# The exact meaning of the angular-momentum and spin operators in quantum mechanics

Abstract : The theory of angular momentum and spin in quantum mechanics seems to defy common-sense intuition. We render the theory intelligible again by pointing out that this apparent impenetrability merely stems from an {\em undue} parallel interpretation of the algebraic expressions for the angular-momentum and spin operators in the group representation theory of SO(3) and SU(2). E.g. the correct meaning of ${\hat{L}}_{z} = {\hbar\over{\imath}}\,(x{\partial\over{\partial y}} - y{\partial\over{\partial x}} )$ is not that it is the operator for the $z$-component $L_{z}$ of the angular momentum ${\mathbf{L}}$, but rather the expression of the operator for the angular momentum ${\mathbf{L}}$ when it is aligned with the $z$-axis. Hence what we are used to note (erroneously) as ${\hat{L}}_{z}$ is not a scalar but a vector operator. The same applies {\em mutatis mutandis} for the spin operators. In the correct interpretation, the whole algebraic formalism is just the group representation theory for the rotations of three-dimensional Euclidean geometry. It is thus mere, elementary high-school mathematics (in a less usual, more technical guise) and as such totally exempt of any physics, let alone quantum mysteries. The change of interpretation has no impact on the algebraic results, such that they remain in agreement with experimental data. It is all only a matter of the correct geometrical meaning of the algebra. All these statements are proved within the framework of the group representation theory for SO(3) and SU(2) which is the basic tool used to describe rotational motion in quantum mechanics.
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https://hal.archives-ouvertes.fr/hal-03323780
Contributor : Gerrit Coddens Connect in order to contact the contributor
Submitted on : Saturday, September 11, 2021 - 1:27:29 PM
Last modification on : Wednesday, September 15, 2021 - 3:24:34 AM

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• HAL Id : hal-03323780, version 2

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Gerrit Coddens. The exact meaning of the angular-momentum and spin operators in quantum mechanics. 2021. ⟨hal-03323780v2⟩

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