There are two mysterious looking correspondences involving ADE groups. McKay correspondence between McKay graphs characterizing tensor products for finite subgroups of SU(2) and Dynkin diagrams of affine ADE groups is the first one. The correspondence between principal diagrams characterizing inclusions of hyper-finite factors of type II

_{1}(HFFs) with Dynkin diagrams for a subset of ADE groups and Dynkin diagrams for affine ADE groups is the second one.

I have considered the interpretation of McKay correspondence in TGD framework already earlier but the decision to look it again led to a discovery of a bundle of new ideas allowing to answer several key questions of TGD.

- Asking questions about M
^{8}-H duality at the level of 8-D momentum space led to a realization that the notion of mass is relative as already the existence of alternative QFT descriptions in terms of massless and massive fields suggests (electric-magnetic duality). Depending on choice M^{4}⊂ M^{8}, one can describe particles as massless states in M^{4}× CP_{2}picture (the choice is M^{4}_{L}depending on state) and as massive states (the choice is fixed M^{4}_{T}) in M^{8}picture. p-Adic thermal massivation of massless states in M^{4}_{L}picture can be seen as a universal dynamics independent mechanism implied by ZEO. Also a revised view about zero energy ontology (ZEO) based quantum measurement theory as theory of consciousness suggests itself.

- Hyperfinite factors of type II
_{1}(HFFs) and number theoretic discretization in terms of what I call cognitive representations provide two alternative approaches to the notion of finite measurement resolution in TGD framework. One obtains rather concrete view about how these descriptions relate to each other at the level of 8-D space of light-like momenta. Also ADE hierarchy can be understood concretely.

- The description of 8-D twistors at momentum space-level is also a challenge of TGD. 8-D twistorializations

in terms of octo-twistors (M^{4}_{T}description) and M^{4}× CP_{2}twistors (M^{4}_{L}description) emerge at imbedding space level. Quantum twistors could serve as a twistor description at the level of space-time surfaces.

**McKay correspondence in TGD framework**

Consider first McKay correspondence in more detail.

- McKay correspondence states that the McKay graphs characterizing the tensor product decomposition rules for representations of discrete and finite sub-groups of SU(2) are Dynkin diagrams for the affine ADE groups obtained by adding one node to the Dynkin diagram of ADE group. Could this correspondence make sense for any finite group G rather than only discrete subgroups of SU(2)? In TGD Galois group of extensions K of rationals can be any finite group G. Could Galois group take the role of G?

- Why the subgroups of SU(2) should be in so special role? In TGD framework quaternions and octonions play a fundamental role at M
^{8}side of M^{8}-H duality. Complexified M^{8}represents complexified octonions and space-time surfaces X^{4}have quaternionic tangent or normal spaces. SO(3) is the automorphism group of quaternions and for number theoretical discretizations induced by extension K of rationals it reduces to its discrete subgroup SO(3)_{K}having SU(2)_{K}as a covering. In certain special cases corresponding to McKay correspondence this group is finite discrete group acting as symmetries of Platonic solids. Could this make the Platonic groups so special? Could the semi-direct products Gal(K)×_{L}SU(2)_{K}take the role of discrete subgroups of SU(2)?

**HFFs and TGD**

The notion of measurement resolution is definable in terms of inclusions of HFFs and using number theoretic discretization of X^{4}. These definitions should be closely related.

- The inclusions
*N*⊂*M*of HFFs with index*M*:*N*<4 are characterized by Dynkin diagrams for a subset of ADE groups. The TGD inspired conjecture is that the inclusion hierarchies of extensions of rationals and of corresponding Galois groups could correspond to the hierarchies for the inclusions of HFFs. The natural realization would be in terms of HFFs with coefficient field of Hilbert space in extension K of rationals involved.

Could the physical triviality of the action of unitary operators

*N*define measurement resolution? If so, quantum groups assignable to the inclusion would act in quantum spaces associated with the coset spaces*M*/*N*of operators with quantum dimension d=*M*:*N*. The degrees of freedom below measurement resolution would correspond to gauge symmetries assignable to*N*.

- Adelic approach provides an alternative approach to the notion of finite measurement resolution. The cognitive representation identified as a discretization of X
^{4}defined by the set of points with points having H (or at least M^{8}coordinates) in K would be common to all number fields (reals and extensions of various p-adic number fields induced by K). This approach should be equivalent with that based on inclusions. Therefore the Galois groups of extensions should play a key role in the understanding of the inclusions.

- The huge symmetries of "world of classical words" (WCW) could explain why the ADE diagrams appearing as McKay graphs and principal diagrams of inclusions correspond to affine ADE algebras or quantum groups. WCW consists of space-time surfaces X
^{4}, which are preferred extremals of the action principle of the theory defining classical TGD connecting the 3-surfaces at the opposite light-like boundaries of causal diamond CD= cd× CP_{2}, where cd is the intersection of future and past directed light-cones of M^{4}and contain part of δ M^{4}_{+/-}× CP_{2}. The symplectic transformations of δ M^{4}_{+}× CP_{2}are assumed to act as isometries of WCW. A natural guess is that physical states correspond to the representations of the super-symplectic algebra SSA.

- The sub-algebras SSA
_{n}of SSA isomorphic to SSA form a fractal hierarchy with conformal weights in sub-algebra being n-multiples of those in SSA. SSA_{n}and the commutator [SSA_{n},SSA] would act as gauge transformations. Therefore the classical Noether charges for these sub-algebras would vanish. Also the action of these two sub-algebras would annihilate the quantum states. Could the inclusion hierarchies labelled by integers ..<n_{1}<n_{2}<n_{3}.... with n_{i+1}divisible by n_{i}would correspond hierarchies of HFFs and to the hierarchies of extensions of rationals and corresponding Galois groups? Could n correspond to the dimension of Galois group of K.

- Finite measurement resolution defined in terms of cognitive representations suggests a reduction of the symplectic group SG to a discrete subgroup SG
_{K}, whose linear action is characterized by matrix elements in the extension K of rationals defining the extension. The representations of discrete subgroup are infinite-D and the infinite value of the trace of unit operator is problematic concerning the definition of characters in terms of traces. One can however replace normal trace with quantum trace equal to one for unit operator. This implies HFFs and the hierarchies of inclusions of HFFs. Could inclusion hierarchies for extensions of rationals correspond to inclusion hierarchies of HFFs and of isomorphic sub-algebras of SSA?

See the article TGD view about McKay Correspondence, ADE Hierarchy, and Inclusions of Hyperfinite Factors.

For a summary of earlier postings see Latest progress in TGD.

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