is the Hamiltonian.

If we want to describe the same physical system from a Quantum Mechanical perspective, we introduce a Hilbert space whose equivalence class of normalised elements correspond to the physical states. The algebra of observables on the Hilbert space form a non-commutative algebra with the time evolution described by Schrodinger's Equation. So I understand the motivation for introducing both formalisms but how do I relate these constructions?

How is the Hilbert Space related to the phase space manifold? The process of quantization replaces the poisson brackets with commutators but I don't understand what this implies for the underlying structures. Most people say that it's a hopeless task to go from Classical Mechanics to Quantum Mechanics because you've lost a lot of information but it's not at all clear to me whether this is actually the case. If anything it feels like Quantum Mechanics arises from discarding away some information from classical description, thereby quantizing it. And even if this is not the case and you do in fact lose information, the process of quantization proves that there is a systematic way of recovering this information because Quantization works independent of the specific system under consideration.

Now I know that every manifold can be understood through its algebra of functions but the algebra of functions on a manifold is always defined pointwise so its naturally commutative. In Quantum Mechanics we start with non-commutative algebra of functions so I'm assuming you can't think of it as an algebra of functions over some geometric manifold. If they're not related in any way, then how can anyone convincingly say that Quantum Mechanics reduces to Classical Mechanics on the macroscopic scale.

• – Qmechanic Mar 12 '17 at 22:40
• Indeed, phase-space quantum mechanics will allow you to compare apples with apples, instead of with oranges. In that formulation, it is easier to see how the classical entropy of a system exceeds its quantum entropy, once a proper shift of the integration measures is readjusted; this indicates forfeiture of information in the $\hbar \to 0$ limit. – Cosmas Zachos Mar 13 '17 at 0:51