LIGO diary--report #4

by Wm. Robert Johnston
last updated 2 October 2001

Monday, 16 July 2001: This report is mostly just odds and ends. Tomorrow we will resume efforts to isolate the source of the RF noise. Dr. Ray Weiss from MIT is currently here at the site; in many ways this entire project is his brainchild. At the daily meeting this morning he discussed details of the seismic noise problem at the site here:

Very minute vibrations, most likely from various human activities in the area (and mostly in the daytime), are causing over ten times as much noise here at the Louisiana site that at the LIGO site in Washington. The greatest amount of vibration is for the mirror at the end of the Y-arm (the south end). This is the part closest to Highway 190 and I-12, and it is plausible but not confirmed that this traffic is a source of the problem. Another possible source is logging activities. Whatever the source (or sources) it could be 10 miles from the site (to give you an idea of how far these tiny vibrations carry).

Whatever it is, the result--if not corrected--would be that on average about once an hour, any science data collecting would be cut off by a vibration that causes them to "lose lock." This means that one or more mirrors shake so much that the electronics systems no longer have their position finely controlled. Regaining lock is a time-consuming procedure. Additionally, this amount of noise is a serious problem for science data gathering in general.

The current suggestion is to strengthen the system that fine-tunes the positions of the mirrors so that it can handle these bigger "shakes." This whole issue is one of the major activities right now. Others include the ongoing preparations to get the system adjusted and reassembled for a "mock data challenge" this fall. This will be a practice run of the software that processes data, as well as a test for all the hardware.

Some of you have had questions regarding prior reports. Please let me know of things that require further explanation. These reports will form the basis of an explanation of LIGO to go on my web site, with high school students and other laypersons being the target audience.

For example, the description in report #2 of the layout is admittedly hard to follow without a diagram:

1 = laser
2 = mirrors/optics to shape laser beam
3 = beam splitter mirror
4 = recycling mirror (one for each arm)
5 = end station mirror (one for each arm)
6 = light detector
between 1 and 3, above laser path = where I'm working

Path of laser light: 1 to 2 to 3 to 4 (both) to 5 (both) to 4 (both) to 3 to 6

Parts 1-4 and 6 are all in the LVEA, in the corner station building. The two paths from 4 to 5 are the 4-kilometer arms. The laser beam is actually trapped between mirrors 4 and 5 in each arm an average of 50 back-and-forth trips before getting back to 3 to 6. The area where I have been checking for RF signals is beside the laser path where the word "here" is. Note that these are the main mirrors: if you count all the lesser mirrors and lenses, I think they number in the dozens.

Someone asked, "how do you split a laser beam?" Try this: take a flashlight and shine it straight at a window. Some of the light goes straight through, but you'll notice that some is reflected back at you. You just split a flashlight beam!

Now shine the flashlight at a 45-degree angle to the window pane. Some of the light continues in the same direction as before on the other side of the window (look for it). Some of the light is reflected on your side of the glass, in a direction at a right angle to the direction you're shining the flashlight (look for it). You have now split the flashlight beam into two beams which are perpendicular to each other. The beam splitter mirror does what the glass did: it is positioned at a 45-degree angle, reflecting one beam to the side and transmitting another beam through in the original direction. (Can we say, "It's all done with mirrors"?)

Also, in report #2 I gave links to two pictures. You can go to to browse the photo collection.

No rain this weekend. I continued my pedestrian explorations of Hammond. Hammond had a population of 17,639 according to last year's census. The city's growth picked up when factories were built in 1861 to produce materials for Confederate soldiers. Southeastern Louisiana University was started about 1926; it is at the north end of the city, and that is where our apartments are. The university has about 15,000 students during the school year, most of whom commute from Baton Rouge and other area cities. There is relatively little commercial development around the campus, however; the north half of the city is mostly residential, with quiet tree-lined streets. They have one mall at the south end of town, near I-12.

Hammond seems to have many churches. Both Sundays to date I went to the morning service at First Baptist Church of Hammond, in the middle of town. They have an attractive building, average attendance a bit over one hundred, and a pastor within a couple of weeks of completing a doctoral dissertation. I met several people in sunday school who had had contact with LIGO, either through SLU, through involvement in the construction, or have interest in the research.

Image credits: Wm. Robert Johnston, © 2001 (all).

© 2001 by Wm. Robert Johnston.
Last modified 2 October 2001.
Return to Home. Return to Relativistic physics.