nLab
quantization

Context

Physics

physics, mathematical physics, philosophy of physics

Surveys, textbooks and lecture notes


theory (physics), model (physics)

experiment, measurement, computable physics

Contents

Idea

By quantization is meant some process that

  1. reads in a system of classical mechanics, or rather of prequantum data in the form of an Lagrangian/action functional and or a phase space equipped with symplectic structure

  2. and returns a corresponding system of quantum mechanics.

Motivation from classical mechanics and Lie theory

We indicate here a systematic motivation of quantization by looking at classical mechanics formalized as symplectic geometry from the point of view of Lie theory (Fiorenza-Rogers-Schreiber 13, Nuiten 13).

\,

Quantization of course was and is motivated by experiment, hence by observation of the observable universe: it just so happens that quantum mechanics and quantum field theory correctly account for experimental observations where classical mechanics and classical field theory gives no answer or incorrect answers. A historically important example is the phenomenon called the “ultraviolet catastrophe”, a paradox predicted by classical statistical mechanics which is not observed in nature, and which is corrected by quantum mechanics.

But one may also ask, independently of experimental input, if there are good formal mathematical reasons and motivations to pass from classical mechanics to quantum mechanics. Could one have been led to quantum mechanics by just pondering the mathematical formalism of classical mechanics? (Hence more precisely: is there a natural Synthetic Quantum Field Theory?)

The following spells out an argument to this effect. It will work for readers with a background in modern mathematics, notably in Lie theory, and with an understanding of the formalization of classical/prequantum mechanics in terms of symplectic geometry.

So to briefly recall, a system of classical mechanics/prequantum mechanics is a phase space, formalized as a symplectic manifold (X,ω)(X, \omega). A symplectic manifold is in particular a Poisson manifold, which means that the algebra of functions on phase space XX, hence the algebra of classical observables, is canonically equipped with a compatible Lie bracket: the Poisson bracket. This Lie bracket is what controls dynamics in classical mechanics. For instance if HC (X)H \in C^\infty(X) is the function on phase space which is interpreted as assigning to each configuration of the system its energy – the Hamiltonian function – then the Poisson bracket with HH yields the infinitesimal time evolution of the system: the differential equation famous as Hamilton's equations.

Something to take notice of here is the infinitesimal nature of the Poisson bracket. Generally, whenever one has a Lie algebra 𝔤\mathfrak{g}, then it is to be regarded as the infinitesimal approximation to a globally defined object, the corresponding Lie group (or generally smooth group) GG. One also says that GG is a Lie integration of 𝔤\mathfrak{g} and that 𝔤\mathfrak{g} is the Lie differentiation of GG.

Therefore a natural question to ask is: Since the observables in classical mechanics form a Lie algebra under Poisson bracket, what then is the corresponding Lie group?

The answer to this is of course “well known” in the literature, in the sense that there are relevant monographs which state the answer. But, maybe surprisingly, the answer to this question is not (at time of this writing) a widely advertized fact that has found its way into the basic educational textbooks. The answer is that this Lie group which integrates the Poisson bracket is the “quantomorphism group”, an object that seamlessly leads to the quantum mechanics of the system.

Before we spell this out in more detail, we need a brief technical aside: of course Lie integration is not quite unique. There may be different global Lie group objects with the same Lie algebra.

The simplest example of this is already one of central importance for the issue of quantization, namely, the Lie integration of the abelian line Lie algebra \mathbb{R}. This has essentially two different Lie groups associated with it: the simply connected translation group, which is just \mathbb{R} itself again, equipped with its canonical additive abelian group structure, and the discrete quotient of this by the group of integers, which is the circle group

U(1)=/. U(1) = \mathbb{R}/\mathbb{Z} \,.

Notice that it is the discrete and hence “quantized” nature of the integers that makes the real line become a circle here. This is not entirely a coincidence of terminology, but can be traced back to the heart of what is “quantized” about quantum mechanics.

Namely, one finds that the Poisson bracket Lie algebra 𝔭𝔬𝔦𝔰𝔰(X,ω)\mathfrak{poiss}(X,\omega) of the classical observables on phase space is (for XX a connected manifold) a Lie algebra extension of the Lie algebra 𝔥𝔞𝔪(X)\mathfrak{ham}(X) of Hamiltonian vector fields on XX by the line Lie algebra:

𝔭𝔬𝔦𝔰𝔰(X,ω)𝔥𝔞𝔪(X). \mathbb{R} \longrightarrow \mathfrak{poiss}(X,\omega) \longrightarrow \mathfrak{ham}(X) \,.

This means that under Lie integration the Poisson bracket turns into an central extension of the group of Hamiltonian symplectomorphisms of (X,ω)(X,\omega). And either it is the fairly trivial non-compact extension by \mathbb{R}, or it is the interesting central extension by the circle group U(1)U(1). For this non-trivial Lie integration to exist, (X,ω)(X,\omega) needs to satisfy a quantization condition which says that it admits a prequantum line bundle. If so, then this U(1)U(1)-central extension of the group Ham(X,ω)Ham(X,\omega) of Hamiltonian symplectomorphisms exists and is called… the quantomorphism group QuantMorph(X,ω)QuantMorph(X,\omega):

U(1)QuantMorph(X,ω)Ham(X,ω). U(1) \longrightarrow QuantMorph(X,\omega) \longrightarrow Ham(X,\omega) \,.

While important, for some reason this group is not very well known, which is striking because it contains a small subgroup which is famous in quantum mechanics: the Heisenberg group.

More precisely, whenever (X,ω)(X,\omega) itself has a compatible group structure, notably if (X,ω)(X,\omega) is just a symplectic vector space (regarded as a group under addition of vectors), then we may ask for the subgroup of the quantomorphism group which covers the (left) action of phase space (X,ω)(X,\omega) on itself. This is the corresponding Heisenberg group Heis(X,ω)Heis(X,\omega), which in turn is a U(1)U(1)-central extension of the group XX itself:

U(1)Heis(X,ω)X. U(1) \longrightarrow Heis(X,\omega) \longrightarrow X \,.

At this point it is worth pausing for a second to note how the hallmark of quantum mechanics has appeared as if out of nowhere simply by applying Lie integration to the Lie algebraic structures in classical mechanics:

if we think of Lie integrating \mathbb{R} to the interesting circle group U(1)U(1) instead of to the uninteresting translation group \mathbb{R}, then the name of its canonical basis element 11 \in \mathbb{R} is canonically ”ii”, the imaginary unit. Therefore one often writes the above central extension instead as follows:

i𝔭𝔬𝔦𝔰𝔰(X,ω)𝔥𝔞𝔪(X,ω) i \mathbb{R} \longrightarrow \mathfrak{poiss}(X,\omega) \longrightarrow \mathfrak{ham}(X,\omega)

in order to amplify this. But now consider the simple special case where (X,ω)=( 2,dpdq)(X,\omega) = (\mathbb{R}^2, d p \wedge d q) is the 2-dimensional symplectic vector space which is for instance the phase space of the particle propagating on the line. Then a canonical set of generators for the corresponding Poisson bracket Lie algebra consists of the linear functions pp and qq of classical mechanics textbook fame, together with the constant function. Under the above Lie theoretic identification, this constant function is the canonical basis element of ii \mathbb{R}, hence purely Lie theoretically it is to be called ”ii”.

With this notation then the Poisson bracket, written in the form that makes its Lie integration manifest, indeed reads

[q,p]=i. [q,p] = i \,.

Since the choice of basis element of ii \mathbb{R} is arbitrary, we may rescale here the ii by any non-vanishing real number without changing this statement. If we write ”\hbar” for this element, then the Poisson bracket instead reads

[q,p]=i. [q,p] = i \hbar \,.

This is of course the hallmark equation for quantum physics, if we interpret \hbar here indeed as Planck's constant. We see it arises here merely by considering the non-trivial (the interesting, the non-simply connected) Lie integration of the Poisson bracket.

This is only the beginning of the story of quantization, naturally understood and indeed “derived” from applying Lie theory to classical mechanics. From here the story continues. It is called the story of geometric quantization. We close this motivation section here by some brief outlook.

The quantomorphism group which is the non-trivial Lie integration of the Poisson bracket is naturally constructed as follows: given the symplectic form ω\omega, it is natural to ask if it is the curvature 2-form of a U(1)U(1)-principal connection \nabla on complex line bundle LL over XX (this is directly analogous to Dirac charge quantization when instead of a symplectic form on phase space we consider the the field strength 2-form of electromagnetism on spacetime). If so, such a connection (L,)(L, \nabla) is called a prequantum line bundle of the phase space (X,ω)(X,\omega). The quantomorphism group is simply the automorphism group of the prequantum line bundle, covering diffeomorphisms of the phase space (the Hamiltonian symplectomorphisms mentioned above).

As such, the quantomorphism group naturally acts on the space of sections of LL. Such a section is like a wavefunction, except that it depends on all of phase space, instead of just on the “canonical coordinates”. For purely abstract mathematical reasons (which we won’t discuss here, but see at motivic quantization for more) it is indeed natural to choose a “polarization” of phase space into canonical coordinates and canonical momenta and consider only those sections of the prequantum line bundle which depend only on the former. These are the actual wavefunctions of quantum mechanics, hence the quantum states. And the subgroup of the quantomorphism group which preserves these polarized sections is the group of exponentiated quantum observables. For instance in the simple case mentioned before where (X,ω)(X,\omega) is the 2-dimensional symplectic vector space, this is the Heisenberg group with its famous action by multiplication and differentiation operators on the space of complex-valued functions on the real line.

Quantization of field theory

An interesting subclass of quantum field theories is thought to arise from prequantum field theory via a process called quantization. This process reads in certain – usually differential geometric – data, interprets this data as specifying the dynamics of some physical system, and spits out the quantum field theory that encodes the time evolution of this system.

Historically, it was an approximation to the true time evolution that was originally found and studied, by Newton, Maxwell, Einstein and others. This is now known as “classical physics”. The true dynamics in turn is “quantum physics”.

In view of this, quantization is often understood as a right inverse to the procedure that sends the full quantum dynamics to its classical limit. As such it is not well defined, i.e. unique, when it exists. Additional structures sometimes make it unique.

algebraic deformation quantization

dimensionclassical field theoryLagrangian BV quantum field theoryfactorization algebra of observables
general nnP-n algebraBD-n algebra?E-n algebra
n=0n = 0Poisson 0-algebraBD-0 algebra? = BD algebraE-0 algebra? = pointed space
n=1n = 1P-1 algebra = Poisson algebraBD-1 algebra?E-1 algebra? = A-∞ algebra

There have been found some formalizations of at least some aspects of quantization, notably there is algebraic deformation quantization and geometric quantization.

duality between algebra and geometry in physics:

algebrageometry
Poisson algebraPoisson manifold
deformation quantizationgeometric quantization
algebra of observablesspace of states
Heisenberg pictureSchrödinger picture
AQFTFQFT
higher algebrahigher geometry
Poisson n-algebran-plectic manifold
En-algebrashigher symplectic geometry
BD-BV quantizationhigher geometric quantization
factorization algebra of observablesextended quantum field theory
factorization homologycobordism representation

These traditional formulations are geared towards mechanics (as opposed to field theory). Even in that restricted application, it seems that a more comprehensive understanding is still missing

The goal is to get closer to a systematic theory of quantization.

(Gukov-Witten 08, p.4)

In the context of field theory the conceptual issues become even more severe. For example the class of field theories called Yang-Mills theory is a core ingredient in the standard model of particle physics but the quantization of Yang-Mills theory (see there for more) poses famous open problems – of which of course that there isn’t yet a comprehensive theory of what this even means is not the least.

Quantization as an index map

Here we survey aspects of quantization which give rise to an index map or something similar, that is, a push-forward in generalized cohomology. More on this is in (Nuiten 13) and at motivic quantization.

The general pattern here is this:

Path integral by integration against the Wiener measure

For instance the archetypical example of the quantization of the charged particle propagating on a Riemannian manifold XX proceeds (in Wick rotated form as it appears in the worldline formalism for corresponding QFT) like so: write [exp(S kin(γ))Dγ][\exp(-S_{kin}(\gamma)) D\gamma] for the Wiener measure on paths in XX, then the integral kernel of the time evolution propagator of the quantum particle is given by the path integral (see there for more)

γP x in,x outXtra (γ)[exp(S kin(γ))Dγ] \int_{\gamma \in P_{x_{in}, x_{out}}X} tra_{\nabla}(\gamma) \; [\exp(- S_{kin}(\gamma)) D\gamma]

which is the integration of the parallel transport/holonomy functional (of the given connection that models the background gauge field) against the Wiener measure.

Quantization by partition function/index in supersymmetric quantum mechanics

More generally, for given any phase space equipped with a prequantum line bundle (L ω,)(L_\omega, \nabla), the corresponding geometric quantization is, if XX admits a compatible spin^c structure, the index of the spin^c Dirac operator coupled to this bundle, hence the space of quantum states is

=index(D ). \mathcal{H} = index( D_{\nabla} ) \,.

For more on this see at geometric quantization – As Index of the Spin^c Dirac operator.

Here the Dirac operator represents a class in K-homology

[D]KK(X,) [D] \in KK(X, \mathbb{C})

and this is now the “measure” against which the K-theory class

[L ω]KK(,X) [L_\omega] \in KK(\mathbb{C}, X)

of the prequantum line bundle is being integrated, by the index map hence the composition in KK-theory:

[](L ωC(X)D)KK(,). [\mathcal{H}] \simeq (\mathbb{C} \stackrel{L_\omega}{\to} C(X) \stackrel{D}{\to} \mathbb{C} ) \in KK(\mathbb{C}, \mathbb{C}) \,.

By the discussion at index this is equivalently the partition function of an auxiliary supersymmetric quantum mechanics system with background gauge field \nabla:

sTr(exp(tD 2))t>0. \mathcal{H} \simeq sTr( \exp(-t D^2) ) \;\;\;\; \forall t \gt 0 \,.

The appearance of an auxiliary and already quantized field theory here may seem circular, but is in fact part of a deeper pattern of quantization by the holographic principle, where rich quantum theories arise as boundary field theories of higher dimensional topological field theories. In the above case the higher dimensional theory is secretly the non-perturbative version of the Poisson sigma-model associated with the original phase space Poisson manifold. More details on this are indicated at extended geometric quantization of 2d Chern-Simons theory. In one dimension higher the Witten genus arises this way as the quantization of the heterotic string 2d QFT regarded as the boundary field theory of the M2-brane in Horava-Witten theory. (See (Nuiten13) for more.)

Cohomological formulation in BV-formalism

The choice of spin^c structure in the above is really the choice of a Poincaré duality (see at Poincaré duality algebra for more) which exhibits the orientation in generalized cohomology by a combined Atiyah duality/Thom isomorphism.

The analogue of such a choice of Poincaré duality between cohomology and homology in BV-formalism is the choice of a volume form that identified the de Rham complex with the BV-complex of multivector fields. See at BV-BRST formalism – Homological integration.

Again, here the kinetic action functional is part of the measure, now under the duality part of the BV-operator. The quantum master equation is now the analog of the orientability condition. Homological BV-quantization is then obtain by assing to the homology groups of this BV-operator, hence again by “pushing it to the point”.

This is the setup in which one can derive Feynman diagram rules form cohomologicalquantization, see at Feynman diagram – Refereces – In homological BV-quantization.

Examples

References

A general survey is in

  • S. Twareque Ali, Miroslav Engliš, Quantization Methods: A Guide for Physicists and Analysts, Rev. Math. Phys.17:391-490,2005 (arXiv:math-ph/0405065)

A general geometrically inclined introduction can be found in

  • Sean Bates, Alan Weinstein, Lectures on the geometry of quantization, pdf

  • Klaas Landsman, Mathematical topics between classical and quantum mechanics, Springer Monographs in Math. 1998. xx+529 pp.

A proposal for a full formalization of the notion of quantization for “finite” theories such as Dijkgraaf-Witten theory is in

A historical discussion by one of the nnlabizants is here: mathlight:quantization. See also Urs’s manifesto at Mathematical Foundations of Quantum Field and Perturbative String Theory.

The quantization via the A-model-method is described in

The perspective of geometric pre-quantization as a canonical construction in higher Lie theory is discussed in

A discussion of quantization as cohomological/motivic quantization of higher prequantum field theory is in

Some discussion of quantization in terms of Bohr toposes is in

  • Kunji Nakayama, Sheaves in Quantum Topos Induced by Quantization (arXiv:1109.1192)

Revised on February 3, 2014 14:46:58 by Urs Schreiber (89.204.130.154)