Two kinds of reciprocity

The Taijitu, the emblem of the action of the Pole with respect to the world, and of the reciprocal action with respect to the Pole, inevitably reminds us of the most universal figure in physics; we are naturally referring to the ellipse —or rather, it should be said, to the idea of the generation of an ellipse with its two foci, since here there is no eccentricity. The ellipse appears in the orbits of the planets no less than in the atomic orbits of the electrons, and in the study of the refractive properties of light it gives rise to a whole field of analysis, ellipsometry. Kepler’s old problem has scale invariance, and plays a determining role in all our knowledge of physics from the Planck constant to the furthest galaxies.

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