Spin and direction of motion

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How this two are related? I read somewhere that the spin is always parallel to the direction of motion of particle. If the direction of motion changes by some magnetic field (Lorentz force) what happens to tha spin? Will its axis direction change too?
 

Fredrik

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I read somewhere that the spin is always parallel to the direction of motion of particle.
That's true for massless particles, but not for massive particles.
 
197
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Why it's true for massless particles? Is there any explanation to this?
Btw thnx for your answer!
 

Fredrik

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There is, but it's very complicated. It's a part of the mathematical theory of massless representations of the Poincaré group. If you're a bit of a masochist, check out section 2.5 in Weinberg's "The quantum theory of fields" (vol. 1).
 

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