### Does quantum gravity look like a mission impossible because of our cherished beliefs?

Bee writes about testing quantum gravitation with title

*Finding space-time quanta in the cosmic microwave background: Not so simple*. Bee's article reflects nicely the basic cherished assumptions of quantum gravity theorists. These belief's indeed lead to think that there are no experimental and empirical guidelines helping in the construction of quantum theory of gravitation.

Bee consider's the possibility that early cosmology might allow to test whether quantum aspects of gravitation could have been important at that time and possibly left signatures to the cosmic microwave background. Here strong quantum gravitational effects are sought and they might have been present during very early cosmology. The quantum field theory inspired belief system describes very early cosmology in terms of inflation and BICEP collaboration was for some time though to have detected quantum gravitational effects in CMB.

Bee tells also about a model of Juan Maldacena for a violation of cosmological Bell inequalities, which could demonstrate quantal character of correlations in cosmological scales as remnants of quantal correlations in extremely short scales during the very early cosmology. Detectors for finding these correlations are not possible and Maldacena considers the possibility that some other field than inflaton field might have preserved information about these correlations. Maldacena is not very optimistic. Another proposal would measure what has been called "quantum discord" in CMB temperature fluctuations. Authors end up with negative conclusion: it is not possible to distinguish between classical and quantal correlations.

The problem with the standard approach is that it makes several implicit assumptions, which do not have real experimental justification. Let us consider some of them.

- What was good in super string models that one saw the problem in a wider perspective. The original goal was to unify all interactions instead of only quantizing gravitation by building a TOE. In loop quantum gravity one only tries to quantize gravity- to my opinion this is a fatal mistake. Extending the horizon brings in enormous amounts of information, all of physics instead of mere gravitational physics according to the - possibly badly wrong - world view we have. It is now clear that string models were not enough.

- The notion of Planck length as a fundamental scale is one of the implicit assumptions inspired by the reductionistic world view and having no other justification than that it emerges from dimensional analysis. The fact that Planck length is proportional to the square root of Planck constant however suggests that it is an emergent quantal notion. The real fundamental constant making sense classically should be some length representable as geometric size for some "internal" space.

- By accepting the fundamental symmetries - Poincare invariance and color symmetry - one ends up with a unique candidate for a new theory: space-times are identified as 4-surfaces in H=M
^{4}× CP_{2}and color and Poincare symmetries act - not at the level of space-time - but at the level of imbedding space. This solves also the "energy problem" of general relativity by making energy, momentum and angular momentum Noether charges. H=M^{4}× CP_{2}predicts just standard model quantum numbers and is also twistorially completely unique.

- Giving up of the Planck length dogma leads to a cascade of discoveries forcing to give up several cherished dogmas. Space-time as region of Minkowski space slightly curved by presence of matter is replaced with many-sheeted space-time. This changes the entire world view: GRT space-time is only an approximation - one can however keep Equivalence Principle, and General Coordinate Invariance and also generalize holography to a strong form of holography. Also strings emerge but as entities differing from those of superstring models. One can say that string world sheets and partonic 2-surfaces as sub-manifolds of space-time surface are "genes" of space-time. They are enough for purely quantal description but space-time is required to have classical description necessitated by quantum measurement theory. An important implication is the failure of reductionism and its replacement with a fractal world view allowing highly non-trivial applications of the theory in all scales including quantum biology.

- Also the assumption that space-time surface has metric with Minkowskian signature must be generalized by allowing Euclidian regions identifiable as 4-D "lines" of generalized Feynman diagrams. Cosmology is dominated by cosmic strings (identified as as space-time surfaces with have 2-D M
^{4}projection, string world sheet) during the primordial period: space-time in the usual sense (4-DM^{4}projection) does not exist during this period. The counterpart of inflationary period corresponds to the transition from string dominated phase to the phase in which many-sheeted space-time allowing approximate description in terms of GRT exists. Cosmology as a whole is replaced with a fractal Russian doll cosmology.

- It is to be emphasized that all this follows from the observation that Planck constant cannot be a parameter characterizing classical physics as space-time geometry: the size scale of CP
_{2}replaces Planck length as the fundamental scale rather than a purely formal constant making Einstein action or stringy action dimensionless. Planck length emerges from TGD as a parameter analogous to quantum diffusion constant D= L_{p}c characterizing the fluctuations for the distance between two partonic 2-surfaces connected by random light-like 3-surface defining the orbit of partonic 2-surface. This light-like randomness provides classical justification also for p-adic thermodynamics.

- By accepting the fundamental symmetries - Poincare invariance and color symmetry - one ends up with a unique candidate for a new theory: space-times are identified as 4-surfaces in H=M
- Another standard ad hoc assumption is that macroscopic - to say nothing about astroscopic - quantum coherence is not possible although there are some exceptions. The assumption about the absence of macroscopic quantum coherence is based on the reductionistic dogma believed to reduce the physics step by step to Planck length scales. Gravitational quantum coherence would be present only in Planckian scales. Only the recent proposals about wormholes as correlates for quantum entanglement start to challenge this dogma. Also the emergence of quantum biology has forced to challenge this belief.

- Super string models provide a candidate for the description of quantum gravitation in these scales and were for some time thought to provide unification of all basic interactions. The problem turned out to be that one can say practically nothing about the physics in long length scales - the landscape problem makes this possible.
The challenge of describing gravitation also in long length scales inspired the ad hoc notion of effective field theory description relying on Einstein's equations assuming some number of dimensions to be compactified. The original 4-D theory is only replaced with 10-, 11- or 12-D theory: you can pick your favorite. Instead of a reduction to something simpler, one obtains much more complex situation. The situation becomes hopeless in practice.

- One could however think differently. Gravitation is the dominating interaction in long length scales and quantum classical correspondence suggests that gravitational quantum coherence could be actually long ranged - even astroscopic. What happens if one assumes that gravitation is macrocopically quantum coherent?
The basic coupling constant in the gravitational interactions between masses M and m is the parameter GMm/hbar c. This parameter becomes larger than unity of Mm is larger than Planck mass squared and quantum perturbation theory does not work. Gravitation becomes strong in quantum sense to be distinguished from strong in classical sense (strong space-time curvature). One must either give up all hopes about perturbative description, assume that quantum coherence in these scales is not possible, or must be ready to update the beliefs about what quantum theory is.

- Planck constant h is what characterized quantum theory. The assumption - just a cherished assumption - is that it has single value. What if it has a spectrum of values: say multiples of the ordinary Planck constant h: h
_{eff}=n×h? The subscript "eff" appears because it is better to allow the possibility that only an effective parameter is in question.The basic coupling constant in gravitational interactions becomes GMm/n× hbar c: the scaling by integer n can make it smaller than unity so that perturbation theory converges. Could it be that Mother Nature is theoretician friendly and in a situation, which would be non-perturbative comes in rescue and makes a phase transition increasing the value of h

_{eff}? This phase transition would scale up all quantum lengths by a factor n and give rise to a macroscopic and even astroscopic quantum coherence.

- Nottale proposed long time ago that the notion of gravitational Planck constant define as h
= GMm/v _{0}, where v_{0}<c is a parameter with dimensions of velocity makes sense in planetary system and that one can explain the radii of planetary orbits in terms of gravitational variant of hydrogen atom. v_{0}<c would be however different for inner and outer (those outside of Earth) planets.Nottale did not assume that macroscopic quantum coherence is in question: this is a TGD ispired assumption. h

characterizes the system formed by central mass M and small mass m (it could be elementary particle) and in TGD one can assign it with the magnetic flux tube mediating gravitational interactions. Bohr ended to Bohr atom from the condition that infrared catastrophe meaning that electron spirals down to atomic nucleus is avoided: in the gravitational case this "electric collapse" is replaced with gravitational collapse and avoided by a variant of Bohr quantization. Quite generally, Planck constant would be a parameter characterizing many-particle system rather than fundamental constant: that this is not the case is an impression created if one does not realize that h _{eff}can have also a non-minimal value.More than decade later the hierarchy of Planck constants can be understood mathematically as being related to the fractal hierarchies of sub-algebras of super-conformal algebras isomorphic to the entire algebra extending ordinary super-conformal algebras. The hierarchies of inclusions of hyper-finite factors of von Neumann are closely related. Also the space-time correlates for the hierarchy of Planck constants are reasonably well understood.

- When one has several profound problems, it is sometimes much easier to solve all of them at one blow rather than trying to develop a separate solution to each of them. There is indeed also a second deep problem besides quantizing gravity: the mystery of dark matter (and of dark energy). Various proposals for understanding dark matter in terms of a couple of exotic particles have been excluded by experiments one after another but elementary particle physicists still stubbornly try to solve the problem by infinitesimal approach by postulating a couple of new particles. The stubborn attempt to find standard SUSY at LHC is second example from top-down approach trying to fit right hand shoe to left hand leg. Nature just laughs for the rational looking decisions of committees about the laws that Universe should obey.
The notion of effective Planck constant suggests an obvious identification of dark matter: dark matter would correspond to phases of matter with a non-standard value h

_{eff}of Planck constant. Entire hierarchy of dark phases would reside at magnetic flux tubes made possible by many-sheeted space-time. In particular, one can speak about dark matter and dark gravitons at gravitational magnetic flux tubes. This leads also to a vision about quantum biology in which gravitational Planck constant plays a key role.The model of dark matter in TGD sense has evolved to the level of detail allowing to make proposals for new technologies: in particular, a model for cold fusion emerges. This is not just a new funny techno thing: our survival as a civilization depends crucially on finding new energy resources to replace the old and depleting ones.

- Super string models provide a candidate for the description of quantum gravitation in these scales and were for some time thought to provide unification of all basic interactions. The problem turned out to be that one can say practically nothing about the physics in long length scales - the landscape problem makes this possible.

For a summary of earlier postings see Links to the latest progress in TGD.

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