2 ms·

Annoyingly I've always struggled with the precise definition of spin, but to my understanding angular momentum is only conserved if you take spin into account a

Annoyingly I've always struggled with the precise definition of spin, but to my understanding angular momentum is only conserved if you take spin into account as well. Being quantum mechanical there is some ambiguity in the direction, but in principle it is a vector (except it's a superposition of several vectors, and this superposition is also equal to superpositions of different vectors, I think there is usually one 'pure' vector, but that might just be after measurement).

galimaufry 7y ago
> angular momentum is only conserved if you take spin into account as well. Do you know an example of a process that moves angular momentum from one kind of spin to the other?
- aeternum 7y ago
  Thinking of it as different 'kinds' of spins isn't quite right. It's more akin to the direction or axis of the spin being changed, and simply measuring the spin along a certain access will change it: https://en.wikipedia.org/wiki/Spin_(physics)#Measurement_of_spin_along_the_x-,_y-,_or_z-axes https://en.wikipedia.org/wiki/Spin_(physics)#Measurement_of_...
- kmm 7y ago
  There's an experiment that transfers that angular momentum all the way up to macroscopic levels. By magnetizing a cylinder of iron, all the spins start pointing in the same direction. By conservation of angular momentum, the cylinder itself has to start spinning in the opposite direction. I'm very fond of this experiment, because it magnifies a strange quantum phenomenon to the classical level. Spin being an intrinsically quantum mechanical concept, I'm afraid the microscopic mechanism by which that transfer occurs will only be explainable in a quantum mechanical context. Here it will appear as a term in the Hamiltonian coupling the spin of an electron to its motion in a potential. https://en.m.wikipedia.org/wiki/Einstein%E2%80%93de_Haas_effect https://en.m.wikipedia.org/wiki/Einstein%E2%80%93de_Haas_eff...

galimaufry 7y ago

> angular momentum is only conserved if you take spin into account as well. Do you know an example of a process that moves angular momentum from one kind of spin to the other?

aeternum 7y ago
Thinking of it as different 'kinds' of spins isn't quite right. It's more akin to the direction or axis of the spin being changed, and simply measuring the spin along a certain access will change it: https://en.wikipedia.org/wiki/Spin_(physics)#Measurement_of_spin_along_the_x-,_y-,_or_z-axes https://en.wikipedia.org/wiki/Spin_(physics)#Measurement_of_...
kmm 7y ago
There's an experiment that transfers that angular momentum all the way up to macroscopic levels. By magnetizing a cylinder of iron, all the spins start pointing in the same direction. By conservation of angular momentum, the cylinder itself has to start spinning in the opposite direction. I'm very fond of this experiment, because it magnifies a strange quantum phenomenon to the classical level. Spin being an intrinsically quantum mechanical concept, I'm afraid the microscopic mechanism by which that transfer occurs will only be explainable in a quantum mechanical context. Here it will appear as a term in the Hamiltonian coupling the spin of an electron to its motion in a potential. https://en.m.wikipedia.org/wiki/Einstein%E2%80%93de_Haas_effect https://en.m.wikipedia.org/wiki/Einstein%E2%80%93de_Haas_eff...