by Wm. Robert Johnston
last updated 1 August 2001
LIGO is Laser Interferometry Gravitational Wave Observatory. It is part of a $360 million NSF project to detect gravitational waves. The one I am at is about 7 kilometers north of Livingston, Louisiana (between Baton Rouge and Hammond).
Einstein's theory of general relativity describes space and time as being inseperable "space-time"; further, it says that what we call gravity is actually a curvature in this space-time caused by matter, like the Earth. This theory has been confirmed by a variety of experiments. The theory also calls for the existence of such things as black holes and gravitational waves. Gravitational waves (GWs) are ripples in space-time and can come from things like black holes and/or neutron stars spiralling into each other, from the formation of black holes, from rotation of neutron stars, and other things.
GWs produce slight (for us) distortions in space. They are hard to detect because they don't interact with matter. The calculated effect of GWs on Earth from sources like those in the above is stretching and compressing of space by about one part in one trillion billion. I have an illustration of this here. (So how do we know they exist? A pair of neutron stars was discovered in 1974 and are observed to be spiralling in towards each other. This means they are losing energy, and the amount of loss is exactly what Einstein's theory predicts they would lose to GWs.)
LIGO will begin attempting to detect GWs next year. The LIGO observatory consists of two tubes, each 4 kilometers long, connected in the shape of an "L". A laser beam generated at the corner of the "L" is split, sent down each arm, and trapped in each arm between two mirrors. The two separate beams are then compared. A GW will produce an unequal change in length of the two arms, which should (in principle) be revealed by that comparison.
In practice, the challenge is that the expected change in length is very small--over the 4 kilometer arm length, the length change is perhaps one-one thousandth of the diameter of the nucleus of an atom. The facility is at the frontiers of technology to try to isolate such a small signal. The beam tubes are pumped down to one of the best vacuums on Earth; the mirrors are suspected in a complex suspension system to eliminate stray vibrations. LIGO actually will use two sites (Louisiana and Washington); signals from the two sites will be cross checked. Thus, when both sites show the same jiggle you can be more confident that it was a GW--as opposed to the vibration from a semi-truck hitting the bumps on I-12, 11 kilometers away, or from a stray air molecule bumping one of the mirrors. They will also use some complex computer programs to separate a real GW signal from all this stray noise.
This last part in particular is primarily where UT-B's physics department is involved. I'm one of four students from UT-B that are getting to visit this summer with the professors from UT-B. We each get put to work on whatever project where they need workers. For example, the other student still here (Doug) is helping measure vibrations at the end of one arm of the "L" where they think trucks on a road one mile away are producing vibrations.
It seems the plan is for me to start helping on one task next week--I don't understand it yet, but this is how I understand it now: the mirrors at the corner and the two ends of the "L" have to be maintained perfectly in length. Besides the elaborate suspension system, there is a feedback system to actively adjust the lengths. Alongside the laser is a radio transmission which is used to monitor the length; if the mirrors move out of position, the radio signal is used in the feedback system to direct tiny servos (electromagnets) to nudge the mirror back into position. Now somewhere around the laser there are some stray radio frequency emissions that are interfering with the signals in the feedback system. What they want to have me do is map the stray emissions around the laser, so they can hopefully track down the source and fix the problem.
Now anyone in the room around the laser, for regulatory and safety reasons, has to have an elaborate eye exam before being allowed there and after leaving employment at the facility. (This lets them document whether your eye was really effected.) The soonest I could be scheduled for the exam is Monday the 9th, so in the meantime I'm mostly doing reading here trying to understand what's going on.
Some misc. notes: we are staying at apartments run by Louisiana State University, 43 kilometers away in Hammond. The apartments were built in the last few years and are nice. The University seems nearly shut down for summer. Weather has been nice; partly cloudy and sparsely scattered thunderstorms the four days I've been here.
Friday, 6 July 2001: An interesting issue raised at yesterday's daily meeting was that a segment of I-12 just west of Livingston is irregular but is not due to be resurfaced by the State of Louisiana for a few years; discussion strayed to whether they could be pursuaded to hasten the repairs, thus eliminating this potential source of "noise" for the LIGO detector. Just imagine, getting your highway repaired because of the search for gravitational waves!
Image credits: LIGO/Caltech, 2000 (top), Wm. Robert Johnston, © 2001 (middle/bottom)
© 2001 by Wm. Robert Johnston.
Last modified 1 August 2001.
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