Carleton Professor, Students Research Gravitational Waves with LIGO

Associate Professor of Physics Nelson Christensen has spent the last 20 years chasing waves. Now, his search may be nearing an end.

16 September 2003
Artist's conception of gravitational waves.
Artist's conception of gravitational waves.Photo:

Associate Professor of Physics Nelson Christensen has spent the last 20 years chasing waves. Now, his search may be nearing an end.

Recent advances in technology have for the first time allowed scientists to attempt direct detection of gravitational waves. Though the waves—ripples in the fabric of space and time—have proved an elusive quarry, the payoffs of persistence are considerable: a confirmed detection would secure some of Einstein’s most important predictions and pave the way for even more stringent tests of general relativity.

Unfortunately, the effects of gravitational waves are difficult to measure. The foremost detector, the Laser Interferometer Gravitational-Wave Observatory (LIGO), is the most sensitive observatory to date. But its task remains daunting—even the most violent cosmic collision only produces a wave that squeezes and releases the Earth by less than the width of an atom.

As a result, the technology must be fine-tuned to an exquisite level of sensitivity, and this summer, Carleton seniors Adam Libson (Rosemont, Pa.) and David Steussy (Indianapolis, Ind.) joined Christensen in a LIGO team working to that end.

“This is the way we expose students to real science,” Christensen said. “We can and do involve undergrads in the research and that is critical. They don’t have to go on in gravitational wave physics for this to be successful . . . but it’s this exposure to real research that gives them skills they can carry on.”

Gravitational waves are caused by cataclysmic events and were first predicted by Einstein in 1916. The idea may seem complicated to non-physicists, but in fact, gravity wave behavior is similar to that of electromagnetic waves—in other words, the light that we see every day. Just as accelerating charged particles produces light waves, gravitational waves are thought to come from huge, accelerating masses. The moving masses create a traveling gravity field, and it is this minute oscillation that is known as a gravity wave.

LIGO’s bid for a direct detection is promising enough to have garnered an international collaboration of more than 400 scientists. In addition, similar detectors are being initialized in several European countries and in Japan, and plans are underway for the projects to merge as one organization. This kind of cooperation is a necessity, Christensen said. “It is critical for us to detect something and believe it . . . everyone has to play together. You can’t really talk about competition.”

Carleton was probably the first liberal arts college to join LIGO’s collaboration, Christensen said, which was otherwise composed of large research universities. But as the project has gained momentum it has attracted several other institutions of Carleton’s size and type. Libson and Steussy, whose work Christensen calls “very advanced,” are enthusiastic about the opportunity to be involved with such significant research as undergraduates. “The signal processing techniques I’m learning here could be pretty useful in just about any branch of physics,” Steussy said.

Libson, who is considering pursuing a career in gravitational wave research, worked with Christensen last summer as well. “Gravitational radiation really fascinates me and this is a really exciting time to be involved with it because of the LIGO project,” he said.

The students’ data analysis requires painstaking attention to detail, a product of the experiment’s design. The observatory uses lasers as measuring instruments, recording the minute movements of four hanging masses which are suspended, two each, in an L-shaped structure of long, low tubes. A gravitational wave passing through the earth should shrink the distance between the masses in one arm of the L and stretch that distance in the other arm. In that event the two returning laser beams would be out of phase, creating an interference pattern that would reflect the gravity wave’s passing.

The tiny, precise measurements are vulnerable even to long-distance tremors. “Anything that will make these masses move will create a signal . . . the ground shakes, electrical systems glitch, thunderstorms happen and lightening strikes, there are big discharges of magnetic fields,” Christensen said. Culprits in the past have included local logging activity, passing freight trains, and even heavy surf pounding against ocean shores miles away.

To compensate, LIGO tracks thousands of seismic events and other environmental factors which might affect readings. Libson and Steussy have spent their summer days sorting through this massive collection of data, developing increasingly refined ways to extract the signal from the noise.

“In experimental physics you need to know a lot about a lot of different things. You need to be able to build the detector, you need to understand your environment,” Christensen said. “So in a sense you have to act like an electrical engineer and a geologist and a mechanical engineer. Then you have to know how to deal with the data, and so you have to have the statistics down.”

“And none of it’s easy. It’s all very difficult, but it’s all very interesting,” he said.