• LIGO is crazy, and other stuff

    Posted on June 22nd, 2009 Isaac Angert 1 comment

    Greetings from the deserts of the Pacific Northwest! Yes, I said deserts. Washington State has the reputation of being a very wet and rainy place, so you wouldn’t think that there’d be any deserts anywhere near it, much less in the state itself. Eastern Washington however, is very dry. As they say here, the Seattle area magically steals all the rain…but it’s no magic, just mountains and a bit of thermodynamics!

    I arrived here in Richland, WA (my REU town) after two weeks and one day on the road. I drove ~9,000 miles total and my truck really needs an oil change. I visited the Grand Canyon, Yellowstone, the Redwoods and Olympic National Park among other things. I took few pictures of the bigger tourist sights (eg old faithful, Grand Canyon)–they are some of the most photographed places in the world after all. I will mention what the pictures don’t tell you: old faithful smells like a giant fart because of the sulfur. It was overall, a really great trip. I really enjoyed the scenery in the Northwest, but Wyoming gets the overall top spot for favorite place.

    I’m writing after my first full week here at LIGO. So far we’re working on settling in and getting the necessary paperwork done. I’ve been given a look at my project: mostly I will be building electronic equipment. I was handed a stack of circuit diagrams that I need to assemble. At this point I need to give a big thank-you to Dr. Goggin’s electronics class, without which I would be pretty lost right now.

    The tumbleweeds around here are quite a sight. For the benefit of those of you who are, like me, from back east: a tumbleweed is a sagebrush plant that has died and been dried out. Eventually the stalk breaks in the wind and the whole plant blows across the desert, often for hundreds of miles. The sagebrush grow fairly round, facilitating their tumbling. They range from about the size of a softball to larger than a beachball. Cars on the highway that are traveling directly into the wind often collect a large pile of tumbelweeds on their grilles. Overall I must say that I find them quite comical–there’s just something about how they bounce across the desert…

    It’s very dry out here. The sky one day was brown from blowing dust and they have signs along the road telling us that it’s illegal to throw burning material out of your car. We were notified one day over the PA system that there was a brush fire along the main road that runs between the site and the town, and that we all should take an alternate route home. This of course prompted stories from the staff of brush fires, which apparently turn tumbleweeds into hurtling balls of flame.

    LIGO itself is pretty amazing. For those who need an introduction, LIGO is a giant michelson interferometer with 4km long arms. It was built as part of an attempt to detect gravity waves originating from distant, energetic cosmic events. A passing gravity wave moves the mirrors in the interferometer, causing a detectable change in the output light. The difficulty in the whole project is that we expect (based on the predictions of general relativity) that a passing gravitational wave will deform the distance between the mirrors in the 4km arms by about 10^-18 or 10^-19 meters depending on the strength of the source and its distance from earth. What is 10^-19 meters? well, it’s about a billion times smaller than the diameter of an atom or a thousand times smaller than the diameter of an atomic nucleus. Insane.

    Most of the work done here at LIGO is focused on eliminating sources of noise in the interferometer. To a large degree the entire LIGO project is a massive exercise in control theory. The complexity of the systems used to keep out external effects is amazing. The equipment is first built on its own concrete slab that is physically separated from the main foundation and rest of the building. Things like the HVAC system are even located a good distance away from the interferometer to reduce vibrations. The optics are isolated from the ground by pendulums so that a vibration in the ground will be damped out. The pendulum isolation system is only effective for higher frequency motions (> ~1Hz). Even though LIGO is not trying to detect gravity waves with such low frequency, it is still necessary to counter low frequency motions so that the arms are maintained at a resonant length for the laser light inside (the interferometer is “locked”). Effects such as tidal stretching of the ground and thermal expansion during the day are some of the main causes of low frequency motion of the mirrors. In fact, the effect of tidal forces is about 200um over the 4km length of the arms–a big deal when you are trying to detect motion on the order of 10^-19m! Low frequency motions are compensated for by servo motors that move the mirrors to keep them at the correct distance.

    Oh ya…we have really big lasers. LIGO is currently using a 30W 1064nm laser, which will be upgraded to a 200W laser as part of the Advanced LIGO project’s improvements. The quantum mechanical signal to noise ratio goes as N/Sqrt(N) where N is the number of photons, so increasing the power will decrease this type of noise. Since LIGO is a power recycled interferometer, the amount of power bouncing around in the arms can be in the neighborhood of 10,000-100,000W. This massive amount of light causes thermal heating of the mirrors, which deforms them and changes their optical properties. To keep the mirrors functional, a secondary laser system was built with the sole purpose of heating the mirrors in a pattern that will compensate for the thermal deformation by the main laser. For example, the second laser might by aimed into an annular pattern around the main beamspot. (in keeping with the philosophy that any problem caused by lasers can be solved with more lasers.)

    I think I’ve rambled enough, sorry for the length of this post! I do have to give a big thanks to Ian Noble at ISU who was nice enough to give me a place to sleep and a free meal when I drove through Pocatello, ID. Also Ian, I passed a sign when I got out here that said I was entering the most potato producingest county in America, and I’ve been told that the area here produces more potatoes than even Idaho…HA!

    Here’s some pics from the latter part of my road trip: (thumbnailed this time!)

    The Grand Canyon:

    Hoover Dam:

    The Olympic Mountains in Washington state:

    The Oregon Coast:

    The snowblower was very colorful:

    Mountains in southern Colorado:

    Mt St. Helens:

    A buffalo on the road in Yellowstone National Park:


    1 responses to “LIGO is crazy, and other stuff” RSS icon

    • Hardy har har Isaac. Sure, that county may grow more potatoes that any SINGLE county in Idaho, but Idaho grows about 20,000 more potatoes (units of hundredweight-specific weight of potato farmers?) than Washington. That is why our license plates say Idaho-famous potatoes.

      You won the battle, but Idaho wins the war.
      Also, your REU seems absolutely perfect for you.

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