In 1916, Einstein predicted the existence of gravitational waves in his general theory of relativity. However, due to technological limitations, the existence of gravitational waves has been inferred only through indirect observations. Scientists hope to change that with the LIGO facility at Hanford, Washington.
LIGO (Laser Interferometer Gravitational Wave Observatory) began as a joint project of the California Institute of Technology and the Massachusetts Institute of Technology, and is largely funded by a grant of nearly $400 million from the National Science Foundation. In March 2002, Washington State University was approved for membership in the LIGO Scientific Collaboration. WSU assistant professor of physics Sukanta Bose works on locating cosmological sources for gravitational waves, and is formulating strategies for detecting them. Assistant professor of physics Guy Worthey will use LIGO data for his research on the evolution and the population of stars. In all, nearly 400 scientists are involved with LIGO worldwide.
In his general theory of relativity, Einstein theorized that moving objects warp and curve “space-time.” According to Worthey, the theory predicts that cataclysmic events like the explosion of neutron stars or the birth of a black holes generate gravitational waves which radiate outward in every direction.
Moving at the speed of light, the waves progress by expanding and contracting distances on an extremely small scale. Unlike radio, X-ray, and other electromagnetic waves, passing through matter does not weaken gravitational waves. Their strength diminishes only with distance, and most have traveled millions of light years before reaching Earth. Bose says even a strong wave would alter a segment of space the length of the Earth’s diameter by a factor of only 10-17, which is a change as small as a hydrogen atom’s diameter. But it is this expanding and contracting of distance that LIGO scientists are trying to detect.
Since the fractional change in distance caused by a gravitational wave is more detectable over increasing distances, a laser interferometer’s sensitivity depends largely on its size; the interferometer at Hanford is one of the world’s largest. LIGO consists of two pipes, each four feet in diameter and each 2.5 miles long. The pipes connect at a 90-degree angle at the observatory’s control center, which houses a precision laser. Upon striking a beam splitter, equal portions of the laser beam are directed into the two pipes. At the end of each pipe, the laser beams strike a mirror and bounce back toward the beam splitter. In this way, says Worthey, the beams can bounce back and forth between the mirrors and beam splitter a hundred or more times per millisecond. When a gravitational wave passes through LIGO, it will cause the distances between the mirrors and beam splitter to alternately lengthen and shorten. The differences in the two lengths will become noticeable when the two separate laser beams are combined, producing an oscillating pattern of light. Photodiodes, which turn light into electric current, will then convert the pattern of light into an electric wave. In this fashion, LIGO scientists hope to detect and study gravitational waves that would change the distance between the Earth and the sun by only a millionth of a meter, says Bose.
But LIGO will do much more than confirm another prediction of Einstein’s general theory of relativity. Gravitational waves will enable us to look at the universe in an entirely new way. To date, the universe has been explored only through the use of optical, radio, infrared, and X-ray telescopes, each of which has revealed something new. Because gravitational waves are drastically different from electromagnetic waves, Bose says, they will enable us to understand the universe from yet another perspective–to gather new information, for example, about neutron stars, black holes, and supernovas that could lead to the discovery of hitherto unknown aspects of the universe.
The most exciting potential of gravitational waves lies in what they can reveal about the creation of the universe. Unlike electromagnetic waves, which were unable to escape the compact early universe until some 300,000 years after the Big Bang, gravitational waves will enable scientists to study the universe milliseconds after its creation, according to Worthey. Says Bose, “Using gravitational waves, we would be able to pin down the age of the universe much more accurately and study how the universe evolved over time.”
Because LIGO needs to be incredibly sensitive to detect gravitational waves, scientists have to eliminate or isolate a wide variety of background noise. Seismic vibrations are one of the main disturbances. Through ground vibration, LIGO can detect rush-hour traffic in Richland 14 miles away, and can every few seconds sense waves hitting the Washington coast hundreds of miles to the west. LIGO can also sense the pull of the sun and moon and detect aircraft flying over the facility.
LIGO scientists continue to develop ways in which to isolate gravitational waves from background noise. But the most important instrument for filtering out background noise may be a nearly identical facility located 1,900 miles away near Livingston, Louisiana. By operating the two instruments simultaneously, scientists can filter out background noises that are not detected at both locations. The distance between the two facilities will also enable scientists to triangulate the position of the source of a gravitational wave by comparing its time of arrival at each facility. Facilities in Germany and Japan will also help in the triangulation process.
LIGO’s first scientific run took place in September 2002 and lasted 17 days. Even though no gravitational waves were detected, Bose says, the run was seen as a success, because it enabled scientists to make improvements in the instruments. Many of those improvements were tested in LIGO’s second scientific run, which started in February and continued for eight weeks in collaboration with facilities across the world. With each run, LIGO scientists will continue to improve the instrument until it is able to detect the expanding and contracting of distances caused by gravitational waves.
Eighty-seven years ago Einstein predicted the existence of gravitational waves, but he said they would be too small to measure. Today LIGO scientists will have to prove him wrong in order to prove him right.