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I've never really been...convinced...of the statement
$$\bf{J}=\bf{L}+\bf{S}$$
I've always just gone along with it, but I've never seen why this is "right". So I guess now's as good a time as any to ask.
Thinking about this from a "classical" perspective (which obviously is not correct, but perhaps I can at least show where my doubt comes from), if the L stands for the angular momentum of the particle with respect to the center of mass, and the S stands for the angular momentum of the particle "spinning" around (again, obviously not right), then the two should be measured from different coordinate origins (e.g. L measured from the proton in a Hydrogen nucleus if we are looking at the electron, and S is measured from the "center" of the electron). So, I can not motivate the correctness of this statement from naive classical analyses.
Looking at this mathematically (e.g. from the analysis in Ballentine chapter 7), we have that the ##\bf{L}## operators act on the physical space while the ##\bf{S}## operators act on the internal space.
Ballentine then says something along the lines of, the total rotation operator ##e^{in_\alpha J_\alpha }## must be in the form:
$$e^{in_\alpha J_\alpha }=e^{in_\alpha L_\alpha }e^{in_\alpha S_\alpha }$$
From which the statement ##\bf{J}=\bf{L}+\bf{S}## is true if the L's and S's commute. But I'm not convinced that the above formula is a simple product, and not a direct product. The two different operators operate in different spaces, and so, shouldn't it be a direct product? If I express my state function as a 2 component vector e.g. ##\Psi_i (x,t), i=1,2##, for example, the rotation dealing with ##\bf{L}=-i\hbar \bf{x}\times\nabla## must be applied to each component individually, while the rotation dealing with S applies to my 2 component vector as a whole. The whole J=L+S thing doesn't make sense to me taken as an operator equation since L is a differential operator, and S is a matrix. What's the sum of a derivative and a matrix supposed to mean? Unless I am now constructing a 2x2 diagonal matrix for ##\bf{L}##? I'm confused. =/ This has always bothered me.
$$\bf{J}=\bf{L}+\bf{S}$$
I've always just gone along with it, but I've never seen why this is "right". So I guess now's as good a time as any to ask.
Thinking about this from a "classical" perspective (which obviously is not correct, but perhaps I can at least show where my doubt comes from), if the L stands for the angular momentum of the particle with respect to the center of mass, and the S stands for the angular momentum of the particle "spinning" around (again, obviously not right), then the two should be measured from different coordinate origins (e.g. L measured from the proton in a Hydrogen nucleus if we are looking at the electron, and S is measured from the "center" of the electron). So, I can not motivate the correctness of this statement from naive classical analyses.
Looking at this mathematically (e.g. from the analysis in Ballentine chapter 7), we have that the ##\bf{L}## operators act on the physical space while the ##\bf{S}## operators act on the internal space.
Ballentine then says something along the lines of, the total rotation operator ##e^{in_\alpha J_\alpha }## must be in the form:
$$e^{in_\alpha J_\alpha }=e^{in_\alpha L_\alpha }e^{in_\alpha S_\alpha }$$
From which the statement ##\bf{J}=\bf{L}+\bf{S}## is true if the L's and S's commute. But I'm not convinced that the above formula is a simple product, and not a direct product. The two different operators operate in different spaces, and so, shouldn't it be a direct product? If I express my state function as a 2 component vector e.g. ##\Psi_i (x,t), i=1,2##, for example, the rotation dealing with ##\bf{L}=-i\hbar \bf{x}\times\nabla## must be applied to each component individually, while the rotation dealing with S applies to my 2 component vector as a whole. The whole J=L+S thing doesn't make sense to me taken as an operator equation since L is a differential operator, and S is a matrix. What's the sum of a derivative and a matrix supposed to mean? Unless I am now constructing a 2x2 diagonal matrix for ##\bf{L}##? I'm confused. =/ This has always bothered me.