Physical meaning of multipole moment

Question

Is there a physical interpretation for multipole moments?

For a quantity governed by the Laplace equation ($\nabla^2 \omega = 0$), I understand that the general solution is given by the multipole expansion. In 2D, the exterior multipoles are given by

\begin{align} \omega(r,\theta) = C_0 \ln r + \sum_{k = 1}^\infty \dfrac{C_k \cos(k\theta+\beta)}{r^k} \end{align}

$C$ is the multipole moment and $\beta$ is the orientation of the multipole.

The magnitude of $C_0$ can is the strength of the monopole. In electromagnetics this is physically interpreted as the enclosed charge. In potential fluid flow, the monopole is a mass source and $C_0$ is interpreted as the mass flux.

$C_1 = \epsilon C_0$ is the dipole moment in the limit where $\epsilon \to 0$ while $\epsilon C_0$ remains constant. $\epsilon$ is the separation between the two opposite signed monopoles. $C_0$ has a straight forward interpretation, but what is the physical interpretation of $\epsilon C_0$? In electromagnetics it appears to be a length times charge. And in potential flow it appears to be a length times mass flux. Also, what is the physical interpretation of higher order multipole moments?

Answer 1

First of all, don't think of multipole moments as separate things that have their own individual meaning. Instead, think of them as parts of one thing. Once we have all the parts written down, we can start naming and organizing each one to determine its contribution to the whole.

Now, for your question

Is there a physical interpretation for multipole moments?

Yes! And they don't necessarily have to do anything with electrostatics, spherical harmonics or geometric series. A multipole expansion of some object in some basis is saying Hmmm, I have this funny shaped thing that isn't an elementary mathematical function, but I want to express it as a sum of elementary functions. (Fourier or someone said you can always do this with enough ink and parchment.)

So you first pick your basis, whether it be sine waves or exponentials or polynomials or the like, and then you start adding more and more terms of that basis, beginning with the lowest order (simplest) component.

For a non-mathematical example, consider this drawing of a cartoon sheep:

Step 1 is very simple since it's basically an oval with ripples. Right off the bat, with the very first thing we drew we have expressed some 75% of what the sheep will look like. This is important in multipole expansions: the lowest order terms dominate. If Step 1 were a square or a triangle then the whole sheep would likely be unrecognizable.

Step 2 does two things: it adds to the drawing and it slightly modifies some of what Step 1 did. You may have heard this called a corrective term or a higher order term. This would be the second term in your multipole expansion.

Step 3 adds less to the picture than Step 2, but look how far we've come. With just the first three multipoles, I'm betting a large percentage of people would recognize our animal as a sheep already. If instead of drawing a sheep with blobs we were constructing an E&M field with Legendre Polynomials, this is about where we stop since we have a good physical view of what's happening (this is the trade off of simplicity vs. accuracy present in all multipole expansions).

Additional steps just add more detail, filling out the gaps in the data at the cost of doing more work and keeping track of more pencil marks.

What is the physical interpretation of higher order multipole moments?

In E&M, we break the arbitrary looking charge distribution into multipoles with the hopes of drawing as much of the sheep as we need with as few details as possible. The multipoles look like:

• 0. the monopole, the offset that affects the E field in all directions the same way

• 1. the dipole, the description of how different two halves of the field would be if you drew a symmetry line right down the center

• 2. the quadrupole, a similar concept to the dipole, but instead of affecting two directions differently, you affect four directions

And you can keep going to as high of a multipole as you want (technically, you need to go to infinity to perfectly redraw an arbitrary sheep, but this isn't useful when we're just trying to predict system certain properties to within a finite precision to begin with).

Summary

A multipole expansion of anything is just breaking it down into a preferred basis. If we picked a good basis, we only need the first few multipoles because after that we're just touching up details we'll never need. Some multipoles are so useful we give them names, like the E&M charge distribution that affects everything isometrically (total charge) and antisymmetrically (dipole).