## Saturday, November 17, 2007

### Modest Understanding of Lie Groups Part 0: U(1)

This semester I had the pleasure to take a very little nice course on mathematics, mathematics for physicists that is. What this means is that half the course we dealt with lie groups and the remaining month or so we studied path integrals. Now, why is this interesting? It just happens to be the sexiest mathematics available to me at this point.

In case you didn't know, finding symmetries in physics leads to a deeper understanding of the phenomena at hand. This is obvious to any undergraduate student facing for the first time electromagnetism. The most basic problem of this course is finding the electrical field a distance d above an very long line of uniform density charge. Needless to say, you want to know how much the line would pull (or repel) a charge, should you feel like putting one a distance d above it. Of course you don't need to understand much about physics to eventually see that it doesn't matter where you place it, as long as it is a distance d perpendicular to the cable. Clearly this is because the line of charge is very long and this places are practically the same to the line. From this information you then can guess that the electric field must only depend on the coordinate perpendicular to the line, a trivial conclusion, but proves the point just fine I guess.

Again, why am I talking about this? Turns out our most precious tool (for the moment) allowing us understanding the world, the Standard Model, is based on symmetry groups. Namely it is usually represented by SU(3)xSU(2)xU(1). Let's start by understanding the simplest part of this: U(1). Imagine a circle, or rather, its points:
In this figure, A and B are two points on the circle. All the points on this circle are characterized by some properties. For example, if a point on the circle is represented by the vector
$X = \begin{bmatrix}x\\y\end{bmatrix}$
the point
$\begin{bmatrix}x'\\y'\end{bmatrix}=\begin{bmatrix}\cos \theta & -\sin \theta \\ \sin \theta & \cos \theta\end{bmatrix}\begin{bmatrix}x\\y\end{bmatrix}$
is also on the circle! Having seen this, it's easy to see that the points on a circle with the operation of addition (since each point is characterized by an angle, we can understand it as the sum of their angles) form a group. If we see this circle on the complex plane, a point on the circle can be represented by a complex number
$e^{ia}$

and a rotation about the center of the circle will be given by multiplying this number by the following phase factor
$e^{i\theta}$
This will take us
$e^{ia}\rightarrow e^{i\theta}e^{ia}=e^{i(a+\theta)}$
that is, another point on the circle (closure). If this is new to you, try and find the identity and inverse elements.
We can see this phase factor as a 1X1 matrix, and call it U. It's clear then that in this case
$UU^*=1$
But in general for bigger matrices
$UU^\dagger=1$
I hope then to have explained how this implies that the complex numbers of norm 1 form a group under the operation of multiplication. This group is the most simple I can think of for now, the U(1) group (unitary matrices of rank 1, which satisfy the last equation).
Tune in next time for a brief explanation on all the other classic groups.

## Saturday, November 03, 2007

Last may Symmetry magazine launched a contest for inventing new particles, and the results are out in the latest edition. One of the particles is so relevant for this humble site that it deserves special mention: the blogino, it's creator Jacobo Konigsberg from Fermilab says about it:
Particles created by non-abelian Blog-Blog interactions. Bloginos typically are produced in a very excited state and with a high degree of spin. Even though all their properties have not yet been determined, it is commonly agreed that they exhibit considerable truthiness. They also have the annoying ability to propagate into extra dimensions, away from the blogosphere, and generate lots of phone calls.

The allmighty blogino, the coolest particle around only behind...

The rockon "discovered" by Ike Hall from Fermilab was, hands down, my favorite particle:

Responsible for such things as face-melting guitar solos, heart-pumping rhythms, screaming vocals, and hair bands. Observation of the rockon over the airwaves has been on the decline since 1995.

Yep, that particle really rocks. It's particullary close to me since rock is what I most like in life!

## Thursday, November 01, 2007

### Comet Holmes from OAN-SPM

Comet Holmes is an old folk for astronomers, discovered in 1892 and with a period of 5.9 years, it was discovered in an outburst during which the comet brigthened to a magnitude around 4-5.

A similar outburst happened last october, from magnitude 17 to 2.5! It is still visible at the dusk (or dawn) if you look at Perseus, maybe some binoculars will be necesary. I won't delve further into it because there are many posts about it in the blogosphere (1)(2)(3), rather I'll show you a nice pic of this comet.

Use this sky chart to look for Comet Holmes, it depicts the sky at a latitude adequate for most of North America and Europe.

Alan Watson, an astronomer at Centro de Radiastronomía y Astrofísica, UNAM was at OAN-SPM (National Astronomical Observatory at San Pedro Martir), and used the 1.5 meter telescope to take this nice picture of comet Holmes in the R band (meaning light was filtered to only allow "red" light arrive the detectors).

The exposition time was 10 seconds. Look at the displacement between the core and the coma.