Preposterous Universe

Wednesday, April 06, 2005
Fundamental physics in the U.S.

Scientists who work on fundamental physics, especially in the U.S., are feeling a kind of urgency these days -- we have to hurry up and get as much research done as we can before the government puts us completely out of business. Belle Waring complains about the shutdown of the Voyager mission, which is indeed a shame, if mostly for sentimental reasons. A much harder hit is NASA's cancellation of the Astrophysics Data Analysis and Long Term Space Astrophysics programs. These programs were a main way to support young scientists (grad students, postdocs, junior faculty) working on theory and data analysis with broad application to NASA's satellite observatories. In other words, in the midst of a golden age of new theories and experiments, we are strangling the field at the point where new blood is entering.

For those of you with more Earth-based concerns, you should know that the U.S. is also basically abandoning experimental particle physics (pdf version if that one is inaccessible). The Tevatron at Fermilab will run through the end of the decade, after which there is basically nothing left in the budget for high-energy physics in the U.S. By that time the focus will move to the Large Hadron Collider at CERN in Geneva, and the traditional brain-drain of bright physicists from Europe to the U.S. will reverse its direction. My real interest is in the health of the field, not in maintaining U.S. dominance, but it will be hard for the field to stay very healthy if the U.S. isn't a major player. Our best hope for a turnaround is if the U.S. makes a serious bid to host the International Linear Collider; but that's a long way off, and the tea leaves don't look so promising. (Update: Just noticed that Peter wrote about the same thing.)

But okay, I don't want to be gloomy all the time, so here's some good news: the LIGO gravitational-wave observatory continues to make progress toward their design goals. LIGO, the Laser Interferometric Gravitational-wave Observatory, consists of two facilities -- one in Hanford, Washington, and the other in Livingston, Louisiana. Each facility shoots lasers down two four-kilometer evacuated tubes, where they bounce off suspended mirrors and come back. By comparing the phases of the light from each tube, you can look for tiny changes in their length, which would signal a passing gravitational wave.

Of course, you're looking for really tiny changes in length; about one part in 1021 or so. Which, over four kilometers, adds up to significantly less than the size of a single atomic nucleus. So you have to be pretty sensitive. LIGO has been operational for a few years now, and they are steadily beating down the noise curve -- the amount of irreducible jiggle in the detector that you can't get rid of. The idea is that anything you observe on top of the noise is an actual signal, such as a pair of inspiraling neutron stars giving off gravitational waves. According an update by David Shoemaker in the most recent Matters of Gravity, the LIGO folks are making significant progress in eliminating various noise sources, such as trucks rolling by.

Here's the graph of noise versus frequency, showing both the goal (solid line at bottom) and what levels they have achieved over time. (Click for larger size.) As you see, they are getting there, and have already decreased the noise by something like three orders of magnitude over the last couple of years. LIGO may or may not see anything in its current configuration; a planned upgrade to "Advanced LIGO" is much more likely to actually detect a gravitational wave. Once they do, it will open a completely new window onto the universe.

Ideas on culture, science, politics.
Sean Carroll

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