The Affinity of Mirror Symmetry to Algebraic Geometry: Going Beyond Formalism

symmetry-07-01633-g005

1617T345fibreandbaseOLD2

Even though formalism of homological mirror symmetry is an established case, what of other explanations of mirror symmetry which lie closer to classical differential and algebraic geometry? One way to tackle this is the so-called Strominger, Yau and Zaslow mirror symmetry or SYZ in short.

The central physical ingredient in this proposal is T-duality. To explain this, let us consider a superconformal sigma model with target space (M, g), and denote it (defined as a geometric functor, or as a set of correlation functions), as

CFT(M, g)

In physics, a duality is an equivalence

CFT(M, g) ≅ CFT(M′, g′)

which holds despite the fact that the underlying geometries (M,g) and (M′, g′) are not classically diffeomorphic.

T-duality is a duality which relates two CFT’s with toroidal target space, M ≅ M′ ≅ Td, but different metrics. In rough terms, the duality relates a “small” target space, with noncontractible cycles of length L < ls, with a “large” target space in which all such cycles have length L > ls.

This sort of relation is generic to dualities and follows from the following logic. If all length scales (lengths of cycles, curvature lengths, etc.) are greater than ls, string theory reduces to conventional geometry. Now, in conventional geometry, we know what it means for (M, g) and (M′, g′) to be non-isomorphic. Any modification to this notion must be associated with a breakdown of conventional geometry, which requires some length scale to be “sub-stringy,” with L < ls. To state T-duality precisely, let us first consider M = M′ = S1. We parameterise this with a coordinate X ∈ R making the identification X ∼ X + 2π. Consider a Euclidean metric gR given by ds2 = R2dX2. The real parameter R is usually called the “radius” from the obvious embedding in R2. This manifold is Ricci-flat and thus the sigma model with this target space is a conformal field theory, the “c = 1 boson.” Let us furthermore set the string scale ls = 1. With this, we attain a complete physical equivalence.

CFT(S1, gR) ≅ CFT(S1, g1/R)

Thus these two target spaces are indistinguishable from the point of view of string theory.

Just to give a physical picture for what this means, suppose for sake of discussion that superstring theory describes our universe, and thus that in some sense there must be six extra spatial dimensions. Suppose further that we had evidence that the extra dimensions factorized topologically and metrically as K5 × S1; then it would make sense to ask: What is the radius R of this S1 in our universe? In principle this could be measured by producing sufficiently energetic particles (so-called “Kaluza-Klein modes”), or perhaps measuring deviations from Newton’s inverse square law of gravity at distances L ∼ R. In string theory, T-duality implies that R ≥ ls, because any theory with R < ls is equivalent to another theory with R > ls. Thus we have a nontrivial relation between two (in principle) observable quantities, R and ls, which one might imagine testing experimentally. Let us now consider the theory CFT(Td, g), where Td is the d-dimensional torus, with coordinates Xi parameterising Rd/2πZd, and a constant metric tensor gij. Then there is a complete physical equivalence

CFT(Td, g) ≅ CFT(Td, g−1)

In fact this is just one element of a discrete group of T-duality symmetries, generated by T-dualities along one-cycles, and large diffeomorphisms (those not continuously connected to the identity). The complete group is isomorphic to SO(d, d; Z).

While very different from conventional geometry, T-duality has a simple intuitive explanation. This starts with the observation that the possible embeddings of a string into X can be classified by the fundamental group π1(X). Strings representing non-trivial homotopy classes are usually referred to as “winding states.” Furthermore, since strings interact by interconnecting at points, the group structure on π1 provided by concatenation of based loops is meaningful and is respected by interactions in the string theory. Now π1(Td) ≅ Zd, as an abelian group, referred to as the group of “winding numbers”.

Of course, there is another Zd we could bring into the discussion, the Pontryagin dual of the U(1)d of which Td is an affinization. An element of this group is referred to physically as a “momentum,” as it is the eigenvalue of a translation operator on Td. Again, this group structure is respected by the interactions. These two group structures, momentum and winding, can be summarized in the statement that the full closed string algebra contains the group algebra C[Zd] ⊕ C[Zd].

In essence, the point of T-duality is that if we quantize the string on a sufficiently small target space, the roles of momentum and winding will be interchanged. But the main point can be seen by bringing in some elementary spectral geometry. Besides the algebra structure, another invariant of a conformal field theory is the spectrum of its Hamiltonian H (technically, the Virasoro operator L0 + L ̄0). This Hamiltonian can be thought of as an analog of the standard Laplacian ∆g on functions on X, and its spectrum on Td with metric g is

Spec ∆= {∑i,j=1d gijpipj; pi ∈ Zd}

On the other hand, the energy of a winding string is (intuitively) a function of its length. On our torus, a geodesic with winding number w ∈ Zd has length squared

L2 = ∑i,j=1d gijwiwj

Now, the only string theory input we need to bring in is that the total Hamiltonian contains both terms,

H = ∆g + L2 + · · ·

where the extra terms … express the energy of excited (or “oscillator”) modes of the string. Then, the inversion g → g−1, combined with the interchange p ↔ w, leaves the spectrum of H invariant. This is T-duality.

There is a simple generalization of the above to the case with a non-zero B-field on the torus satisfying dB = 0. In this case, since B is a constant antisymmetric tensor, we can label CFT’s by the matrix g + B. Now, the basic T-duality relation becomes

CFT(Td, g + B) ≅ CFT(Td, (g + B)−1)

Another generalization, which is considerably more subtle, is to do T-duality in families, or fiberwise T-duality. The same arguments can be made, and would become precise in the limit that the metric on the fibers varies on length scales far greater than ls, and has curvature lengths far greater than ls. This is sometimes called the “adiabatic limit” in physics. While this is a very restrictive assumption, there are more heuristic physical arguments that T-duality should hold more generally, with corrections to the relations proportional to curvatures ls2R and derivatives ls∂ of the fiber metric, both in perturbation theory and from world-sheet instantons.

Spinorial Algebra

Pascal-g1

Superspace is to supersymmetry as Minkowski space is to the Lorentz group. Superspace provides the most natural geometrical setting in which to describe supersymmetrical theories. Almost no physicist would utilize the component of Lorentz four-vectors or higher rank tensor to describe relativistic physics.

In a field theory, boson and fermions are to be regarded as diffeomorphisms generating two different vector spaces; the supersymmetry generators are nothing but sets of linear maps between these spaces. We can thus include a supersymmetric theory in a more general geometrical framework defining the collection of diffeomorphisms,

φi : R → RdL, i = 1,…, dL —– (1)

ψαˆ : R → RdR, i = 1,…, dR —– (2)

where the one-dimensional dependence reminds us that we restrict our attention to mechanics. The free vector spaces generated by {φi}i=1dL and {ψαˆ}αˆdR are respectively VL and VR, isomorphic to RdL and RdR. For matrix representations in the following, the two integers are restricted to the case dL = dR = d. Four different linear mappings can act on VL and VR

ML : VL → VR, MR : VR → VL

UL : VL → VL, UR : VR → VR —– (3)

with linear map space dimensions

dimML = dimMR = dRdL = d2,

dimUL = dL2 = d2, dimUR = dR2 = d2 —– (4)

as a consequence of linearity. To relate this construction to a general real (≡ GR) algebraic structure of dimension d and rank N denoted by GR(d,N), two more requirements need to be added.

Defining the generators of GR(d,N) as the family of N + N linear maps

LI ∈ {ML}, I = 1,…, N

RK ∈ {MR}, K = 1,…, N —– (5)

such that ∀ I, K = 1,…, N, we have

LI ◦ RK + LK ◦ RI = −2δIKIVR

RI ◦ LK + RK ◦ LI = −2δIKIVL —– (6)

where IVL and IVR are identity maps on VL and VR. Equations (6) will later be embedded into a Clifford algebra but one point has to be emphasized, we are working with real objects.

After equipping VL and VR with euclidean inner products ⟨·,·⟩VL and ⟨·,·⟩VR, respectively, the generators satisfy the property

⟨φ, RI(ψ)⟩VL = −⟨LI(φ), ψ⟩VR, ∀ (φ, ψ) ∈ VL ⊕ VR —— (7)

This condition relates LI to the hermitian conjugate of RI, namely RI, defined as usual by

⟨φ, RI(ψ)⟩VL = ⟨RI(φ), ψ⟩VR —– (8)

such that

RI = RIt = −LI —– (9)

The role of {UL} and {UR} maps is to connect different representations once a set of generators defined by conditions (6) and (7) has been chosen. Notice that (RILJ)ij ∈ UL and (LIRJ)αˆβˆ ∈ UR. Let us consider A ∈ {UL} and B ∈ {UR} such that

A : φ → φ′ = Aφ

B : ψ → ψ′ = Bψ —– (10)

with Vas an example,

⟨φ, RI(ψ)⟩VL → ⟨Aφ, RI B(ψ)⟩VL

= ⟨φ,A RI B(ψ)⟩VL

= ⟨φ, RI (ψ)⟩VL —– (11)

so a change of representation transforms the generators in the following manner:

LI → LI = BLIA

RI → RI = ARIB —– (12)

In general (6) and (7) do not identify a unique set of generators. Thus, an equivalence relation has to be defined on the space of possible sets of generators, say {LI, RI} ∼ {LI, RI} iff ∃ A ∈ {UL} and B ∈ {UR} such that L′ = BLIA and R′ = ARIB.

Moving on to how supersymmetry is born, we consider the manner in which algebraic derivations are defined by

δεφi = iεI(RI)iαˆψαˆ

δεψαˆ = −εI(LI)αˆiτφi —– (13)

where the real-valued fields {φi}i=1dL and {ψαˆ}αˆ=1dR can be interpreted as bosonic and fermionic respectively. The fermionic nature attributed to the VR elements implies that ML and MR generators, together with supersymmetry transformation parameters εI, anticommute among themselves. Introducing the dL + dR dimensional space VL ⊕ VR with vectors

Ψ = (ψ φ) —– (14)

(13) reads

δε(Ψ) = (iεRψ εL∂τφ) —– (15)

such that

ε1, δε2]Ψ = iε1Iε2J (RILJτφ LIRJτψ) – iε2Jε1I (RJLIτφ LJRIτψ) = – 2iε1Iε2IτΨ —– (16)

utilizing that we have classical anticommuting parameters and that (6) holds. From (16) it is clear that δε acts as a supersymmetry generator, so that we can set

δQΨ := δεΨ = iεIQIΨ —– (17)

which is equivalent to writing

δQφi = i(εIQIψ)i

δQψαˆ = i(εIQIφ)αˆ —– (18)

with

Q1 = (0LIH RI0) —– (19)

where H = i∂τ. As a consequence of (16) a familiar anticommutation relation appears

{QI, QJ} = − 2iδIJH —– (20)

confirming that we are about to recognize supersymmetry, and once this is achieved, we can associate to the algebraic derivations (13), the variations defining the scalar supermultiplets. However, the choice (13) is not unique, for this is where we could have a spinorial one,

δQξαˆ = εI(LI)αˆiFi

δQFi = − iεI(RI)iαˆτξαˆ —– (21)