notes from Hanford

LIGO, sitting out here in the desert, staring up into space, listening carefully for a signal -- it reminds me of SETI a little bit, the project that is listening for radio transmissions from other civilizations. Like with SETI, it is unlikely that the current LIGO installation will detect anything; but it will be a big deal if it does. It's actually quite a poor comparison, because we're very sure that Gravitational Waves -- what LIGO is listening for -- exist, and it's merely a manner of building a sufficiently sensitive listening device. Hearing from an honest-to-god ET would be a Much Bigger Deal indeed. Maybe what makes me relate the two projects is the degree to which both are inherently peaceful.

Big Physics has historically been all about accelerators -- machines that you might call "atom smashers" even though they don't smash atoms anymore (they've moved on to much smaller things to smash), and today there are huge accelerator projects (Fermilab near Chicago, CERN near Geneva, SLAC at Stanford) and they are entirely peaceful operations. But the history of accelerator physics is not quite so peaceful. The first big accelerators were invented and built at Berkeley, and with them came the ability to smash atoms together, forming exotic new elements and the field of nuclear science. A young Glenn Seaborg stumbled across Plutonium there, some others stumbled across Fission, and suddenly everyone involved was booking train tickets for Los Alamos and Chicago.

In the American nuclear archipelago, Hanford Site may or may not be the Big Island, but it's certainly the biggest mess.

From http://www.columbiariverkeeper.org/history.htm: In January 1943, Hanford was chosen as a site for the government's top-secret Manhattan Project. The mission was to produce plutonium for the nuclear bomb. It was selected because of it's remoteness, its abundant water for reactor cooling, and its plentiful electricity from hydroelectric dams. In the spring of 1943, 1,200 residents were evacuated from the towns of Hanford, White Bluffs, and Richland. Access was denied to Native Americans who had historically used the lands for hunting, food gathering and religious purposes. The world's first three plutonium production reactors were quickly built with a work force of 51,000. Just 27 months after construction started Hanford-produced plutonium provided the explosive charge for the worlds first nuclear detonation in Alamogordo, New Mexico. Not long after, the Nagasaki bomb was powered by concentrated plutonium manufactured at Hanford.

Weapons-production activities at Hanford came to an end with the end of the Cold War in the 1980's (so recent!). Now the big project at Hanford Site is cleaning up the mess, to the extent that it can be cleaned up, at a monetary cost of ~ $100 billion.

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LIGO has nothing to do with all of this, however, except that it was built here at Hanford. LIGO is a big L-shaped contraption, with each arm of the "L" four kilometers (2.5 miles) in length. Laser beams travel through the L, bouncing off of mirrors at each end. The whole thing is a giant — and very complicated — interferometer, identical in principle to (but vastly larger and more sensitive than) the interferometer that Michelson used to disprove the existence of the Ether. In LIGO the idea is that a passing gravitational wave (coming from a galaxy far, far away) will stretch space, changing the path lengths in the arms of interferometer, resulting in a measurable signal.

The whole thing becomes very complicated because Gravitational Waves are very weak: a 4km arm of LIGO is expected to change in length by 1/1000th of the diameter of a proton!

Why is LIGO built way out here at Hanford? Largely for the same reason the Government chose to put a plutonium factory here: remoteness. With the extreme sensitivity of the LIGO machine, it would be completely overwhelmed by the vibrations from city traffic in a big city. The other reason is just that there is space here to build such a large machine. [I also wonder whether it's important that the other LIGO observatory, at Livingston, LA, be as far away as possible, or have some other relative positioning requirements.]

Driving directions to LIGO: "The key turn-off along this route is unsigned, so we cannot recommend it unless you know the surrounding area well. There is a jog in SR-240 at the North end of Richland. Coming from Richland, you can jog to the left and follow SR-240 about 10 miles out into the countryside. Eventually you will see the Yakima River along the left side of the road, which is your signal that you are getting close. You will see a weir in the river, near where the river takes a sharp bend. This is Horn Rapids. Look for a big green sign just before a four-way intersection. The sign will indicate that SR-240, to Vantage is straight ahead and SR-225 to Benton City is to your left. The sign gives no indication whatsoever that there is a two-lane highway, SR-10 that goes to the right. (This is probably an intentional oversight harking back to the days when Hanford was a secret and very secure site. We recently have gotten assurances from the Washington State Department of Transportation that this road does not exist, although many of our employees drive it every day!)

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Adrian pointed me to a nice anecdote about Wilson, a Director of the Fermilab accelerator. He was asked by Congress how Fermilab would contribute to national defense, so as to justify its cost:

[I]n 1969, when Wilson was in the hot seat testifying before the Congressional Joint Committee on Atomic Energy, Sen. John Pastore demanded to know how a multimillion-dollar particle accelerator improved the security of the country. Wilson said the experimental physics machine had "nothing at all" to do with security, and the senator persisted.

"It has only to do," Wilson told the lawmakers, "with the respect with which we regard one another, the dignity of men, our love of culture. It has to do with: Are we good painters, good sculptors, great poets? I mean all the things we really venerate in our country and are patriotic about. It has nothing to do directly with defending our country except to make it worth defending." (http://www.news.cornell.edu/releases/Jan00/RRWilson_obit.hrs.html)

LIGO Day 1

After a reasonably sumptuous breakfast at the hotel, Adrian and I drove out to the LIGO site. It's really closer than I had expected. The city of Richland, such as it is, peters out and quickly we're out on a straight stretch of 2-lane highway seemingly in the middle of nowhere. Steam is billowing up from somewhere a few miles off the road -- a reactor still running? The snow-covered desert is really quite pretty; the snowy hills blend right into the sky. At a bend in the Yakima river, we turn right onto the road that doesn't exist. A few miles later we're at the LIGO site, with a big sign out front with the LIGO logo.

There's a small collection of buildings at the "corner station" (the elbow of the "L"). One where we check in, where there are some offices too. Another has a tall roll-up door; this is some kind of assembly area. And the third building in the corner station itself. Inside everything is clean and well organised; it seems new.

The first half of the day was mostly the formalities (signing papers, reading safety information) and practicalities (getting keys, being shown around) of being introduced to LIGO, and the dozen-or-so people working on site. The Control Room is like Mission Control, with five projector displays on the wall (only the three forward ones currently in use), showing the current Interferometer status. Overhead, two large digital clocks display the time, one in UTC and one in local time. On two rows of desks there are numerous computer terminals, each with two large flatscreen displays. Almost all SUN Sparc machines. Closed-circuit TV displays show the area surrounding the LIGO center, and others show the LASER spots inside the interferometer. Most of the stations have the usual roller-chairs, but a few large inflated "fitness balls" are available too, lightening the atmosphere a bit.

At lunchtime we attended a meeting of the collaboration. Like at Big Physics labs everywhere, the meeting is done by teleconference on a sort of speaker phone system with PowerPoint slides projected onto the wall. The other LIGO observatory, in Livingston, Louisiana, is on the phone. As are, I think, the collaborators at MIT and Caltech. The meeting is, as meetings tend to be, very boring. They are planning when to do some big upgrades, planning for the next several years.

Daniel took Adrian and I into the clean-room environment that houses the corner station to get us acquainted. There's an automatic boot scrubber before we enter the 'garb' room, and then we put on booties and LASER safety goggles. Earlier Adrian had drawn for me a schematic of the LIGO observatory. The input laser is only a few watts, but the arms of the interferometer form resonant cavities, which build up a laser power on the order of kilowatts. The laser is infrared. We have proximity cards to enter the cleanroom area. We enter one-by-one, and the computer notes that we're now each "inside". Entering an area without using the proximity cards activates interlocks and automatically shuts down the lasers. The safety system is to protect people from lasers, and the cleanroom system is to protect the ultra-high-vacuum system and the clean optics from the dirt that rides in on people.

Once inside the clean area, the vacuum system of the corner station is visible. The beamlines are housed in a vacuum pipe about 1m in diameter. The optics systems are housed in large metal cylinders along the beamline that look like giant tin cans welded into the system. Beside the beamline are the usual rackmount systems familiar to any physics installation: crates containing VMW-bus backplanes, with cards sprouting patch cables. Analog signals are routed into digitizers. The digital signals are eventually stored in a sort of distributed shared memory, which is reflected between machines using gigabit ethernet over optical fiber. The air is clean and it's easy to breathe.

Daniel showed us around the electronics and we set about our mission. For various reasons, there is an interest in looking at a signal at 37.5 kHz -- noise tends to cancel out at this frequency, so it might be a good place to hunt for a signal. To gain access to this signal, software digital signal processing is used to shift this signal down to baseband, so that it can be stored at a reasonable sample rate. Our task is to verify the frequency response of some "whitening" (equalizer) boards, connect signals from the interferometer output photodiodes to this board, make sure the data acquisition (DAQ) is working properly, record data from this frequency range, and then look at it. The DAQ system seems to run on VxWorks (I believe the same OS that ran on the Mars rover).

I really have to brush up on my signal processing... FIR and IIR filters, Butterworth filter design, complex baseband signal representations / quadrature, control systems design. LIGO is really all about control loops: The main operating mode of the interferometer is to keep the interferometer output locked at a "dark fringe" (no light output; total destructive interference) by using actuators to move the mirrors; and the feedback signals to these actuators becomes the output signal of the detector. There are several resonant cavities, most of which are kept resonant by active control systems.

In between times, Adrian explains all about how LIGO works, and what we will be doing. ("And tomorrow, Dark Energy," he says. I don't know what Dark Energy has to do with anything...)