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In a recent Letter [1] the authors claimed that they discovered electromagnetic fields with “spin angular momentum typical of spin-3=2 objects.” In this Comment I prove that this claim is based on a total misunderstanding. The factor 3=2 has been obtained as a numerical coefficient in the ratio γ 1⁄4 Jz=W of the z component of the total angular momentum to the beamenergy per unit length. This ratio has the dimension of time. Therefore, the numerical coefficient 3=2 in Eq. (14) is completely arbitrary since it depends on the unit of time. The authors have chosen arbitrarily 1=ω as this unit, but the wave period T 1⁄4 2π=ω would be equally good. In the absence of a distinguished time scale in Maxwell theory, all efforts to assign a physical meaning to the numerical coefficient in the dimensional quantity are pointless. Moreover, I prove that the value 3=2 is as good as any other real number. In order to show that any value of γ can easily be obtained, I use the complex combination of field vectors named the Riemann-Silberstein vector [2],

physical review letters

Comment on “Role of Radial Charges on the Angular Momentum of Electromagnetic Fields: Spin- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>3</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:math> Light”

Comment on “Role of Radial Charges on the Angular Momentum of Electromagnetic Fields: Spin- 3/2 Light”