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Is there a quantum theory of magnetism? 
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#1
Mar1112, 05:04 PM

P: 50

After asking my teachers the same question, I came away with the impression that the classical field model was as far as magnetism went, as if after Maxwell's macroscopic laws there had been no development into a quantum theory of magnetism. Were they wrong? Additionally, if they were to some extent, is there a mediating particle governing magnetism? And how would QED explain the phenomenon?



#2
Mar1112, 07:14 PM

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PF Gold
P: 1,783

QED is the quantum theory of the electromagnetic field. Electric and magnet theories are unified. (It just is too much of a mouthful to say "Quantum Electromagnetic Dynamics".) The photon has both magnetic and electric effects and mediates both components of the unified field.



#3
Mar1212, 10:59 PM

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PF Gold
P: 2,085

Ferromagnetism eluded microscopic derivation from classical principles and was only understood after QM was developed and applied to the problem. Ferromagnetism arises ultimately from longrange couplings between electron spins (themselves a QM concept). Numerous other QM principles play important roles: atomic shell theory and the Pauli exclusion principle, exchange interactions, and Bloch walls, to name a few.



#4
Mar1212, 11:17 PM

P: 1,583

Is there a quantum theory of magnetism?



#5
Mar1312, 12:04 PM

P: 165

Quantum field theory, or Quantum electrodynamics. It is often referred to as the "2nd quantization" or the quantization of the electromagnetic field. It is a beautiful and powerful theory.



#6
Mar1312, 01:37 PM

P: 110

As R. Faynamn was mentioned in his famous lectures (Vol 2, 34):



#7
Mar1512, 12:03 AM

P: 30

Yes, the fundamental interactions that drive magnetism are completely and totally quantum mechanical and have absolutely no analogy in classical physics. They derive from some subtle interplay of Fermion statistics, Pauli exclusion principle, quantum tunneling and Coulomb repulsion.
Unfortunately this makes it extremely difficult to make any explanation of magnetism from a classical perspective. Actually there's some interview Feynman did where he basically goes on a bit of a tirade over this exact question because the interviewer wants a simple answer and Feynman basically says he can't give it to him. I'm sure it's on youtube somewhere. 


#8
Mar1512, 04:47 PM

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PF Gold
P: 1,783

The business about the Fermion statistics, PEP, and tunneling is utter nonsense! Magnetism is part of electromagnetism and is explained wholly within the gauge theory of their bosonic mediator the photon. Its effects on other systems may involve e.g. charges, and their statistics, et al but that is secondary. That's the quantum mechanical part. The photon field has a perfectly well behaved classical limit in the electromagnetic field described by a classical gauge theory. But the gauge business is not necessary to understand and apply the physical theory. 


#9
Mar1512, 05:50 PM

P: 30




#10
Mar1512, 05:56 PM

P: 30

EDIT: In fact if you type Quantum Magnetism into wikpedia it apparently redirects you straight to "Spin Models" (i.e. Heisenberg model, Hubbard model, etc.) 


#11
Mar1512, 06:35 PM

P: 198

Also, check out virtual particles



#12
Mar1512, 06:47 PM

P: 30




#13
Mar1512, 07:23 PM

P: 29




#14
Mar1612, 09:43 PM

Sci Advisor
PF Gold
P: 1,783

I can go into e.g. the Josephson effect but that sheds no light on the basics of quantized electrical fields and charges. Likewise invoking condensed matter phenomena (a topic in and of itself) doesn't shed any light on the basics of the quantum theory of magnetism (which is of course simply the quantum theory of electromagnetism). The excerpt I quoted and criticized is without merit. To whit... The fundamental interactions (fundamental, not derivative!) driving the magnetic field is the electromagnetic interaction. As it is one of our two long range forces it has a perfectly compatible classical and quantum description. It is not true that it has "no classical analogue". The statement you made is false. As I read it either you were anxious to invoke many irrelevant esoteric topics or you were not reading the thread and understanding the level of the OP question. In either event (or for whatever reason you made the statement, still given its fallacy) your statement did not in any way serve the purpose of enlightening the OP or other readers. That is why I jumped on it so firmly. Yes there are rich areas of research on condensed matter invoking the magnetic component of particleparticle electromagnetic interaction. Beautiful stuff. But it isn't fundamental (to magnetism as an effect) but rather derivative. One must first have the quantum theory of (electro)magnetic interaction before one can begin the theory of magnetically coupled spin networks, and ferro,para,dia,antiferro...magnetism in condensed matter systems. Not to mention thermodynamics and quantum stat mec. with particle statistics playing its part. 


#15
Mar1612, 09:48 PM

Sci Advisor
PF Gold
P: 1,783




#16
Mar1712, 02:27 PM

P: 50

Bickering aside, thanks for enlightening me.



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