Monday, 15 September 2008


As promised, a report on my trip to LIGO Hanford Observatory (1:30pm, second Saturday of the month, entry free, approx. 2.5 hours). Our tour opened with an explanatory movie, the gist of which I'll relate for clarity. LIGO (Laser Interferometer Gravitational Observatory) is an experiment to detect gravity waves, the shadowy warpings of space and time given out when massive bodies interact. The textbook example is that of binary neutron stars, pairs of absurdly massive dead stars which circle each other, losing energy by emitting gravity waves until they collide in a final burst of energy. They hope that by measuring the gravity waves from sorts of events, in combination with conventional electromagnetic (light, radio, x-ray) signals, they can get a better understanding of how gravity works on a large scale. The oh-so-topical Large Hadron Collider is chasing gravity too, from the opposite end (the tiny subatomic interactions which give rise to gravity in the first place).

Each of LIGO's observatories consists of a pair of 4km arms, meeting at one end to form a right angle. These form a gigantic Michelson interferometer. A single laser beam is generated at the intersection, split, and sent out along the arms. The lasers reflect off mirrors, and return to the intersection, then are recombined and sent to a detector. If the arms are of equal length, then the recombined beams interfere to zero. If the arms are different lengths, then they fail to interfere properly and there's an appreciable signal. A passing gravity wave distorts space-time so that the arms are of different lengths (or equivalently, time passes at different rates) and thus there's a measurable signal. (CalTech's own info page.)

These are impressively sensitive instruments. After the film, our tour guide demonstrated an interferometer maybe a foot across, tugging on the metal frame with a string. The minute warping of the apparatus caused the laser pointer to fade in and out, while more delicate motions, such as nearby footsteps, were made plain by connecting up a light sensor in the laser's path to a loudspeaker. By scaling up the instrument, even more minute changes can be picked up, an elegant bit of "big science". In principle, LIGO is sensitive down to less than the radius of a proton.

Like the footsteps of our guide, any number of mundane things can change the lengths of the arms, or just wiggle the mirrors around, and our tour seemed to revolve around their efforts to wipe out this unwanted noise. First and foremost, there are two LIGO observatories, one in Hanford in Washington state, the northwest of the United States, and one in Livingston, Louisiana in the southeast of the United States. Therefore any signal detected at one and missed at the other can be assumed to be background noise. (Another upshot is that the two detectors are in slightly different planes in space, so the relative strengths of the two signals can be used to triangulate the source of a gravity wave, at least in principle.)

Secondly, and perhaps most obviously from my perspective as a visitor, they built one of the observatories at Hanford. As my earlier post relates, Hanford lent itself to the atomic weapons program due to its utter isolation (and water supply, of course). It took us about twenty unnerving minutes of driving in the nuclear reservation to get out there. This helps to cut out traffic noise. Presumably out of necessity rather than anything else, the Louisiana counterpart detector is surrounded by logging, and unlike the Washington sands, the local ground carries vibrations of just the right frequencies to mess with the mirrors' fancier stabilising equipment.

That's where things get less elegant, more ingenious. The movie related that the mirrors hang from wires, which attach to vibration-isolated platforms, and sets of teeny electromagnets can prod the mirrors back and forth to counterbalance any unwanted wiggles. The control systems are designed to anticipate certain kinds of wobbles and damp them out. Another example of the device's sensitivity: the atoms in the suspension wires jiggle around because of their own heat, making the wires vibrate at their resonant frequencies like violin strings. On the opposite end of the scale, they brought in a freshman to write an application which would automatically account for the one-foot tidal warping of the Earth's surface as the sun and moon circle overhead.

This opening preamble was probably a little long for my taste, although it was a great chance to quiz a member of the LIGO staff. Now we got a wander across the site to the intersection of the beams themselves. Did I mention that we're in the desert? And the arms are 4km long? As much as I would've liked to see the mirrors up close, that's a hell of a hike, and the drop-in tour doesn't go there. Our tour guide was happy to relate anecdotes and field even more questions, in spite of the weather. Honestly, we were suffering a little by the end. Come in the winter, if you insist on the afternoon drop-in like we did.

Our tour concluded in the control room, an appealingly dim-lit NORAD-esque chamber, ablink with readouts and spectra from LIGO. There aren't any genuine signals yet. At its current 25 million-light-year range (bigger than the entire local group of galaxies), they're only expecting to see colliding neutron stars every ten years, give or take, but upgrades set for the next couple of decades should expand its range tenfold, raising the frequency of detections a thousand times (the volume covered goes up with the cube of range), so that certain events should be detected daily. To be honest, I think LIGO isn't an operation instrument yet, and it's in a prolonged setup phase until these upgrades are complete. Until then, they've certainly built the world's most sensitive seismometers, as we got to watch individual trucks popping up as blips on the control room readouts.

Where next? The obvious way to escape all those rumbles is to put the detectors in space. That's the idea behind LISA, whose arms will be links between satellites in orbit of the sun. This will allow them to study gravitational waves of frequencies which are inescapably concealed by noise on the Earth. I wish them all the best of luck. As for the tour, it was perhaps a little too limited in terms of what we got to see on the site, and there was much repetition between the tour and the video, but if you're keen to ask questions (or one of the other visitors is), there's a lot to learn. (The tour guide, if anything, was too keen to talk, apparently oblivious to the blasting heat.) And it's not every day you get to clamber around on top of a scientific tool. (My fiance was disappointed when it turned out there was not actually any lego, though.)

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