Quantum classical correspondence states that all aspects of quantum states should have correlates in the geometry of preferred extremals. In particular, various elementary particle propagators should have a representation as properties of preferred extremals. This would allow to realize the old dream about being able to say something interesting about coupling constant evolution although it is not yet possible to calculate the Mmatrices and Umatrix. Hitherto everything that has been said about coupling constant evolution has been rather speculative arguments except for the general vision that it reduces to a discrete evolution defined by padic length scales. General first principle definitions are much more valuable than ad hoc guesses even if the latter give rise to explicit formulas.
In quantum TGD and also at its QFT limit various correlation functions in given quantum state code for its properties. These correlation functions should have counterparts in the geometry of preferred extremals. Even more: these classical counterparts for a given preferred extremal ought to be identical with the quantum correlation functions for the superposition of preferred extremals.
 The marvelous implication of quantum ergodicity would be that one could calculate everything solely classically using the classical intuition  the only intuition that we have. Quantum ergodicity would also solve the paradox raised by the quantum classical correspondence for momentum eigenstates. Any preferred extremal in their superposition defining momentum eigenstate should code for the momentum characterizing the superposition itself. This is indeed possible if every extremal in the superposition codes the momentum to the properties of classical correlation functions which are identical for all of them.
 The only manner to possibly achieve quantum ergodicity is in terms of the statistical properties of the preferred extremals. It should be possible to generalize the ergodic theorem stating that the properties of statistical ensemble are represented by single spacetime evolution in the ensemble of time evolutions. Quantum superposition of classical worlds would effectively reduce to single classical world as far as classical correlation functions are considered. The notion of finite measurement resolution suggests that one must state this more precisely by adding that classical correlation functions are calculated in a given UV and IR resolutions meaning UV cutoff defined by the smallest CD and IR cutoff defined by the largest CD present.
 The skeptic inside me immediately argues that TGD Universe is 4D spin glass so that this quantum ergodic theorem must be broken. In the case of the ordinary spin classes one has not only statistical average for a fixed Hamiltonian but a statistical average over Hamiltonians. There is a probability distribution over the coupling parameters appearing in the Hamiltonian. Maybe the quantum counterpart of this is needed to predict the physically measurable correlation functions.
Could this average be an ordinary classical statistical average over quantum states with different classical correlation functions? This kind of average is indeed taken in density matrix formalism. Or could it be that the square root of thermodynamics defined by ZEO actually gives automatically rise to this average? The eigenvalues of the "hermitian square root " of the density matrix would code for components of the state characterized by different classical correlation functions. One could assign these contributions to different "phases".
 Quantum classical correspondence in statistical sense would be very much like holography (now individual classical state represents the entire quantum state). Quantum ergodicity would pose a rather strong constraint on quantum states. This symmetry principle could actually fix the spectrum of zero energy states to a high degree and fix therefore the Mmatrices given by the product of hermitian square root of density matrix and unitary Smatrix and unitary Umatrix having Mmatrices as its orthonormal rows.
 In TGD inspired theory of consciousness the counterpart of quantum ergodicity is the postulate that the spacetime geometry provides a symbolic representation for the quantum states and also for the contents of consciousness assignable to quantum jumps between quantum states. Quantum ergodicity would realize this strongly selfreferential looking condition. The positive and negative energy parts of zero energy state would be analogous to the initial and final states of quantum jump and the classical correlation functions would code for the contents of consciousness like written formulas code for the thoughts of mathematician and provide a sensory feedback.
How classical correlation functions should be defined?
 General Coordinate Invariance and Lorentz invariance are the basic constraints on the definition. These are achieved for the spacetime regions with Minkowskian signature and 4D M^{4} projection if linear Minkowski coordinates are used. This is equivalent with the contraction of the indices of tensor fields with the spacetime projections of M^{4} Killing vector fields representing translations. Accepting ths generalization, there is no need to restrict oneself to 4D M^{4} projection and one can also consider also Euclidian regions identifiable as lines of generalized Feynman diagrams.
Quantum ergodicity very probably however forces to restrict the consideration to Minkowskian and Euclidian spacetime regions and various phases associated with them. Also CP_{2} Killing vector fields can be projected to spacetime surface and give a representation for classical gluon fields. These in turn can be contracted with M^{4} Killing vectors giving rise to gluon fields as analogs of graviton fields but with second polarization index replaced with color index.
 The standard definition for the correlation functions associated with classical time evolution is the appropriate starting point. The correlation function G_{XY}(τ) for two dynamical variables X(t) and Y(t) is defined as the average G_{XY}(τ)=∫_{T} X(t)Y(t+τ)dt/T over an interval of length T, and one can also consider the limit T→ ∞. In the recent case one would replace kenotau with the difference m_{1}m_{2}=m of M^{4} coordinates of two points at the preferred extremal and integrate over the points of the extremal to get the average. The finite time interval T is replaced with the volume of causal diamond in a given length scale. Zero energy state with given quantum numbers for positive and negative energy parts of the state defines the initial and final states between which the fields appearing in the correlation functions are defined.
 What correlation functions should be considered? Certainly one could calculate correlation functions for the induced spinor connection given electroweak propagators and correlation functions for CP_{2} Killing vector fields giving correlation functions for gluon fields using the description in terms of Killing vector fields. If one can uniquely separate from the Fourier transform uniquely a term of form Z/(p^{2}m^{2}) by its momentum dependence, the coefficient Z can be identified as coupling constant squared for the corresponding gauge potential component and one can in principle deduce coupling constant evolution purely classically. One can imagine of calculating spinorial propagators for string world sheets in the same manner. Note that also the dependence on color quantum numbers would be present so that in principle all that is needed could be calculated for a single preferred extremal without the need to construct QFT limit and to introduce color quantum numbers of fermions as spin like quantum numbers (color quantum numbers corresponds to CP_{2} partial wave for the tip of the CD assigned with the particle).
 What about Higgs like field? TGD in principle allows scalar and pseudoscalars which could be called Higgs like states. These states are however not necessary for particle massivation although they can represent particle massivation and must do so if one assumes that QFT limit exist. pAdic thermodynamics however describes particle massivation microscopically.
The problem is that Higgs like field does not seem to have any obvious spacetime correlate. The trace of the second fundamental form is the obvious candidate but vanishes for preferred extremals which are both minimal surfaces and solutions of Einstein Maxwell equations with cosmological constant. If the string world sheets at which all spinor components except right handed neutrino are localized for the general solution ansatz of the modified Dirac equation, the corresponding second fundamental form at the level of imbedding space defines a candidate for classical Higgs field. A natural expectation is that string world sheets are minimal surfaces of spacetime surface. In general they are however not minimal surfaces of the imbedding space so that one might achieve a microscopic definition of classical Higgs field and its vacuum expectation value as an average of one point correlation function over the string world sheet.
For details and background see the chapter The recent vision about preferred extremals and solutions of the modified Dirac equation, or the article with the title "Do geometric invariants of preferred extremals define topological invariants of spacetime surface and code for quantum physics?".
