Tuesday, March 15, 2005

Physics at the south pole

I learned about two really cool experiments today, which I wanted to highlight. I don't follow experimental physics nearly as well as I should, given that I agree with what David Polizter said in his Nobel lecture: "I must say that I do regard theoretical physics as a fundamentally parasitic profession, living off the labors of the real physicists." In general, experiments are what drives the field. Certainly areas with active experimental programs are usually healthier than those without. Condensed matter physics is one example, another is neutrino physics, which is what I'm going to post about.

The colloquim speaker today was Buford Price from Berkley. Dr. Price has many interests, it would be impossible to sum up his whole talk here, even if I could have taken notes at the pace he was going. Instead, I'll say a few words about the amazing neutrino experiment that he was involved in.

Neutrinos are very weakly interacting particles, they can travel vast distances without interacting with anything. The are copiously produced in many astronomical objects that we are interested in studying, from the "mundane" (supernovae) to the more exotic (gamma ray bursters). Many groups are now engaged in studying neutrinos, produced in all sorts of situations. But traditional detectors have trouble studying the highest energy neutrinos. Enter AMANDA.

The AMANDA-II Telescope consists of 19 strings of optical modules buried between 1 and 1.5 miles beneath the snow surface of the geographic south pole. The total number of OMs in the array is 680, although only about 600 are fully operational at any given moment. Cherenkov light is converted to electrical signals by the photomultiplier tubes within each OM.

Buried under the south pole, experimentalists have turned ultra-pure ice into the worlds largest telescope. Because of its size, AMANDA is able to see neutrinos that have much higher energies than more modest, man-made detectors. So far, they've only seen an isotropic distribution of high energy neutrinos, no point source can be resolved.

This of course raises the question: What do you do when your giant telescope made of ice isn't sensitive enough? Yup, you guessed it, you build a bigger, more sensitive, telescope. This one is called ICECUBE. The idea is basically the same as AMANDA, you drill holes in the ice, two or three kilometers down. Then you lower your detectors down on strings. What's different is the scale, ICECUBE occupies a cubic kilometer of space. To put that in perspective, if you stacked up every human being living or dead (all of them) they'd only take up about a third of it. With its size ICECUBE will be sensitive to neutrinos with PeV energies.