Neutron stars, space-time ripples and gravitational waves all sound more like something from the next “Star Trek” movie than serious science, but a team of University of Mississippi physicists has joined a worldwide collaboration to study these far-out phenomena.
The UM researchers have joined the Laser Interferometer Gravitational-wave Observatory scientific collaboration, which includes more than 500 scientists from 47 institutions globally working to detect gravitational waves for the first time.
Predicted by Albert Einstein as part of his theory of general relativity 91 years ago, these invisible “ripples” in space-time are accepted by many astrophysicists but have never been actually observed. The LIGO collaboration, backed by the National Science Foundation, could unlock unimaginable scientific advances in astrophysics.
“In Einstein’s theory, space and time are described as a single entity called space-time,'” said Mark Cavaglia, assistant professor of physics and astronomy and principal investigator of the UM LIGO team. “Just like a stretched piece of fabric is distorted by a heavy object placed upon it, the space-time geometry is continuously distorted by the presence of mass or energy.”
When large masses – such as neutron stars, exploding stars or black holes – move rapidly, the space-time becomes stirred by their motion, producing gravitational waves that start traveling outward at the speed of light, roughly 186,000 miles per second, Cavaglia explained.
“An interferometer is a device that measures the stretching of space-time by means of a controlled laser beam,” said Vitor Cardoso, a post-doctoral research associate who works closely with Cavaglia.
An interferometer is shaped like a giant “L,” with two long “arms” that have mirrors at both ends and a beam splitter at their common corner. Laser light enters the arms at the beam splitter and bounces between the mirrors repeatedly before returning to the entry point.
“If the two arms have identical lengths, the interference between the light beams will direct all of the light back toward the laser,” Cavaglia said. “If there is any difference between the lengths of the two arms, some light will travel to where it can be recorded by a photodetector. A gravitational wave hitting the interferometer causes the length of the two arms to vary in a predictable way.”
LIGO uses three giant interferometers of this kind. Two (with arm lengths of two kilometers and four kilometers) are near Hanford, Wash. The third (with an arm length of four kilometers) is near Livingston, La. The UM LIGO team travels the four-hour drive to Louisiana where they monitor the detectors and search for gravitational waves emitted by massive black holes.
UM physicists have studied astrophysical sources of gravitational waves for years. Their scholarship has been recognized by their peers at other institutions and their findings published extensively in academic journals. Cavaglia recently was appointed a visiting scientist at Livingston Laboratory.
The UM LIGO team also includes graduate students Jun-Qi Quo of Shandong Province, China, and Julio Gonzalez Tafoya of Tepic Nay, Mexico.
“I believe it is very important for the Department of Physics and Astronomy to have joined the LIGO scientific collaboration,” said Thomas Marshall, UM chair and professor of physics and astronomy. “Our membership in LIGO will bring substantial benefit to our faculty and students. Proximity to the LIGO Louisiana site with its new outreach center will also increase outreach opportunities.”
LIGO was designed and constructed by scientists from the California Institute of Technology and Massachusetts Institute of Technology and by industrial contractors. Construction was completed in 1999, and the detectors began operating in 2001.
Although not yet detected directly, the influence of gravitational waves on a binary pulsar (two neutron stars orbiting each other) has been measured accurately, and the measurements were found to agree with predictions Einstein made in 1916. Scientists therefore have great confidence that gravitational waves do exist. American physicists Joseph Taylor and Russel Hulse won the 1993 Nobel Prize in Physics for their studies in this field.