The most accurate theory we have

It's often said that Quantum Electrodynamics is the most accurate theory we have in physics. This is based on a few things, first it agrees with a large number of experiments with good precision. It also reduces to standard classical electromagnetism in the classical limit, and classical E&M is a very well tested theory. Finally, there are a handful of things where the agreement between theory and experiment is truly spectacular. Of these, the crown jewel are the QED predications of the magnetic moments anomaly (MMA) of the muon and the electron.

For example, the best current experimental value for the muon MMA is

amu(exp) = 11 659 208 (6) x 10^{-10}.

First off, it's amazing that this can be measured so precisely. What's even more amazing is that the theory prediction for this quantity is

amu(theory) = 11 659 187(8) x 10^{-10}.

The difference is

2.1(1.0) X 10^{-9}.

Now, it's not zero, and that might be meaningful, but it's far more likely that the theory part has a larger error than suspected. In particular, the dominant part of the error in the theory is the low energy QCD contribution. That's something that can, in principle, bet computed using lattice QCD. But what's more remarkable than the small difference is the agreement. To a very high level of accuracy, the theory agrees with experiment. The same is true for the electron.

Obviously, to produce theoretical predictions like this requires a lot of work. This work started in the forties, when Schwinger computed the first approximation for the MMA. Since then, people have computed, order by order in perturbation theory, this quantity. And no person has done more than Tom Kinoshita to improve the prediction. Tom gave a talk last week on his work (with collaborators) in this field.

At the first order in perturbation theory, the exact solution (\alpha/(2\pi)) is easy to find. It's a standard exercise. As you go to higher and higher orders though, the calculation quickly becomes very hard. The second order calculation was completed a few years after the first, and the third was many years after that. The final results for the fourth order calculation weren't finalized until last year.

Tom is still at it. With the fourth order calculation done, he and his collaborators have moved on to the fifth order electron MMA. A new generation of experiments should make this calculation necessary. For those in the know, the fifth order calculation requires evaluation of around 12000 five loop Feynman diagrams. Currently the only practical way you can do this is to evaluate them numerically. Like my own work, much of this invovles figuring out ways to automate as much of the computation as possible.

This doesn't have much to do with lattice QCD, but it's certainly inspirational to know that theory and experiment agree so well. And it's nice to know that when I'm sweating over a two-loop calculation, somebody else is dealing with five-loops.

A Brief History of Rhyme

The world of physics jokes is fairly small. You can get a good impression of them by wandering around a physics department and looking for the people who put them up on their doors. For the most part, they're mostly "in" jokes. For example, one gives a transtlation table between phrases you commonly find in physics papers (for example, "it can be shown") and what is "really meant" ("my grad student worked this out, I have no idea if it's correct"). You basically have to be a professional physicist to find this sort of thing funny.

Even worse are the jokes in various sub-fields of phyiscs. For example, in my field, high-energy physics, the most recent joke was this preprint posted on April first. This is really funny, if you've had two graduate level courses in quantum field theory and are familier with the latest work in string theory. Otherwise, you'd be hard pressed to distinguish it from any other paper in the field.

There are a few physics jokes that can be apprciated by everybody though. The best has been around for a few years, the strange case of MC Hawking. The premise of this site is that noted theoretical physicist Steven Hawking has had a modestly sucessful second career as a gangsta rapper. Needless to say, not everybody will actually find this funny, given
Prof. Hawking's disability, he is confined to a wheelchair, cannot move anything apart from his head and hand, and speaks only with the aid of a computer. However, Dr. Hawking (the real one, not the fictional gangsta rapper) takes the joke in good humour.

What's more, the guy who is actually responsible for the joke (Ken Leavitt-Lawrence, not a physicist) has some reasonable skills putting together rhymes and beats, and the speech program he uses sounds exactly like the real Steven Hawking. The MC Hawking website used to have a few mp3 files of various MC Hawking raps which you could download. Demand was so high that a CD has been released, "A Brief History of Rhyme: MC Hawking's Greatest Hits". As a fan of both theorectial physics and gangsta rap, I had to have it.

The CD arrived yesterday, and I can definatly recommend it. If you've ever wanted to hear Steven Hawking gangsta rapping (and he doesn't hold back on the foul languauge) it doesn't disappoint. Highlights include the classic "All my shootings be drive-bys", in which the Hawkman takes care of some punks from rival MIT, "Bitchslap" which
details how MC Hawking deals with a disrespectful TA, and the duo "The Big Bizang" and "Entropy" which explain the Big Bang theory and the second law of thermodynamics via rap. The only track missing is the ode to first person shooter video games, Quakmaster. You can get that on the MC Hawking website.

My personal fave is "F**k the Creationists", which is particularly appropriate given the sillyness going on these days in Kansas (and elsewhere).