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Question:
Does light interact with (i.e., scatter from) uncharged subatomic particles?



Replies:
Short answer: No. Exception: particles that are actually made up of still smaller charged particles, in such a way that the whole is neutral - an example is the neutron, which does have a magnetic moment and so does interact a very small amount with electromagnetic radiation. Reason: the only reason something would scatter light is if the light wave (consisting of electric and magnetic fields) causes it to move - electric fields interact directly only with charged particles, and magnetic fields interact only with things that have some magnetic property which in turn has to be due to the motion of charged particles (if the particle itself is not charged).

Arthur Smith


Dr. Smith is correct from a classical particle perspective, but quantum field theory allows for the existence of an extra degree of freedom for "particles" (i.e. the excitations of the field) which is called "spin," although it really is not (the particle is not actually spinning). The important point here about quantum spin --- which has a number of unusual properties --- is that it can endow the particle with a magnetic dipole moment, with which the particle can interact with electromagnetic fields, albeit much more weakly than if it were charged. The spin on a neutron (which is uncharged), with which neutrons can interact with light, does not come from the orbital motion of the quarks which make it up, but from the addition together of the quarks' spin, which is of the QFT origin I just mentioned.

Generally speaking, any distribution of matter which has *some* electromagnetic moment --- electric charge, electric or magnetic dipole, quadrupole, octupole, etc. moment --- can interact with an electromagnetic field. The interaction goes moment by moment, i.e. a charge interacts with a constant electric potential, a electric or magnetic dipole moment with a constant electric or magnetic field, a quadrupole moment with the gradient ("slope") of a field, and so on. The point lurking in the back of the questioner's mind is probably that one cannot generate anything higher than a monopole moment (i.e. a charge) from a point particle, so if you have no charge, you should have no moments at all. Classically this is true. In QFT, though, it is not.

christopher grayce


Well, I suppose I would better respond! The only uncharged elementary particles that we know of are the neutrinos, (plus some of the particles that cause interactions like the photon itself). The neutrinos have spin, but no magnetic moment, and so do not interact (directly) with light. However, there is always the possibility of indirect interactions - in fact, it is possible for two photons to collide with one another and interact - but this always has to involve the existence of charged intermediaries (for example temporary creation of an electron-positron pair). Anyway, in a direct sense uncharged elementary particles will not interact with photons, but in an indirect way they can... the resulting interaction is usually very small however.



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