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The one-loop "vacuum" Heisenberg-Euler coupling of four electromagneticfields can lead to interesting collective effects in the collision of twophoton clouds, on a time scale orders of magnitude faster than one estimatesfrom the cross-section and density. We estimate the characteristic time formacroscopic transformation of positive to negative helicity in clouds that areinitially totally polarized and for depolarization of a polarized beamtraversing an unpolarized cloud.Comment: Recapitulates much that is in hep-ph/0402127, with new results in the  last section, and the first section drastically reduced in view of the  previous work of Kotkin and Serbo. Typo corrected in eq. 1

Collective Polarization Exchanges in Collisions of Photon Clouds