by Malcolm Saunders
A group of BPAA members and friends drove to Hanford to tour the LIGO (Laser Interferometer Gravitational wave Observatory) site the weekend of Nov. 8 http://www.ligo.caltech.edu/. After touring the LIGO site we drove on to Goldendale, planning to watch the lunar eclipse from the observatory/state park there. We caught a ferry about 8 in the morning and arrived at the Hanford LIGO site about 1 p.m. I have never been to Hanford before. I was surprised to see how open it is. There is not so much as a gate controlling access. But then, there is nothing secret about gravity wave research. We expected a brief talk and tour lasting about 1½ or 2 hours. What we were given was about 4 hours of lecture and tour by the head of the laboratory. He did a very good job.
View of the “corner building”, the intersection of the two 4 km arms of the interferometer and the location of the controls.
Dr Raab speaking to the tour group. We are standing on a bridge that crosses over the detector. You can see one arm of the detector (a 10 ft diameter concrete tube) receding into the distance.
The LIGO project is a joint research project by CalTech and MIT. There are two large research facilities, one in Louisiana and one at Hanford. At the heart of each site is a large Michelson interferometer. These are contained in "L" shaped buildings 4 kilometers on a side. The interferometers are really detectors, not observatories. They are designed to detect gravity waves but they can’t tell us anything about where the gravity wave comes from and they can’t create any sort of image such as we excpect from an observatory telesecope. However, if a gravity wave is detected at each of two detectors, the difference in the time of arrival at the two sites can be used to determine something about the direction the wave is moving. With more than two detectors more information becomes available. As it happens, there are other detectors: one in Italy, one in Germany and one in Japan. There may be a detector constructed in Australia in the future.
At LIGO, a Michelson interferometer (http://en.wikipedia.org/wiki/Interferometer) is used to look for oscillations in space-time as predicted by general relativity. A laser beam is split using a half silvered mirror into two beams which are sent at right angles to each other down the length of the two arms of the building. At the end of their 4 km journeys they are reflected off mirrors and sent back to the origin where they are recombined. If one of those two arms has changed size relative to the other arm, the two halves of the laser light beam will no longer be in phase. Out of phase light beams will dim or extinguish each other. This is detectable with a photocell.
The Hanford LIGO instrument is up and running but it has not reached its full design potential yet. The operators have spent months, with more months to come, identifying and eliminating or compensating for noise sources. This is necessary because the instrument is looking for extremely small vibrations, 10-18 meters. There are many sources of interfering noise. These include trucks passing on the highway 10 miles away, highway and general industrial noise in the tri-cities about 25 miles away, passing airplanes, tidal distortions of the shape of the Earth (about 6 inches but only twice per day). Apparently the single biggest noise source is ocean waves, even though Hanford is about a 4-hour drive from the nearest salt water. As the operators identify and eliminate successively smaller noise sources the instrument becomes capable of detecting fainter and fainter gravity waves. Ideally, the mirrors at the ends of the long arms stay perfectly still in space, so that any vibration detected is caused by the movement of space-time itself.
Part of the control room. The large displays on the wall are real time monitors of instrument status and data collected. The center image is power vs frequency vibration data. This is the main data output of the instrument. The rest is devoted to monitoring and controlling the instrument. We were told that most of the people in the room were working on identifying interfering noise sources. There are about 1000 data feeds from the instrument to the control room.
Our tour of LIGO didn’t end until 5:30. By that time the moon was already more than half eclipsed and was above the horizon. We set out to drive to Goldendale, stopping for a quick supper on the way. It was a beautiful drive over nearly deserted roads with occasional views of
the eclipsing moon over our shoulders but it takes about 2 ½ hours to drive from Hanford to Goldendale. We arrived at 8:30 and the observatory closes at 9 pm. We would have enjoyed spending a bit more time touring the Goldendale observatory but, as it worked out, being late didn’t cause us to miss any observing. Clouds had been coming and going all evening and they hadn’t managed to see anything through the telescope.
LIGO public tours are given on the second Saturday of every month. No reservations are needed and the tours are free. I would recommend this outing for anyone who likes science. Just don’t plan on leaving Hanford and driving to Goldendale in time to do astronomy.