# If the source of magnetism is spin, why is magnetism only associated with charged spin particles?

When talking about the source of magnetism, I have been told that there are two answers to this, as listed in this question. 1) we can think about this as the spin of an electron giving rise to the magnetic field (a semi-classical picture of a quantum phenomenon) and 2) magnetism is due to moving charges. The latter doesn't explain much about the source, but rather the realm in which we see this. I have since realized that there are particles that have spin that are not charged, so I am wondering why, if spin is the source of magnetism, why specifically charged particles are associated with the magnetic force? What is going on at the fundamental level here regarding the interaction between charge and spin to create magnetism?

• I'm not sure what you're asking. The fundamental sources of magnetic field are (1) moving charged particles, and (2) particles with magnetic dipole moments. All particles with magnetic dipole moments must have spin, but they don't have to be charged. (For example, the neutron has no charge but creates a magnetic field through its magnetic dipole moment.) Commented Oct 4, 2022 at 20:27
• If a particle does not have charge, it means the particle does not interact with electromagnetic field, thus such particle cannot be a source of electric or magnetic field. Commented Oct 4, 2022 at 20:28
• oh, i did not know that particles that are not charged can create magnetic fields. this seems counterintuitive to me - doesn't this mean they can also create electric fields since you can transform between the two? and the expression for the electric force in my experience always contains charge as a coupling constant (classical field theory) or a constant (e&m in coulomb's law) @knzhou Commented Oct 4, 2022 at 20:29
• I guess it depends on the context. You might say the neutron is not "really" an uncharged particle, but rather a composite of three particles whose charges all cancel out. Commented Oct 4, 2022 at 20:41
• As @knzhou said, neutron is not a particle without inner structure, and we know from classical electrodynamics a charges system can have total zero charge, but nonzero dipole moment. Commented Oct 4, 2022 at 23:20