Spring 2003

Assistant Professor of Physics Steven Penn works on developing gravity wave detectors in the basement of Eaton Hall

by Catherine Williams

When Assistant Professor of Physics Steven Penn looks at the night sky, he sees the same landscape of gently twinkling lights as the rest of us - constellations like the big dipper or Orion. Penn, though, can envision another topography, this one of the cataclysmic events that produce those lights, and even more energetic events, some of which don't produce any light at all - a black hole gobbling another black hole… an asymmetric supernova explosion… the collision of binary neutron stars. And now Penn is involved in an effort to do more than envision these events, but to detect them directly. Penn joined the Hobart and William Smith faculty in 2002 fresh from a Ph.D. at MIT and two postdoctoral positions at the University of Washington and Syracuse University. He is a member of a collaboration of scientists from places like MIT, CalTech and Stanford who have conceived of a new kind of observatory that may reveal the nature of our universe, and possibly confirm a century's worth of scientific hypotheses. Some of the research behind this new observatory is happening in the basement of Eaton Hall under the direction of Penn. To understand Penn's work, you first have to understand a bit about the nature of gravity.

Gravity

Imagine that all the stars in the sky are connected by a complex crochet of strings. The slightest movement of a sun, the tiniest pull of an asteroid and the entire framework flutters. The larger the tug - the collapse of a supergiant star or the collision of two neutron stars merging into an enormous black hole - the greater the radiating disturbance. According to Albert Einstein's General Theory of Relativity, those invisible strings weave together and form the fabric of what physicists call spacetime. The analogy most often used to explain the phenomenon is to picture a trampoline with a bowling ball in the center. The mass of the bowling ball stretches and curves the trampoline so that when you place a golf ball at the edge, it rolls to the center. According to Einstein, smaller objects are not attracted to larger objects via some mysterious force (as Sir Isaac Newton proposed), but instead smaller objects travel through space that is distorted by larger objects.

Those ripples in spacetime, the cosmic aftershocks of monumental space events, are called gravitational waves. Traveling at the speed of light, their power fades the further away from their source they get. But unlike light, which follows the valleys and mountains of the curvature of space, gravitational waves move through and bend space itself. They are potential sources of information about all sorts of mysteries including black holes, gravitons, neutron stars, and possibly even the birth of the universe.

The Observatory

Physicists are reasonably certain that gravitational waves exist. General Relativity, one of the most thoroughly tested theories in physics, predicts their existence. The Nobel Prize was even awarded in 1993 to two physicists who showed that the changing period of a binary pulsar was exactly what one would expect if there are gravity waves. Yet despite this evidence, no one has yet been able to measure these waves directly. Detecting gravity waves is difficult for two reasons. First, while gravity may seem quite strong to us, it is by far the weakest of all forces in nature. In addition, the Universe is so vast that when a cataclysmic event does produce gravity waves, by the time these waves reach the earth, their rippling effect - their ability to alter spacetime - is infinitesimal.

In response, a group of scientists, including Penn, have designed and built an observatory capable of measuring tiny fluctuations in spacetime. With funding provided by the National Science Foundation, these scientists have created the Laser Interferometer Gravitational-Wave Observatory (LIGO).

The LIGO facility is a huge building in the shape of the letter "L" with each arm of the "L" spanning 4 kilometers (roughly 2.5 miles). A laser beam enters the corner of the "L" and is split in two sending a beam up each of the 4 km-long corridors. At the end of each arm of the "L," a reflective mirror made of fused silica bounces the laser beam back to the corner. If a gravitational wave passes through the observatory, the arms will oscillate, stretching and contracting out of phase with each other, thus generating a mismatch in the recombined laser beam and thereby signaling a disturbance in spacetime. The shift LIGO measures is extremely small. Over a length of 4 km, scientists hope to measure a shift of 10-18 meters (a billionth, billionth meter). This feat is similar to measuring the distance from the earth to the sun with an accuracy of one atom. There are observatories at two widely separated sites (one in Louisiana and the other in Washington) to allow scientists to better see the direction of the gravity wave. Having two distant sites also lowers noise. Natural events, like seismic disturbances, ocean waves, land tides and lightening strikes have the potential to jiggle LIGO, but when these things happen at one site, they won't be happening at the other. Any single gravity wave, however, will pass through both sites, enabling scientists to distinguish a gravity wave from noise. Other observatories are being constructed all over the globe and plans are even being made to build a space version.

In a project as large and as important as LIGO, there are dozens of challenges that require researchers to push the boundaries of known science. One of the most pressing though is the mirror at the end of each LIGO arm. "How can we measure gravity waves," asks Penn, "when the thermally induced vibrations in the surface of a test mass mirror have an amplitude one million times greater than the distances we want to measure?"

So Penn and others have dedicated their research to creating mirror masses with higher and higher quality factors. Starting this summer, Penn hopes to involve HWS students in this research. "Students will be measuring the mechanical loss, called 'thermal noise,' in optical and mirror coating materials," he explains. "They also will be refining methods for reducing the loss. These are uncharted waters. No one knows the fundamental loss mechanisms in fused silica."

"This is an extraordinary opportunity for HWS students to be a part of an experiment that will change the face of physics and astronomy," says Professor Donald Spector, chair of the physics department. "We hope to build on the momentum of this project by continuing to grow the physics department. Certainly, if LIGO detects gravitational waves, it will receive the Nobel Prize."