N.33 Set­ting the record straight on align­ment

Some sources claim the spin is un­der an an­gle with the mag­netic field; this is im­pos­si­ble since, as pointed out in chap­ter 4.2.4, the an­gu­lar mo­men­tum vec­tor does not ex­ist. How­ever, the an­gu­lar mo­men­tum com­po­nent along the mag­netic field does have mea­sur­able val­ues, and these com­po­nent val­ues, be­ing one-di­men­sion­al, can only be aligned or anti-aligned with the mag­netic field.

To in­tu­itively grab the con­cept of Lar­mor pre­ces­sion, it may be use­ful any­way to think of the var­i­ous com­po­nents of an­gu­lar mo­men­tum as hav­ing pre­cise nonzero val­ues, rather than just be­ing un­cer­tain. But the lat­ter is the truth.