A change of magnetic field caouses an electric field and an associated potential which is as high as the time derivative of the magnetic field.So is it possible that the change of the spin should be a finite derivative regarding time?
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1$\begingroup$ The question is not very clear: why electric field should affect spin? What change of magnetic field are we talking about? - instantaneous? electromagnetic wave? $\endgroup$– Roger V.Commented Oct 2, 2020 at 13:15
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$\begingroup$ @Vadim in simple cases the spin couples to the B field, but the E field can affect the spin if the particle is moving. For example, the spin of an electron in an atom is coupled to the electric field of the nucleus via the Dirac equation. $\endgroup$– QuilloCommented Oct 2, 2020 at 13:36
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Answering the title:
When a particle changes its spin orientation is it instantaneous?
An elementary particle is a quantum mechanical entity which is described by the numbers in the table, including a fixed spin. The spin of a charged particle will be changing orientation when interacting with electric and magnetic fields. Even though the changes in direction obey classical electromagnetism, there is still the quantum mechanical energy-time uncertainty
that imposes a non instantaneous interaction, given the energies involved. The uncertainty principles are the envelopes of the possible exact solutions of a particular quantum mechanical interaction.
The same is true in all problems where there exists and intrinsic quantum mechanical total spin, as with composites like protons , or even atoms and molecules, i.e. where the quantum mechanical frame is necessary.
