Operational milestone scheduled for LIGO

Scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) will establish "first lock" of the Hanford, WA facility's lasers on October 20.

LIGO, a joint project of MIT and the California Institute of Technology, is designed to capture faint gravitational waves from distant sources in the universe.

The operational milestone will mark the first time that the LIGO detector at Hanford will have simultaneously sent laser light back and forth along its two arms, although full operation to detect gravitational waves is not expected until 2002. The laser light shooting down and back the one-and-a-quarter mile arms achieves the delicate optical interference that will make the detection of gravitational waves possible. This feat, dubbed "first lock" or "locking the interferometer," is similar to the "first light" of a newly commissioned telescope.

LIGO is being built as a national research facility for detecting gravitational waves in the universe. Funded by the National Science Foundation, LIGO comprises three detectors in the United States -- two at Hanford and one near Livingston, LA. The detectors will work in concert to detect gravitational waves, which are distortions of space-time caused by accelerating masses, such as exploding stars or vibrating black holes.

The LIGO detectors are set up in such a way that the very slight distortions of space-time in the vicinity of the detector's arms will cause perpendicular laser beams to go out of phase. Two observatory sites hundreds of miles apart are necessary to get a direction for the event causing the gravitational waves, and also to ensure that the signals come from space and are not a local phenomenon.

To reach maximum sensitivity, LIGO employs a sophisticated computer-based control system to hold mirrors at the ends of the two arms in their proper locations with subatomic precision, while bouncing a laser beam back and forth between them. This "locking" of the interferometer will be the initial full test of the first of three similar detectors being constructed by LIGO.

"This achievement brings us an important step closer to our real goal -- LIGO's first gravitational-wave observations," said Barry Barish, the Linde Professor of Physics at Caltech and LIGO director.

"First lock is a step toward bringing the apparatus to its full operating potential, but still some distance from the beginning of the scientific investigations that will ultimately come about," said Gary Sanders, deputy director of LIGO.


The LIGO detector comprises mirrors suspended in vacuum on fine wires at the corner and end of a long "L." A highly stable laser beam is split, the two halves are sent back and forth about 100 times between the mirrors on the two arms, and then the beams are recombined. A passing gravitational wave will cause very small motions of the mirrors at the ends of the L, which scientists will observe by the changes they cause in the amplitude of the recombined light.

"The challenge is that the predicted motions of the mirrors due to even the strongest gravitational waves are incredibly small, about 10 billionths of the diameter of an atom," explained Rainer Weiss, professor of physics at MIT. Professor Weiss initially proposed building such a detector in 1973, and has worked to that end ever since.

"The detectors require extreme sensitivity to measure such motions, as well as to eliminate all other possible sources of disturbance to the mirrors," he said.


Gravitational waves are an important prediction of Einstein's theory of general relativity. They travel at the speed of light, but are very different from the more familiar electromagnetic waves: light, radio waves and X-rays.

Gravitational waves are a periodic distortion of space itself, expanding distances along one arm while shrinking them along the other, then half a cycle later, shrinking the first arm while expanding the second. The waves that LIGO seeks are created by rapid accelerations of very massive astrophysical objects.

"Many of the sources of gravitational waves that LIGO may detect are difficult or even impossible to study using the familiar electromagnetic spectrum," said Kip Thorne, the Feynman Professor of Theoretical Physics at Caltech and the leading theorist studying gravitational-wave sources. "These include black holes, neutron stars, and possibly even the Big Bang."

The search for gravitational waves has generated worldwide interest. Detectors are being built by the Japanese (the TAMA detector), by Italian and French collaborators (the Virgo project), and by German and British collaborators (the GEO600 project). Also, an Australian consortium has proposed a Southern Hemisphere detector.

"As important as this milestone is, there is still a great deal more to do," emphasized Stan Whitcomb, director of commissioning for LIGO. "The detector control systems must be carefully characterized and tuned to achieve maximum sensitivity and reliable operation. And, of course, this is just the first of three interferometers that we have to commission."

Commissioning of the LIGO detectors will continue through the remainder of 2000 and 2001. Short periods of operation to test different aspects of their operation (engineering runs) will be alternated with installation and commissioning. Full operation to detect gravitational waves will commence at the beginning of 2002.

A version of this article appeared in MIT Tech Talk on October 18, 2000.

Topics: Physics

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