Date: Thu, 9 Jan 2003 13:56:14 -0500 (EST)
From: "Jim Ray (NGS 301-713-2850 x112)" 
To: ivsmail@ivscc.gsfc.nasa.gov
Subject: Scientist measures speed of gravity 

 PRESS RELEASE
 Date Released: Wednesday, January 08, 2003
 University of Missouri

            Scientist Measures Speed of Gravity

 Physicist Who Conceived Experiment Led Effort 

 COLUMBIA, Mo. -- For the first time, scientists have measured the
 speed of gravity, one of the fundamental constants of Albert
 Einstein's 1916 general theory of relativity. Led by Sergei
 Kopeikin, a physicist at the University of Missouri- Columbia, a
 team of scientists took advantage of a rare cosmic alignment on
 Sept. 8 to test Einstein's assumption that gravity moves at the
 speed of light. 

 "Newton thought that gravity's force was instantaneous," Kopeikin
 said. "Einstein assumed that it moved at the speed of light, but
 until now, no one had measured it." 

 Kopeikin worked with Ed Fomalont, a radio astronomer with the
 National Science Foundation's National Radio Astronomy Observatory
 (NRAO) in Charlottesville, Va. On Sept. 8, the scientists measured
 the shift of a quasar, a celestial object that resembles a star.
 Jupiter's gravitational force caused the quasar to shift as
 Jupiter passed by it closely. 

 "We have determined that gravity's propagation speed is equal to
 the speed of light within an accuracy of 20 percent," Fomalont
 said. 

 The scientists presented their findings to the American
 Astronomical Society's meeting in Seattle, Wash. The landmark
 measurement is important to physicists working on field theories
 that attempt to combine particle physics with Einstein's general
 theory of relativity and electromagnetic theory. 

 To conduct the experiment, the scientists used the National
 Science Foundation's Very Long Baseline Array (VLBA), a
 continent-wide, radio-telescope system, and a 100-meter radio
 telescope in Effelsberg, Germany, to make an extremely precise
 observation when Jupiter passed in front of the quasar. The
 observation recorded a slight "bending" of the radio waves coming
 from the quasar because of the gravitational effect of Jupiter.
 The bending resulted in a small change in the quasar's apparent
 position in the sky in addition to the deflection of light
 calculated by Einstein in 1915. 

 "Because Jupiter is moving around the sun, the precise amount of
 the bending depends slightly on the speed at which gravity
 propagates from Jupiter," Kopeikin said. "Since the effect is very
 small, Einstein neglected it in his calculations." 

 In 1999, Kopeikin extended Einstein's theory for light propagation
 to include the gravitational effects of a moving body on light and
 radio waves. Prior to this study, no one had tried to measure the
 speed of gravity because most physicists had assumed that the only
 way to do so was to detect gravitational waves, Kopeikin said. The
 MU scientist realized that if Jupiter moved closely in front of a
 star or radio source, he could test his theory. 

 The VBLA system is a result of a general radio technique known as
 Very Long Baseline Interferometry (VLBI). 

 "I believe this experiment sheds new light on fundamentals of
 general relativity and represents the first of many more studies
 and observations of gravitation available presently with existing
 VLBI technique," Kopeikin said. "We have a lot more to learn about
 this intriguing cosmic force and its relationship to the other
 forces in nature."