Conservation of the spin and orbital angular momenta in electromagnetism

Understand

We review and re-examine the description and separation of the spin and orbital angular momenta (AM) of an electromagnetic field in free space.

• While the spin and orbital AM of light are not separately-meaningful physical quantities in orthodox quantum mechanics or classical field theory, these quantities are routinely measured and used for applications in optics.
• A meaningful quantum description of the spin and orbital AM of light was recently provided by several authors, which describes separately conserved and measurable integral values of these quantities.
• However, the electromagnetic field theory still lacks corresponding locally-conserved spin and orbital AM currents.