Dual electromagnetism: Helicity, spin, momentum, and angular momentum

Understand

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, and, as we show here, is closely related to the separation of spin and orbital degrees of freedom of light (the helicity flux coincides with the spin angular momentum).
• However, in the standard field-theory formulation of electromagnetism, the field Lagrangian is not dual symmetric.
• This leads to problematic dual-asymmetric forms of the canonical energy-momentum, spin, and orbital angular momentum tensors.