I encountered in Facebook (thanks to Ulla) a link to a very interesting article Here is the abstract.
We prove an instance of the Reciprocity Theorem that demonstrates that Kerr rotation, also known as the magneto-optical Kerr effect, may only arise in materials that break microscopic time reversal symmetry. This argument applies in the linear response regime, and only fails for nonlinear effects. Recent measurements with a modified Sagnac Interferometer have found finite Kerr rotation in a variety of superconductors. The Sagnac Interferometer is a probe for nonreciprocity, so it must be that time reversal symmetry is broken in these materials.
I had to learn some basic condensed matter physics. Magneto-optic Kerr effect occurs when a circularly polarized plane wave – often with normal incidence – reflects from a sample with planar boundary. In magneto-optic Kerr effect there are many options depending on the relative directions of the reflection plane (incidence is not normal in the general case so that one can talk about reflection plane) and magnetization. Also the incoming polarization can be linear or circular. Reflected circular polarized beams suffers a phase change in the reflection: as if they would spend some time at the surface before reflecting. Linearly polarized light reflects as elliptically polarized light.
Kerr angle θK is defined as 1/2 of the difference of the phase angle increments caused by reflection for oppositely circularly polarized plane wave beams. As the name tells, magneto-optic Kerr effect is often associated with magnetic materials.
Kerr effect has been however observed also for high Tc superconductors and this has raised controversy. As a layman in these issues I can naively wonder whether the controversy is created by the expectation that there are no magnetic fields inside the super-conductor. Anti-ferromagnetism is however important for high Tc superconductivity. In TGD based model for high Tc superconductors the supracurrents would flow along pairs of flux tubes with the members of S=0 (S=1) Cooper pairs at parallel flux tubes carrying magnetic fields with opposite (parallel) magnetic fluxes. Therefore magneto-optic Kerr effect could be in question after all.
The author claims to have proven that Kerr effect in general requires breaking of microscopic time reversal symmetry. This result happens to be very interesting from the point of TGD. The reason is that twistorial lift of TGD requires that imbedding space M4× CP2 has Kähler structure in generalized sense. M4 has the analog of Kähler form, call it J(M4). J(M4) is assumed to be self-dual and covariantly constant as also CP2 Kähler form, and contributes to the Abelian electroweak U(1) gauge field (electroweak hypercharge) and therefore also to electromagnetic field.
J(M4) implies breaking of Lorentz invariance since it defines decomposition M4= M2× E2 Implying preferred rest frame and preferred spatial direction identifiable as direction of spin quantization axis. In zero energy ontology (ZEO) one has moduli space of causal diamonds (CDs) and therefore also moduli space of Kähler forms and the breaking of Lorentz invariance cancels. Note that a similar Kähler form is conjectured in quantum group inspired non-commutative quantum field theories and the problem is the breaking of Lorentz invariance.
What is interesting that the action of P,CP, and T on Kähler form transforms it from self-dual to anti-self-dual form and vice versa. If J(M4) is self-dual as also J(CP2), all these 3 discrete symmetries are broken in arbitrarily long length scales. If CPT is exact symmetry then the effects of CP and T cancel. If the effect of T is the naively expected If J(M4) changes sign if the effect of T involves additional changes of sign suggested by the anti-unitary character of T, CPT leaves J(M4) unaffected.
One can imagine several consequences at the level of fundamental physics.
For a summary of earlier postings see Latest progress in TGD.