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Light/Subatomic Particle Interactions
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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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Update: June 2012
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