The association of spin and orbital angular momenta of light with its polarization and helical phase fronts is now well established. The problems in linking this with electromagnetic theory, as expressed in Maxwell's equations, are rather less well known. We present a simple analysis of the problems involved in defining spin and orbital angular momenta for electromagnetic fields and discuss some of the remaining challenges. Crucial to our investigation is the duplex symmetry between the electric and magnetic fields.

中文翻译：

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电磁场的旋转和光学角动量的性质

光的自旋和轨道角动量与其偏振和螺旋相位前沿的关联现在已经很好地建立起来。正如麦克斯韦方程所表达的那样，将其与电磁理论联系起来的问题鲜为人知。我们对定义电磁场的自旋和轨道角动量所涉及的问题进行了简单分析，并讨论了一些剩余的挑战。对我们的研究至关重要的是电场和磁场之间的双重对称性。