the angular momentum has 2 kinds – orbital angular momentum , which is caused by a charged particle executing orbital motion, since there are 3 dimension space. and spin , which is an internal degree of freedom to let particle “orbiting” at there.

thus, a general quantum state for a particle should not just for the spatial part and the time part. but also the spin, since a complete state should contains all degree of freedom.

when we “add” the orbital angular momentum and the spin together, actually, we are doing:

where the 1 with L is the identity of the spin-space and the 1 with S is the identity of the 3-D space.

the above was discussed on J.J. Sakurai’s book.

the mathematics of and are completely the same at rotation operator.

where can be either or .

the can only have effect on spatial state while can only have effect on the spin-state. i.e:

the can only have integral value but can be both half-integral and integral. the half-integral value of makes the spin-state have to rotate 2 cycles in order to be the same again.

thus, if the different of and is just man-made. The degree of freedom in the spin-space is actually by some real geometry on higher dimension. and actually, the orbital angular momentum can change the spin state:

but the effect is so small and

but the c is very small, but if we can rotate the state for a very large angle, the effect of it can be seen by compare to the rotation by spin.

the experiment can be done as follow. we apply a rotating magnetic field at the same frequency as the Larmor frequency. at a very low temperature, the spin was isolated and and is equal to . the different in the c will come up at very long time measurement and it exhibit a interference pattern.

if is a complex number, it will cause a decay, and it will be reflected in the interference pattern.

if we find out this c, then we can reveal the other spacial dimension!

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the problem is. How can we act the orbital angular momentum on the spin with out the effect of spin angular momentum? since L and S always coupled.

one possibility is make the S zero. in the system of electron and positron. the total spin is zero.

another possibility is act the S on the spatial part. and this will change the energy level.

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an more fundamental problem is, why L and S commute? the possible of writing this

is due to the operators are commute to each other. by why?

if we break down the L in to position operator x and momentum operator p, the question becomes, why x and S commute or p and S commute?

i will prove it later.

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another problem is, how to evaluate the Poisson bracket? since L and S is not same dimension. may be we can write the eigenket in vector form:

i am not sure.

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For any vector operator, it must satisfy following equation, due to rotation symmetry.

run in cyclic

Thus,

where J is rotation operator. but i am not sure is it restricted to real space rotation. any way, spin is a vector operator, thus

$latex [S_x, L_y] = i \hbar S_z = – [S_y, L_x] $

so, L, S is not commute.

Angus

Apr 28, 2019@ 18:45:39By definition L and S *do* commute – they live on different Hilbert spaces. In dirac notation (if we promote L and S to the new combined space)

[Lz, Sz] = (Lz⊗1)(1⊗Sz) – (1⊗Sz)(Lz⊗1) = 0⊗0

GoLuckyRyan

Jul 26, 2019@ 18:28:31Yes, by definition, but how about in the reality?