### Understanding How to Transpose Without Really Trying: EMII Notes 2014_08_09 Part II

Summary of what's gone on before.  Got through the index notation for gradients and whatnot.  I was left a little bit baffled by the notation for the orthogonal transpose identity.  Consequently, I'm digging back into it.

In this set of notes, the transpose, orthogonal identity, $M_{ki}M_{kj} = \delta_{ij}$, is first hammered out.  It then becomes obvious what's going on.  The dummy summing of the two row indices gave us the equivalent of a matrix multiply where it's row times row instead of row times column.  The rows of a matrix that should have been transposed however are the same as the columns of one that wasn't.  In other words by forcing a different type of matrix multiply, they teased out the transpose for free.

Here's the hammering through bit.

Let's take as an example, the simple rotation matrix about the z axis.  Keep in mind that it has already been explained above why this will work for any orthogonal matrices and that this is just a concrete version that I already happened to work out before figuring out the general case above.

$M_z = \begin{pmatrix} cos \theta & sin \theta & 0\\ -sin \theta & cos \theta & 0\\ 0 & 0 & 1 \end{pmatrix}$

Now, with the multiply defined as above, we will get three resulting matrices for $k=1$, $k=2$, and $k=3$, and then sum them all together.  The k's are fixed, so the only combinations of terms that need to be multiplied are the i's and the j's .  For each k, we'll get the 9 terms indexed by i and j.  So, for the k's, 1 through 3, respectively, we get:

$\begin{pmatrix} cos^2 \theta & cos \theta sin \theta & 0\\ sin \theta cos \theta & sin^2 \theta & 0\\ 0 & 0 & 0 \end{pmatrix}+\begin{pmatrix} sin^2 \theta & -sin \theta cos \theta & 0\\ -cos \theta sin \theta & cos^2 \theta & 0\\ 0 & 0 & 0 \end{pmatrix}+\begin{pmatrix} 0 & 0 & 0\\ 0 & 0 & 0\\ 0 & 0 & 1 \end{pmatrix}$

which adds term by term to give $\delta_{ij}$.

NOTE:  Look at this a lot!  The multiply method of swizzling through all the combinations of i and j tends to slip my mind!

Yurlungur said…
Well done! I remember really struggling with this sort of thing when I was first starting on GR.

It's not terribly enlightening, but I often found it helpful to prove index identities to myself using "brute force" by programming the summation into a computer and confirming the left-hand side is the same as the right-hand side. The reason I would do this is sort of two-fold.

Sometimes it was helpful just to prove to myself the identity worked. I didn't really believe it until I saw it in action.

And sometimes I misunderstood the index notation and got the identity wrong! The brute force would make that clear.
Hamilton Carter said…
Thanks Yurlunger!

I like your idea about programming the identities. I guess I'll get better with practice, but these are pretty difficult for me to see now, and a computer program, as you pointed out would force me to confront the inconsistencies in my thinking about indices more quickly.

### Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

### Lost Phone

We were incredibly lucky to have both been in university settings when our kids were born.  When No. 1 arrived, we were both still grad students.  Not long after No. 2 arrived, (about 10 days to be exact), mom-person defended her dissertation and gained the appellation prependage Dr.

While there are lots of perks attendant to grad school, not the least of them phenomenal health insurance, that’s not the one that’s come to mind for me just now.  The one I’m most grateful for at the moment with respect to our kids was the opportunities for sheer independence.  Most days, we’d meet for lunch on the quad of whatever university we were hanging out at at the time, (physics research requires a bit of travel), to eat lunch.  During those lunches, the kids could crawl, toddle, or jog off into the distance.  There were no roads, and therefore no cars.  And, I realize now with a certain wistful bliss I had no knowledge of at the time, there were also very few people at hand that new what a baby…

### Lab Book 2014_07_10 More NaI Characterization

Summary: Much more plunking around with the NaI detector and sources today.  A Pb shield was built to eliminate cosmic ray muons as well as potassium 40 radiation from the concreted building.  The spectra are much cleaner, but still don't have the count rates or distinctive peaks that are expected.
New to the experiment?  Scroll to the bottom to see background and get caught up.
Lab Book Threshold for the QVT is currently set at -1.49 volts.  Remember to divide this by 100 to get the actual threshold voltage. A new spectrum recording the lines of all three sources, Cs 137, Co 60, and Sr 90, was started at approximately 10:55. Took data for about an hour.
Started the Cs 137 only spectrum at about 11:55 AM

Here’s the no-source background from yesterday
In comparison, here’s the 3 source spectrum from this morning.

The three source spectrum shows peak structure not exhibited by the background alone. I forgot to take scope pictures of the Cs137 run. I do however, have the printout, and…