Monday, February 08, 2021

A test for the quantum character of gravity as a test for quantum TGD

The following represents comments to a popular article describing a proposed experiment possibly conforming the quantum character of gravity. Thanks for the link to Gary Ehlenberg.

The proposal in idealized form is following.

  1. One has two diamonds which are originally in superposition of two opposite spin directions.
  2. One applies magnetic field with gradient. We know that in external magnetic field which is not constant, particle with spin is deflected to a direction which is opposite for opposite sping directions. If one has a superposition of spins then one would obtain superposition of deflected spin states with different spatial positions. The diamond can be said to be at two positions simultaneously. This is the key effect and leads to the question about whether and how it could be described in general relativity.
Consider now the situation in various approaches to quantum gravity.
  1. The situation is problematic from the point of view of classical GRT since the gravitational fields created by the different spin components of superposition would have different distribution of mass. Classical GRT space-time determined by Einstein's equations would not be unique. In order to avoidthis one could argue that state function reduction (SFR) occurs immediatelyso that the spin is well-defined and space-time is unique. This looks very ugly but to my surprise Dyson takes this possibility seriously.
  2. In quantum gravity based on GRTview it seems that one should allow superposition of space-times or at least of 3-spaces. The superposition of 3-geometries would conform with Wheelerian superspace view but leads to the problems with time and conservationlaws quite generally. Wheeler's superspace did not work mathematically.
  3. In the description in terms of quantum gravitons one would give up the GRT picture altogether and string model view is something akin to this. Unfortunately, the attemptsto formulate quantum gravitation as a field theory in empty Minkowski space using Einstein's action fail. The divergences are horrible and renormalization is not possible. String models in turn try to get space-time by spontaneous compactification but this led to the landscape misery.
TGD based picture avoids the problems of these approaches.
  1. In zero energy ontology (ZEO) the quantum states are superpositions of space-time surfaces : not superpositions of general 4-geometries making even not sense in standard QM or of 3-surfaces as they would be in generalization of Wheeler's superspace approach. The space-time surfaces would be preferred extremals of the action defining the theory and one obtains holography from general coordinate invariance: 3-surface determines space-time surface just as the point of Bohr orbit defines the Bohr orbit.
  2. In this picture superposition of states with position of particle depending on spin direction corresponds to superposition of different space-time surfaces. Also the entanglement between various components of the state involving two spin superposed states of the two diamonds involved, is possible and generated in the time evolution. Entanglementwould have as a correlate magnetic flux tubes connecting the diamonds. One would have superposition of space-time surfaces corresponding to different flux tube lengths connecting spin states at various positions. Thereisno reason to expect that entanglement would not be generated. The expected positive outcome of the experiment could be seen as a proof of not only quantum character of gravitations but evidences for the superposition of space-time surfaces and ZEO.
  3. Diamonds are rather large objects and the small value of Planck constant is the trouble in the standard approach. It might be possible to overcome this problem in TGD. TGD predicts that dark matter corresponds to phases of ordinary matter with arbitrarily large values of h_eff at MB glux tubes would have macroscopic quantum coherence in the scale of flux tube and could induce coherence of ordinary matter (diamonds) at their end as controllers of the ordinary matter. This would not be quantum coherence but would make visible the quantum coherence at the level of MB.

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

Articles and other material related to TGD. 

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