28 November 2004

QCD told by one of its own architects

Apologies for not writing anything in depth lately. But if you have a good connection and know some physics, it might be fun to watch Frank Wilczek's "2004 Nobel Prize" MIT colloquium. MIT World Video did a nice job.

In case you hadn't already heard, Frank helped to discover the right theory to describe the strong interaction which binds nuclei together. In particle physics speak, he, David Gross, and David Politzer figured out that to match physical observations (that quarks are "asympotically" free at close distances/high energies), you needed a non-Abelian gauge theory. From the work of others, it was known that the non-Abelian gauge theory had to have SU(3) color symmetry. Today, the theory of the strong interaction is known as quantum chromodynamics (QCD). The talk was good, but mostly I think it was just a nice celebration of Frank's great achievement being recognized. MIT physics alums (2000-present) can be proud!

21 November 2004

Link of the day: cochlear implants

There's a great video of a Fermilab colloquium about cochlear implants. The talk was given by Ian Shipsey, who uses a cochlear implant himself.

It's really cool that Fermilab tapes all their colloquium talks. I wish other universities would do the same. Sure, there are books and papers out there, but nothing beats the experience of listening to another human being tell you something!

Are you subconsciously prejudiced?

To find out, take one of the tests at implicit.harvard.edu. It works by measuring your response time to associating certain words with certain groups of people.

I found out that I have a strong preference for straight over gay people and that I have no or little prejudice associating women with liberal arts and men with science. The latter makes sense as I am a woman physicist!

14 November 2004

How does a watch/clock work?

I haven't had too much time to think about this question yet, but I thought I should make a note of it here to remind myself to do more research. The little I've read on the web indicates that there are two kinds of watches, those that run on a quartz oscillator and integrated circuit (powered by a battery in general) and those that are purely mechanical. Here are some good links about how quartz watches and mechanical watches work. The Swiss watch industry also has a page that shows the internal parts of quartz and mechanical watches. Supposedly over 90% of the world's watches are based on quartz oscillators.

Quartz watches are known to be so accurate that they only change by a minute over a year. But as a physicist, I know that even better time keepers exist -- that's right, atomic clocks. But it's late, so I'll report on those devices another time.

12 November 2004

Today's interesting link

I was reading Michael Nielsen's blog earlier and found an interesting link to Eugene Wallingford's blog. In one entry, Wallingford describes an inspiring talk by computer scientist visionary Alan Kay. The thrust of Kay's talk was that computer science as a field was falling into a lull and forgetting about how to be creative and innovative. What I found especially interesting was Ivan Sutherland's PhD thesis. Apparently, in one very intense year, Sutherland created the first user interface for graphics, object oriented programming, etc. Hopefully, I'll have a chance to read the thesis sometime.

Particle physics

As an undergraduate, my education was heavily influenced by particle physicists. They taught my classes, I did research with them, they were my mentors and heroes. In the end, I decided that particle physics wasn't for me, but I still think about it from time to time.

This year, I finally got around to taking a class in quantum field theory -- the subject that lies at the heart of particle physics. Between reading Peskin and Schroeder, sitting in class, and talking to the lecturer, I am starting to get a clearer picture of what is going on.

I'll just list a few of the interesting things I've learned so far.

1) People often say that quantum chromodynamics (QCD) is a beautiful theory because it only has one adjustable parameter (the strength of the coupling between quarks and gluons). This is because QCD is a non-Abelian gauge theory. Once you add in the fact that QCD obeys SU(3) symmetry and that dimensions of the operators have to have be less than some critical number for the theory to be renormalizable, the QCD Lagrangian is almost completely constrained. However, the constraint that the theory must be renormalizable is debatable. There could be some terms in the Lagrangian which are not renormalizable but are extremely weak in our low-energy observation range.

2) If you read any introductory book on particle physics, the first thing that is mentioned is that lepton and baryon number are conserved. That is why when a neutron decays to a proton and electron, it must also produce an electron neutrino. A muon or tau neutrino is not allowed. It turns out that this conservation might be approximate. The physics we know about (the Standard Model) only has Lagrangians which conserve these numbers. In addition, the dimension of the coupling constants are less than four (I think) which implies that we know about these interactions because they are important at low energies. But there could be other terms in the Lagrangian which only kick in at high energy beyond our current observation range. And these terms might not conserve lepton and baryon number.

3) According to the lecturer of my class, proton decay is a very strong constraint on grand unification theories (GUT). [In most GUTs, at very high energies, protons can couple to other interactions which make them decay.] The fact that the proton lifetime is experimentally limited to being greater than 10^33 years makes these theories unlikely. Of course that is his opinion. He also thought that the non-unification of quark energies was also a strong indicator that GUTs are implausible. I asked him whether future experiments might clear up the mystery. He thought that the Large Hadron Collider at CERN would be able to see something but not with great precision. The Next Linear Collider (an experiment being planned) has the technological power to do precision measurements, but currently no country wants to fund it. All-in-all, a rather depressing outlook!

My conclusion so far is that particle physics is a fascinating subject, but until there are experiments at higher energies, it is impossible to say anything coherent about speculative theories like GUTs. Of course, bona fide particle physicists say the same thing, but I'm just now starting to realize why.

I realize that this post will not make sense to 99.99% of the population. A few months ago, I wouldn't have understood it earlier. I would say that to really understand particle physics at any meaningful depth, it's imperative to take a field theory course.

11 November 2004

Hello world!

The first thing you might be wondering about is the origin of this blog's title. Well, I'm a physicist, but unlike many physicists, my work actually has some practical applications. Specifically, I study superconducting devices that are capable of quantum computation. So that makes me a "quantum mechanical engineer." (Plus, I have a degree in engineering in addition to my physics degree.) I got the idea from Seth Lloyd, a professor at MIT who works on the theory of quantum computation. People often ask him why he's in the mechanical engineering department, and his reply is that he's actually a quantum mechanical engineer.

Perhaps one of these days, when I have more time, I'll try to say a few words about quantum computation. For now though, you can try Michael Nielsen's blog. Michael is a well-known quantum information theorist.

I'm not exactly sure what I will be writing in this blog. I suppose it will be a way for me to keep a scrapbook of random thoughts.