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*From*: John Denker <jsd@av8n.com>*Date*: Tue, 9 Feb 2021 12:01:32 -0700

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.

Radiation.

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

case.

**Follow-Ups**:**Re: [Phys-L] neutrons***From:*"Alex. F. Burr" <aburr@aol.com>

**Re: [Phys-L] neutrons***From:*bernard cleyet <bernard@cleyet.org>

**References**:**[Phys-L] neutrons***From:*David Ward <dward@uu.edu>

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