Konstantin Y. Bliokh, Aleksandr Y. Bekshaev, Franco Nori
The dual symmetry between electric and magnetic fields is an important intrinsic property of Maxwell equations in free space. This symmetry underlies the conservation of optical helicity (chirality). However, the standard field-theory formulation of electromagnetism lacks helicity conservation because the field Lagrangian is not dual symmetric. To resolve this discrepancy between the symmetries of the Lagrangian and Maxwell equations, we put forward a dual-symmetric Lagrangian formulation of electromagnetism. This preserves the form of Maxwell equations, all fundamental conservation laws of standard electromagnetism, but recovers the helicity conservation as one of the Noether currents. Moreover, we show that the dual-symmetric electromagnetism naturally yields a meaningful separation of the spin and orbital degrees of freedom of light, in complete agreement with other recent results.
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http://arxiv.org/abs/1208.4523
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