What did Escher know?


In the interesting physics colloquium that Marcelo Gleiser gave on friday at UQ he used the Escher print above to illustrate CP symmetry in a system which violates both C and P symmetry.
[C is charge conjugation and P is parity].
I found the image here.
Another example of breaking of a symmetry while conserving a combined symmetry concerns vibronic transitions in molecules. For molecules which have inversion symmetry selection rules suggest that electronic transitions that involve no change in parity should not be observed. However, they sometimes are because they are combined with a vibrational transition. Hence, vibronic [= vib(rational)-(elect)ronic] transitions. I think the first observed case of this may have been in benzene.

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