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Re: [Phys-L] neutrons

On 2/9/21 10:14 AM, David Ward wrote:

I recognize that a neutron has a non-zero magnetic moment, that it is
a collection of quarks with a total electric charge of zero. Here's
the student question: If a neutron were made to move, would its
electromagnetic nature result in EM wave propagation?

My first answer is no, but there is that magnetic moment and the
substructure of the neutron. A cursory glance with Google led to one
or two hits where people implied it might emit.

The short answer is yes, if you wiggle a neutron, it will radiate
(except maybe on a set of measure zero, if you wiggle it in just
the wrong way).

I don't have a nice simple explanation of why, which probably
means I don't understand the topic as well as I should ... but
here's the best I can do.

E) You may be familiar a radio antenna in the form of an electric
dipole. Just a wire. Electrons run up and down the wire. The E
field of the electrons does the work.
M) There are also loop antennas. A loop of one or more turns of
wire. AC current flows in the loop. Changing magnetic field.

A neutron looks like a magnetic dipole. So wiggling a neutron
will look more like item (M) than item (E) above. The pattern
of radiation in space will be different from what you would get
from an electric dipole.


Here's a thought that's not simple, but may explain why the
right answer will never be as simple as you would like. This
is important to keep in mind when trying to visualize the
radiation process:

Let's take a step back and look at something that is easier to
deal with, namely a plain old charge. The short answer is, if
you wiggle it, it will radiate. However, beware: What's going on
here is considerably more complicated than you might guess. In
particular, the non-moving charge has a static Coulomb field,
but the wigging the charge produces a field that is *not* what
you would get by just wiggling the Coulomb field. Really not.
In particular, the Coulomb field is 100% longitudinal, while
the radiation field is 100% transverse.

The only way I know how to explain this is to take yet another
step back, and look at the /potentials/. The field in general
contains two pieces:
R) ∂/∂t of the spacelike part of the potential
C) ∂/∂r of the timelike part of the potential

R is the radiation field.
It vanishes for a static charge.
It falls off like 1/r.
C is the Coulomb field.
It falls off like 1/r² so in the far field it is negligible
compared to the radiation field.


Similar words apply to gravitational radiation. You simply
cannot figure out how gravity waves work by wiggling the
Newtonian gravitational field.

Bottom line: Anything that has a field also has a potential.
There is no easy way to figure out the potential by looking
at the static field, because the part you care about for
radiation purposes is annihilated by the ∂/∂t in the static